Dynaflow, Turboglide, Roto Hydra-Matic, and Other Early GM Automatics

The Hydra-Matic, GM’s first fully automatic transmission, was a great success, inspiring a host of rivals — including some within General Motors itself. In this installment of Ate Up With Motor, we look at the origins of Dynaflow and Powerglide, the ambitious but ill-fated Turboglide and Flight Pitch Dynaflow (a.k.a. Triple Turbine), the later Controlled Coupling Hydra-Matic and Roto Hydra-Matic, and more.

AUTHOR’S NOTE: This article, originally written in 2010, has been extensively revised and expanded for 2016.

Dynaflow badge on a 1951 Buick Super Riviera © 2007 Aaron Severson


I say this often, but on an article like this, it bears repeating in boldface type: I CANNOT to tell you how to fix any of these transmissions. I DO NOT sell (or buy) parts and I can’t help you find parts for them! If you have maintenance or repair questions, I strongly recommend that you seek out a factory service manual and/or consult a transmission specialist familiar with early automatics.


As we saw in our first installment, the original Hydra-Matic, introduced in late 1939, was the world’s first really successful fully automatic transmission. By 1952, General Motors’ Detroit Transmission Division had produced more than 2 million Hydra-Matics, which were used by Oldsmobile, Cadillac, Pontiac, and a variety of outside automakers, ranging from Kaiser-Frazer to Muntz. Hydra-Matic was standard on all Cadillacs by the early fifties and went into most Oldsmobiles and more than 80% of Pontiacs.

Notably absent from the list of Hydra-Matic users were GM’s other automotive divisions, Buick and Chevrolet. Instead, between 1948 and 1963, those divisions fielded no fewer than seven distinctly different automatic transmissions, none of them related to the original Hydra-Matic or its successors, which we’ll discuss in more detail later in this article. (Starting in 1954, Chevrolet did offer Hydra-Matic on Series 3100, 3600, and 3800 trucks, but not on passenger cars.) Moreover, Buick and Chevrolet did not use the same transmissions, although their respective designs were conceptually similar in many respects.

This curious divergence may perplex the modern reader accustomed to a world of corporate engines and transmissions, even at GM. At almost any other automaker, then or now, Hydra-Matic (in various light-, medium-, and heavy-duty versions) would have been the automatic transmission until being phased out in favor of something newer and/or better. Even more surprising is the fact that the original impetus for Buick and Chevrolet’s unique automatic transmissions came not from the engineering staffs of those divisions (which in that era still enjoyed considerable autonomy), but rather from one of the principal architects of Hydra-Matic.

Engineer Oliver K. Kelley (often known as “O.K.” Kelley) began his career as an engineer at Cadillac in the late twenties and later worked for GM’s Yellow Truck and Coach Manufacturing subsidiary before joining Earl Thompson’s transmission development group, which by then had become part of the central Engineering Staff. Although Hydra-Matic was a team effort building on ideas Thompson had been developing since 1932, the three patents that most closely reflect the early production versions of the Hydra-Matic transmission were actually in Kelley’s name. When preproduction of the initial Model 180 Hydra-Matic began in mid-1939, Kelley was among the corporate engineers reassigned to Detroit Transmission Division (of which Kelley’s colleague William L. Carnegie became the first chief engineer) to oversee the transition from prototype to mass production.

We may presume, therefore, that Kelley was as familiar as anyone was with the original’s Hydra-Matic’s strengths and various limitations. As we’ve previously discussed, Hydra-Matic was very clever in many respects, but it was by no means a light, compact, or mechanically elegant design and it can’t have been cheap to manufacture. Furthermore, its operation was far from seamless even under the best of conditions, something that would earn the transmission considerable criticism in the years to come. There was obvious room for improvement.

Nonetheless, considering how much money GM had invested in the project, proposing, as Kelley and his colleague George R. Smith did in the summer of 1939, that the corporation begin working on another new and completely different automatic transmission was a bold suggestion indeed — particularly since at that point Hydra-Matic had not yet gone on sale. The most compelling point of Kelley and Smith’s argument, and the likely reason their proposal was not dismissed out of hand, was Hydra-Matic’s substantial production costs. While those might be acceptable for the senior divisions, which could pass the cost along to the customer, Hydra-Matic was expensive enough to be a questionable proposition for Chevrolet. Chevrolet owners were as weary as anyone of shifting gears (as evidenced by Chevrolet’s decision to make a vacuum-assisted shift linkage standard equipment for 1940), but whether the buyer of an $800 Chevy would be willing or able to spend $100 or more for a self-shifting transmission was another matter. The demand was there, but to tap it, Chevrolet would need an automatic transmission that could be priced to sell.

We don’t know what higher-level discussions Kelley and Smith’s proposal may have prompted, but the gist is not hard to guess. Even during the Depression, Chevrolet’s total sales volume had only once fallen below 400,000 units per year, and 1939 sales had been closer to 600,000. If Chevrolet could offer an automatic affordable enough to achieve a take-up of 50% or better, that would mean more than a quarter of a million transmissions a year. Since very few American drivers liked to shift, offering such a transmission would also give buyers a compelling reason to choose Chevrolet over low-priced rivals, so Chevrolet might even stand to increase its market share. With numbers like that, developing an automatic transmission for Chevrolet was likely to be a worthwhile investment even if it didn’t share a single bolt with Hydra-Matic.

The upshot was that Kelley and Smith’s rather daring proposal eventually paid off. In the summer of 1940, as first-year production of Hydra-Matic was winding down, they were transferred to the Engineering Staff as part of a reorganized transmission research team (known in contemporary GM vernacular as a product study group). This worked out particularly well for Kelley. Not only was he once again doing advanced research work — which we have to assume was vastly more interesting than production engineering — he was now leading the team, Earl Thompson having left General Motors about three months earlier.

The initial focus of Kelley’s new group was on torque converters. As Kelley was undoubtedly aware, some Yellow Truck & Coach buses had recently begun offering a Spicer torque converter transmission, a licensed derivative of the Lysholm-Smith unit developed by engineer Alf Lysholm of the Swedish firm Ljungstroms Angturbin AB. Over the previous decade, that transmission and others like it had become increasingly common for bus and railroad use, although to our knowledge, there had not yet been any production automotive applications.

Porsche torque converter cutaway © 2006 BerndB~commonswiki [assumed] (CC BY-SA 3.0 Unported)
A cutaway model of a modern Porsche torque converter. The lower set of blades are part of the stator, which redirects the flow of oil returning from the turbine to the impeller. The springs visible near the center of the image are part of the lockup clutch, which mechanically locks the engine flywheel to the transmission input shaft for greater efficiency at cruising speeds. Some prewar torque converter bus transmissions used lockup clutches, as did Packard’s postwar Ultramatic and some early Borg-Warner automatics, but lockup converters were not common on automotive transmissions until the late seventies. (Photo: “Torque-converter-cutbox-model” © 2006 BerndB~commonswiki [assumed]; resized 2010 by Aaron Severson and used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)

Today, we’re accustomed to thinking of torque converters primarily as clutches, but a torque converter is also a type of infinitely variable transmission. (See the next page for a further explanation.) The bus and rail-car torque converter transmissions of the thirties used the converter primarily as a transmission, sometimes adding a separate clutch to connect the converter to the engine; conventional reduction gears were typically used only for reverse. Such transmissions were capable of providing torque multiplication comparable to Hydra-Matic with no perceptible steps and no need for a complicated hydraulic control mechanism, making them a potentially attractive Hydra-Matic alternative for Chevrolet.

Before Kelley and company had had the time for more than preliminary research, however, outside circumstances shifted their attention to a very different application.


In June 1940, about two months before the establishment of Kelley’s new product study group, GM president William S. Knudsen was summoned to Washington, D.C., where he was asked to oversee the ramp-up of American military production. By then, Europe had been at war for months, a growing number of European nations had fallen to the Nazis, and Great Britain’s position was looking increasingly precarious. Knudsen’s assignment was to enlist domestic industry in the accelerating U.S. rearmament effort.

Late that year, Kelley’s group was asked to shift their attention from a potential Chevrolet automatic to the development of a transmission that could take the place of the conventional gearboxes then used in most U.S. armored fighting vehicles (AFVs). The idea of automatic transmissions for tanks may sound faintly ridiculous, but what is merely annoying in a car — e.g., the need to shift gears — can be positively hazardous for a combat vehicle, particularly a lightly armored one. While Cadillac would shortly adopt Hydra-Matic for use in light tanks (mated, as we explained in Part 1 of this article, to Cadillac V-8 engines), Hydra-Matic had neither the torque multiplication nor the torque capacity needed for heavier AFVs.

Kelley and his team responded to this request by devising a heavy-duty semiautomatic torque converter transmission that was subsequently produced by Allison (then a GM division) under the trademark Torqmatic. The original Torqmatic 900T AFV transmission combined a six-element torque converter (a single impeller, three turbines, and two stators) with two hydraulically controlled planetary gearsets, providing three forward speeds and one reverse. The transmission still had to be shifted manually, but there was no need to de-clutch and little danger of missing a shift. Moreover, the torque converter alone provided a stall ratio of 4.8:1, so a useful amount of torque multiplication was available even in the direct-drive third gear.

M18 Hellcat Tank Destroyer © 2007 User:Dammit (CC BY-SA 2.5 Netherlands)
The M18 Hellcat tank destroyer (officially known as 76mm Gun Motor Carriage M18) was designed and manufactured by Buick, which built 2,507 Hellcats in all. The M18 had a gross weight of about 38,000 lb (about 18 metric tons), was capable of more than 50 mph (80 km/h), and was armed with a 76mm (3-inch) gun (made by Oldsmobile) that gave it the ability to destroy the latest German Panzers. The Hellcat’s principal limitation was extremely thin armor, although in that respect, the M18 was more survivable than some of the U.S. Army’s earlier tank destroyers. Some M18s remained in service for many years after the war’s end. (Photo: “M18 Hellcat side” © 2007 User:Dammit; resized 2010 by Aaron Severson and used under a Creative Commons Attribution-ShareAlike 2.5 Netherlands license)

This transmission was selected for the Buick-developed T-70 tank destroyer, which entered service in 1943 as the M18 Hellcat. The 900T helped to keep the M18’s nine-cylinder air-cooled Continental radial engine within its narrow power band all the way up to the Hellcat’s 50+ mph (80+ km/h) top speed and had the torque capacity to withstand the 972 cu. in. (15,972 cc) engine’s monstrous 940 lb-ft (1,275 N-m) net torque output, which would have made an oily metal milkshake of the Hydra-Matic’s innards. The transmission performed well in the M18 and later in the derivative M39 armored utility vehicle and the M26 Pershing medium tank, both introduced in 1944.

It was obvious early on that the torque converter transmission would also be well-suited to heavy civilian vehicles and equipment. After the war, Allison developed Torqmatic into an extensive and long-running line of heavy-duty torque converter transmissions for different military, commercial, and industrial applications, including trucks, buses, and heavy machinery. (Today, Torqmatic remains a trademark of Allison Transmission, which is no longer owned by General Motors.)


While military work remained the top priority for Kelley and his team (and the auto industry in general) until late in the war, they had not forgotten their original objective. In late 1944, Kelley filed a patent application (U.S. Patent No. 2,606,460) for an automotive torque converter transmission combining a simple planetary gearset with a novel five-element torque converter featuring dual stators and dual impellers. The large primary impeller was driven directly by the engine. The much smaller secondary impeller, which was positioned between the stators and the inlet of the primary impeller, was connected to its larger sibling by an overrunning clutch.

The purpose of this unusual arrangement was to create the equivalent of a single impeller with two distinctly different blade profiles. The primary impeller was optimized for near-stall conditions (i.e., the period of highest torque multiplication, when the turbine was moving very slowly or not all). During that period, the primary impeller acted alone while the small secondary impeller freewheeled idly on its overrunning clutch. The secondary impeller initially spun much faster than the engine, but secondary impeller speed decreased as turbine speed increased. Once the two impellers were turning at the same speed, the overrunning clutch locked them together. The blades of the secondary impeller then acted as extensions of the first, effectively optimizing them for cruising efficiency.

Color diagram of a 1948–1952 Buick Dynaflow transmission © 2016–2017 Aaron Severson
A simplified, somewhat abstracted, definitely not-to-scale diagram of the early dual-impeller Buick Dynaflow, offered from 1948 through 1952. When cruising at a steady speed, the secondary impeller (dark red) functioned as an extension of the primary impeller (red). At times when that extension wasn’t needed or helpful, such as starting from rest, the secondary impeller simply freewheeled on its clutch (fuchsia with heavy black triangle). Incidentally, while a Ravigneaux planetary gearset is difficult to represent in two-dimensional plan view, Dynaflow actually had six planet gears, arranged in an evenly spaced ring (alternating short/long/short). (author diagram)

By mid-1945, Kelley’s group had installed working prototypes of this transmission in several test mules. Although the torque converter transmission had been conceived with Chevrolet in mind, Kelley also showed off the prototypes to engineers at the other automotive divisions to see if any of them were interested in the new design as a potential alternative to Hydra-Matic.

The strongest interest came from Buick chief engineer Charles A. Chayne. Buick had been very resistant to the earlier Automatic Safety Transmission and had declined to adopt Hydra-Matic, which Chayne caustically nicknamed “Hydra-Jerk.” Some of Buick’s antipathy toward Hydra-Matic was probably attributable to divisional pride; some years earlier, the division had spent a lot of time and money on the abortive “Roller,” an infinitely variable friction drive transmission that Buick general manager Harlow Curtice had been obliged to cancel back in 1934. Nonetheless, the concerns about shift harshness were probably not without merit. Unlike Oldsmobile, Cadillac, and Pontiac, which used open driveshafts, Buick (and Chevrolet) in those days used torque tube drive, which combined the enclosed driveshaft and rear axle into a single rigid assembly connected to the transmission via a single U-joint. Since the primary purpose of the torque tube was to transmit drive torque, the mass of the axle assembly would have amplified each of Hydra-Matic’s firm shifts into an uncouth thump, hardly in keeping with Buick’s upscale image.

Chayne and Curtice both sampled the prototype torque converter automatic and found it much more to their liking. It was mechanically straightforward and offered seamless if rather stately acceleration. Chayne subsequently assigned Buick’s own engineers to collaborate with Kelley’s team on the development of a production version of the torque converter transmission for Buick.

Dynaflow badge on a 1949 Buick Roadmaster sedanet © 2008 Aaron Severson
Dynaflow was initially a $206 option available only on Roadmasters; it became standard on Roadmasters and optional on other Buick models for 1949. Cars with Dynaflow had slightly more power than standard-shift cars to make up for the automatic transmission’s substantial slippage and limited torque multiplication. (author photo)

After much development and extensive testing, the new transmission, which Buick christened Dynaflow, was finally announced in January 1948. It went on sale in March as an option for the top-of-the-line Buick Roadmaster. While Dynaflow was mechanically simpler and somewhat lighter than Hydra-Matic, the Buick transmission was no cheaper — initial list price was $206 (more than $2,000 in 2016 dollars), some $20–$30 more than the contemporary Hydra-Matic.

Dynaflow retained the five-element, dual-impeller torque converter of the early prototypes, but the planetary transmission adopted what today is commonly known as a Ravigneaux gearset (after French inventor Pol Ravigneaux, who patented many variations of this layout in the thirties and forties). This comprised two sun gears, six planet gears (three short, three long) on a single planet carrier, and a single annulus (ring gear). The front sun gear was affixed to a brake drum and a multi-disc direct drive clutch. The input shaft from the torque converter turbine passed through the center of the front sun gear (which had a hollow center for that purpose) to drive the rear sun gear. The annulus formed a second brake drum surrounding the sun and planet gears, whose carrier was affixed to the output shaft.

Each of the two drums was surrounded by a contracting band brake. Engaging only the front brake (the low band) would put the gearset in reduction while engaging only the rear brake (the reverse band) provided reverse reduction; gear ratios were +/-1.82:1 respectively. Releasing both brakes and engaging the direct drive clutch locked the input shaft to the front brake drum so that both sun gears (and thus the entire gearset) would rotate together at the same speed as the torque converter turbine. Releasing the direct drive clutch as well as both bands put the transmission in neutral, allowing the gears to turn idly. A mechanical pawl allowed the output shaft to be locked in place to serve as a parking brake; unlike the early Hydra-Matic, there was a separate position for this on the shift quadrant. Other notable Dynaflow features that Hydra-Matic lacked were an oil cooler (in this era used only on heavy-duty Hydra-Matics) and a pair of hydraulic accumulators to damp the shock of clutch or band engagements.

The original Dynaflow is often described as a two-speed automatic, but the only automatic “shifting” the transmission provided was via the torque converter. Like Hydra-Matic, Dynaflow had front and rear oil pumps supplying operating pressure to control the transmission’s clutch, bands, and parking pawl. Unlike its corporate cousin, however, Dynaflow had neither a hydraulic speed governor nor a throttle valve, relying entirely on the position of the selector lever to direct the flow of oil to the appropriate elements for each range. With the selector in Drive, the direct drive clutch remained engaged at all speeds. The driver could manually select Low, which released the clutch and engaged the low band, but the transmission would then remain in that gear until the selector was moved to a different range. This was by design; while it wouldn’t have been difficult to engineer the transmission to start in its reduction gear and shift automatically to and from direct drive, Kelley’s team and their counterparts at Buick wanted normal operation to be as ‘stepless’ as possible.

The tradeoff was performance. While non-automotive torque converters often had stall ratios of 4.00:1 or more, those units were intended for use with heavy-duty engines that spent much of their operating lives at or near full throttle. To provide torque characteristics more suitable for an automotive engine — and to avoid excessive slippage at cruising speeds — the early Dynaflow converter had a stall ratio of only 2.25:1. That was taller (numerically lower) than second gear of a contemporary Hydra-Matic and significantly taller than the 2.67:1 low gear of Buick’s standard three-speed manual transmission.

1949 Buick Roadmaster front 3q © 2008 Aaron Severson
The 1949 Buick Roadmaster was distinguished from lesser Buicks by its longer 126-inch (3,200mm) wheelbase (compared to 121 inches/3,073mm for other models); both figures were down 3 inches (76 mm) from 1948. For 1949, the Roadmaster came standard with Dynaflow and Buick’s biggest 320 cu. in. (5,247 cc) eight, making 150 gross horsepower (112 kW) and 280 lb-ft (380 N-m) of torque. The extra power was offset somewhat by a taller (lower numerical) axle ratio. (author photo)

In partial compensation, Dynaflow-equipped Roadmasters used a special high-compression version of Buick’s 320 cu. in. (5,247 cc) straight eight that added an extra 6 hp (4 kW) and 4 lb-ft (5 N-m) of torque. Even so, Dynaflow’s off-the-line response was lethargic unless you manually selected Low, which could be done at any speed up to about 45 mph (72 km/h). Unfortunately, frequent manual gear changes exacerbated the already heavy fuel consumption and would eventually take their toll on the transmission’s low band and direct drive clutch, which were intended for only occasional use. Buick cautiously described Dynaflow’s reduction gear as “emergency low.”

Despite those shortcomings, Dynaflow was well-suited to the character of postwar Buicks, which emphasized unhurried plushness over performance or road manners. The average Buick buyer of the time was not terribly concerned with fuel economy and welcomed Dynaflow’s lazy smoothness. It was too bad that Buick no longer offered formal cars; Dynaflow lent itself admirably to a processional pace.


Although Buick had beaten Chevrolet to the punch, GM’s largest automotive division had also evaluated the corporate engineers’ torque converter transmission and begun work in 1946 on a production version for Chevrolet. Dubbed Powerglide, it finally debuted as a $159 option for 1950 Chevrolet DeLuxe models.

Powerglide badge on a 1952 Chevrolet Bel Air © 2010 Aaron Severson
Early Powerglide-equipped Chevrolets came with a bored-and-stroked 236 cu. in. (3,859 cc) version of the familiar Stovebolt Six — similar to the engine used in Chevrolet trucks, but with different valve timing, a higher compression ratio, and hydraulic lifters. The bigger engine gave 1950–1952 Chevrolets with Powerglide 105 gross hp (78 kW) compared to the 92 hp (69 kW) of standard-shift cars’ 217 cu. in. (3,547 cc) engine. (author photo)

In its original form, Powerglide was much like the early Dynaflow — not surprising considering that both were production derivatives of the same basic corporate design. Both transmissions used a two-speed Ravigneaux gearset providing 1.82:1 low and reverse ratios. Both had a five-element torque converter with dual stators and dual impellers, although Powerglide’s stall ratio was slightly lower, at 2.20:1. Powerglide’s hydraulic control system also included a vacuum modulator that varied operating pressure based on the engine’s manifold air pressure, a feature Dynaflow didn’t have.

Powerglide’s principal novelty, developed and patented by Kelley and William S. Wolfram (U.S. Patent No. 2,651,918), was an unusual auxiliary fluid coupling, incorporated within the torque converter and sharing the same oil supply. The auxiliary coupling’s “impeller” was actually an additional set of vanes mounted on the turbine torus, a little inboard of the turbine inlet, while the “turbine” was a comparable set of vanes on the primary impeller. Each set of auxiliary vanes was curved in the opposite direction of the corresponding primary blades.

Color diagram of a 1950-1952 Chevrolet Powerglide transmission © 2016–2017 Aaron Severson
A diagram (again, simplified, abstracted, and not to scale) of the early dual-impeller Chevrolet Powerglide, showing the auxiliary coupling. Whenever the turbine spins faster than the impeller (for example, when coasting or push-starting a stalled car), the auxiliary blades on the turbine torus propel oil at the auxiliary blades on the primary impeller torus. This seems like a good place to mention that these diagrams’ representation of fluid coupling/torque converter impellers and turbines depicts the fluid path, not the actual external appearance of the torus discs! (author diagram)

The auxiliary coupling was intended to address a minor but disconcerting flaw of most fluid clutches: a distinct shortage of engine braking when the speed of the output shaft exceeds the speed of the engine (technically known as overrun), such as when coasting or descending a hill with the throttle closed. Under those conditions, the output shaft of an early Powerglide-equipped car would turn the converter turbine, whose auxiliary vanes would transmit that motion to the auxiliary vanes on the primary impeller and attempt to overdrive the engine; the engine’s inertia would then provide a braking effect. The auxiliary coupling also allowed the car to be push-started at speeds as low as 12 mph (19 km/h). The downside was a bit of additional drag within the converter during normal acceleration.

In other respects, Powerglide operated very much like Dynaflow and suffered the same limitations. The performance penalty was even more pronounced with the Chevrolet six than with Buick’s big straight eight, particularly since Powerglide included a taller 3.55:1 axle ratio, compared to 4.11 for Chevrolets with manual shift. Despite the bigger, more powerful engine that was standard with Powerglide, Chevrolets with automatic were more than five seconds slower to 60 mph (97 km/h) than standard-shift cars (assuming a start in Drive) and returned less-than-frugal fuel economy.

1952 Chevrolet Bel Air Powerglide quadrant © 2010 Aaron Severson
The early Powerglide had the same PNDLR shift quadrant as the early Dynaflow. Operation was similar to the Buick transmission in most respects. (Powerglide’s auxiliary coupling required no special controls or driver intervention.) (author photo)

Although these limitations did little to dampen buyer enthusiasm, Chevrolet quickly moved to address Powerglide’s performance shortfall with an extensive redesign of the transmission, introduced for the 1953 model year. The planetary transmission retained the same internal ratios as before, although the low band and clutch were beefed up. However, a completely redesigned hydraulic control system now started in reduction rather than direct drive and executed automatic upshifts and downshifts at speeds up to 42 mph (68 km/h). As in Hydra-Matic, shift points were determined by road speed and throttle position. At the same time, the five-element torque converter and its auxiliary coupling were discarded in favor of a simpler (and undoubtedly cheaper) three-element unit. The stall ratio was reduced slightly, to 2:10:1, but overall starting ratio in Drive was now 3.82:1.

This revised arrangement was a compromise of Kelley’s original vision, but it was much better suited to Chevrolet’s needs and worked well enough for most buyers. Chevrolet would later offer another “pure” torque converter automatic, the Turboglide (discussed in more detail later in this article), but Powerglide would remain the division’s principal transmission well into the sixties.

There were of course a number of design changes along the way. Torque capacity had to be increased several times to cope with progressively larger and more powerful engines. For 1958, Powerglide also got a revised hydraulic system with a new PRNDL shift pattern, reducing the potential confusion for drivers switching between Powerglide-equipped cars and ones with Turboglide, which already used the latter pattern.

For 1960, Chevrolet engineers adapted Powerglide components to create a lightweight automatic transaxle for the rear-engine Corvair. The torque converter of the Corvair Powerglide was mounted at the back of the transaxle, behind the rear axle line, while the planetary gearbox was ahead of the axle. A narrow central shaft passing through the center of the differential pinion allowed the engine to drive the transaxle’s front oil pump. Around the oil pump driveshaft was the main shaft, connecting the torque converter turbine to the planetary gearbox. The output shaft was a hollow sleeve surrounding the main shaft, connecting the gearset’s planet carrier to the differential gears. The Ravigneaux gearset itself was similar to that of the standard Powerglide and shared the same +/-1.82 indirect ratios, but traded the reverse band brake for a multi-disc reverse clutch that performed the same function. The standard Powerglide’s parking pawl was omitted in the interests of cost reduction, but the Corvair unit got a lighter aluminum case and a higher, 2.60:1 stall ratio, providing the lightweight Corvair with surprisingly peppy performance.

Color diagram of a 1960-1969 Chevrolet Corvair Powerglide transaxle © 2016–2017 Aaron Severson
A simplified, abstracted diagram of the Corvair Powerglide. (The earlier disclaimer regarding scale goes double for this one — no attempt was made to make this to scale.) Note the replacement of the earlier Powerglide transmission’s reverse band with a multi-disc reverse clutch performing the same function. The later front-engine, aluminum-case Powerglide adopted the same feature, albeit with more clutch discs for greater torque capacity. (author diagram)

For the 1962 model year, Chevrolet introduced new light-duty and heavy-duty versions of the conventional Powerglide, now also using an aluminum case and adopting a Corvair-style multi-disc reverse clutch. The light-duty unit, used in the compact Chevy II, retained the +/-1.82 ratios, but the heavy-duty unit had a revised gearset with indirect ratios of +/-1.76. The “standard” medium-duty Powerglide used in most full-size Chevrolets adopted most of these changes and the aluminum case for 1963. In this form, and with various further refinements, Powerglide remained in use on various North American models through the 1973 model year.

In 1968, Chevrolet revived the original Powerglide concept with Torque-Drive, a torque converter transmission with a two-speed Ravigneaux gearset and a simple hydraulic control system that included no provision for automatic gear changes. Although similar in operation to the original 1950–1952 Powerglide, Torque-Drive had an aluminum case and three-element torque converter like its latter-day brethren. By sixties standards, Torque-Drive — which Chevrolet now described as semiautomatic — was rather quaint, although its mechanical simplicity enabled Chevrolet to sell it for as little as $68.65, over $100 less than the fully automatic Powerglide. Torque-Drive was available on six-cylinder Camaros through 1970, on the four- and six-cylinder Chevy II and Nova into 1971, and on the first-year Chevrolet Vega.


The 1953 model year also saw the introduction of a heavily revised Buick automatic, dubbed Twin-Turbine Dynaflow. Developed by a group of Buick engineers led by Rudolf J. Gorsky, the twin-turbine transmission was again based on concepts originated in O.K. Kelley’s corporate engineering team; most of the underlying patents (in particular U.S. Patents 2,766,641; 2,782,659; and 3,025,720) were in Kelley’s name. The apparent objectives of the new transmission were to provide additional torque multiplication without hurting part-throttle fuel economy, raising the converter stall speed, or compromising the outstanding smoothness that had always been Dynaflow’s principal virtue.

Twin-Turbine Dynaflow retained the original Dynaflow’s Ravigneaux gearbox, but the torque converter was a new four-element design with a single impeller, a single stator, and — as the name implied — two turbines. The first turbine, which faced the impeller, was essentially a metal ring with closely spaced, slot-like radial vanes around its rim. That ring was pressed into a drum-like support shell, within which was the second turbine, a more conventional bladed torus. The stator, which sat within the first turbine ring, was positioned to receive the ‘backwash’ of oil exiting the second turbine.

Within the turbines’ central hub was an additional planetary gearset. (Insofar as the torque converter and gearbox were separate entities, this gearset was part of the converter.) The annulus of the converter gearset was driven by the first turbine support shell. The gearset’s planet carrier was attached at one end to the second turbine and at the other to the gearbox input shaft. The converter gearset sun gear, meanwhile, was connected to the hub of the stator and shared its one-way clutch. Reverse torque on either element would lock both elements, putting the converter gearset in reduction. This multiplied any torque applied to the first turbine by a ratio of 1.60:1 and forced the second turbine to rotate at 62.5% percent of the speed of the first turbine (i.e., first turbine speed divided by 1.6).

The stream of oil from the impeller would first enter the first turbine, where the oil attempted to impart its angular velocity — that is, to apply torque — to the turbine vanes. Thanks to our old pal, Newton’s Third Law of Motion, whatever torque the oil stream exerted on the turbine vanes would apply an equal and opposite reaction torque on the oil. Since the vanes of the first turbine were open at the back, oil passing through them retained its forward momentum, but the reaction torque effectively reduced the oil’s angular velocity. Oil exiting the first turbine would then enter the inlet of the second turbine, pass through its vanes to the turbine outlet, and then curve back through the stator blades to the impeller.

Up to the point of stall (that is, as long as neither turbine was moving), the oil stream would apply all or nearly all of its torque to the vanes of the first turbine. Consequently, at stall and for a brief period thereafter, the vanes of the first turbine exerted so much reaction torque on the oil stream that the oil exited the first turbine spinning in the opposite direction and therefore opposed the rotation of the second turbine, slightly reducing the net torque on the gearbox input shaft.

Color diagram of a 1953–1954 Buick Twin Turbine Dynaflow torque converter © 2016 Aaron Severson
A diagram (again abstracted, somewhat simplified, and definitely not to scale) of the Twin-Turbine Dynaflow torque converter, omitting the mostly carryover Ravigneaux gearbox. The 1955 single-stator Variable-Pitch Dynaflow converter was very similar, but had two one-way clutches (one sprag-type, one cam-and-roller) rather than one. (author diagram)

Once the first turbine began to move, the oil stream exerted progressively less torque on the first turbine’s vanes, which in essence were now trying to run away from the spinning oil stream. (This is a gross simplification of some rather complicated vector math, but we figure you’re probably confused enough already.) The reduced torque on the first turbine’s vanes meant the vanes also exerted progressively less reaction torque on the oil, allowing the oil stream to enter the second turbine with a somewhat reduced but still positive angular velocity (that is, spinning in the same direction as the impeller) and exert a steadily increasing positive torque on the second turbine’s vanes. To put it another way, discounting slippage, any impeller torque not applied to the first turbine would be applied to the second. Again, torque on the first turbine was multiplied by the planetary gears; torque on the second turbine was not.

As long as the sun gear clutch remained locked, the planetary gearset forced the two turbines to maintain a fixed speed ratio. Therefore, the second turbine could not turn faster (or slower) than 62.5% of the first turbine’s speed. Once the rotational speed of the first turbine was close to impeller speed, however, there was enough torque on the second turbine to cause it, and thus the planet carrier, to overdrive the annulus and the first turbine rather than being driven by them in reduction. That unlocked the sun gear/stator clutch (causing all torque multiplication to cease) and allowed the stator, the sun gear, the annulus, and the first turbine to freewheel idly; the first turbine was now turning fast enough that the oil stream could no longer exert a meaningful amount of torque on the turbine vanes. The torque applied to the second turbine, meanwhile, caused the second turbine (and thus the carrier and the gearbox input shaft) to continue accelerating until it was rotating at close to engine speed.

Like any torque converter, the torque multiplication provided by the dual-turbine converter was continuously variable, peaking at stall and gradually diminishing with increasing turbine speeds. However, there was now significantly more area under the curve thanks to the converter gearset’s additional mechanical advantage. Despite the initial interference between the turbines, which hurt the converter’s efficiency at stall, the converter gearset allowed a higher net stall ratio — now 2.45:1 — and somewhat lower stall speeds. The use of two separate turbines also allowed each to be optimized for its respective operating regime, providing more efficient cruising for better fuel economy without sacrificing off-the-line performance.

Dynaflow shift quadrant on a 1956 Buick Roadmaster convertible © 2010 Aaron Severson
Although the Twin-Turbine and later Variable Pitch Dynaflow both differed internally from the original dual-impeller transmission, the shift quadrant and operation were basically the same from the driver’s perspective. The main difference was that with the 1955 and later Variable Pitch Dynaflow, the stator blades would change position if the accelerator was floored. (author photo)

Twin-Turbine Dynaflow’s principal shortcomings were engine braking and passing response. Even with the converter gearset, there was still little engine braking in Drive; the sun gear clutch would automatically unlock if the output shaft overran the engine. The converter gearset was also of marginal usefulness for passing unless turbine speeds fell significantly below engine speed. Shifting to Low range mitigated both these issues, but Low was really too short to be ideal for passing or mountain driving at highway speeds. Dual-Range Hydra-Matic was much more convenient in those situations.

Buick attempted to address that limitation with the 1955 introduction of Variable Pitch Dynaflow. The revised transmission retained the four-element torque converter and converter gearset, but added Kelley’s latest brainstorm (described in U.S. Patent No. 2,999,400): a variable-pitch feature for the stator blades, similar in principle to a variable-pitch propeller. Rather than being affixed to the stator hub in the usual manner, the stator blades were connected via a series of small crank pins to a servo-controlled annular piston (basically a flat metal ring) that could pivot forward or backward, thus rotating each blade on its crank. Hydraulic pressure on the piston normally held the blades at a low angle relative to the oil stream. Flooring the accelerator, or moving the selector to Low or Reverse, opened a control valve to exhaust one side of the stator servo, flipping the piston to its forward position and cranking the stator blades to a more upright angle. Backing off on the throttle would reengage the servo, causing the piston to flip back to its normal position and thus crank the stator blades back to low angle.

With the blades at their low-angle position, the converter traded some torque multiplication — net stall ratio in low was 2.10:1 — for nominal stall speeds as low as 1,400 rpm, reduced throttle lag, and greater efficiency at cruising speeds. Shifting the stator blades to high angle brought the net stall ratio to 2.50:1 and raised the stall speed to a nominal 2,600 rpm for stronger off-the-line performance and better passing response. Changing the stator pitch in this way wasn’t as effective as an additional reduction gear, but it was helpful nonetheless. The principal drawback was that at very high road speeds, forcing the stator blades to high angle would hurt performance more than it helped.

To take fuller advantage of the new stator, the converter gearset sun gear was divorced from the stator hub and given its own sprag clutch, separate from the stator’s cam-and-roller one-way clutch. Having its own clutch allowed the stator to remain locked after the first turbine freewheeled (further fattening the torque multiplication curve) or to re-lock in response to load without necessarily putting the converter gearset back in reduction.

Although Variable Pitch Dynaflow provided slightly better performance and somewhat better fuel economy than the earlier Twin-Turbine Dynaflow, an effective starting ratio of no more than 2.50:1 in Drive was still marginal for the steadily increasing curb weights (and steadily decreasing axle ratios) of mid-fifties Buicks. This was addressed for 1956 with a revised five-element torque converter that incorporated dual stators as well as twin turbines.

Color diagram of 1956-1963 Buick Variable Pitch Dynaflow - Twin Turbine torque converter © 2016 Aaron Severson
All of Buick’s dual-turbine transmissions from 1956 through 1963 — 1956–1958 Variable-Pitch Dynaflow, 1959 Twin Turbine, and 1960–1963 Turbine Drive — used a torque converter like the one shown in the diagram. Again, the diagram is simplified and not to scale and certain artistic liberties have been taken in the interests of visual clarity. (For example, the front stator hub actually surrounded the variable stator disc rather than sitting behind it.) (author diagram)

The additional stator — confusingly described as the first stator or front stator — was mounted immediately behind the first turbine and looked much like it. However, the stator blades were angled in more or less the opposite direction so as to counteract the reverse torque that had previously compromised the twin-turbine converter’s efficiency near stall speed. Now, oil entering the second turbine at stall increased the torque on the turbine vanes and the planet carrier rather than opposing their rotation. To put it another way, with the turbines stationary or turning slowly, the rotary flow in different points of the converter was now like this:

  • From the impeller outlet to the first turbine inlet: with the engine
  • From the first turbine outlet to the first stator: opposite the engine
  • From the first stator to the second turbine inlet: with the engine
  • From the second turbine outlet to the variable-pitch stator: opposite the engine
  • From the variable-pitch stator to the impeller inlet: with the engine.

Once the first turbine was turning fast enough that the rotary flow of oil out of the first turbine outlet was no longer opposite the engine’s rotation, the first stator would freewheel on its own sprag clutch.

The variable-pitch stator and converter gearset were retained, but the additional stator increased the converter’s net stall ratio to 3.10:1 at a nominal 1,500 rpm with the variable-pitch stator blades in their low-angle position, or 3.50:1 at 2,800 rpm in high position. The variable-pitch stator controls were also modified so that the blades would normally remain at low angle in Low or Reverse rather than automatically switching to high angle in either of those gears.

Even the high-angle stall ratio didn’t quite match the first-gear ratio of the four-speed Hydra-Matic or the step-off ratios of contemporary two-speed torque converter automatics, but the additional torque multiplication made Dynaflow-equipped cars a good deal less sleepy when starting in Drive. More importantly, as far as Buick was concerned, that improved performance was still obtained without any perceptible shift points.

1957 Buick Roadmaster 75 Riviera coupe front © 2007 Aaron Severson
By the mid-fifties, Dynaflow was standard on upper-series Buicks, like this 1957 Roadmaster Riviera hardtop. On the Buick Special, the cheapest and most popular contemporary model, Dynaflow was nominally optional, but very few cars — probably fewer than 5% of all contemporary Buicks — were built without it. With Variable Pitch Dynaflow and the standard 364 cu. in. (5,957 cc) V8, a 1957 Roadmaster was capable of 0-60 mph (0-97 km/h) in a little over 10 seconds and a top speed of perhaps 115 mph (185 km/h), although such performance required using Low gear. Starting in Drive added roughly 2 seconds to 0-60 mph (0-97 km/h) acceleration times. (author photo)

Variable Pitch Dynaflow — renamed Twin Turbine for 1959 and Turbine Drive for 1960 — received a variety of further refinements, including several revisions to the stator blade pitch (making the stall ratios 3.10:1 and 3.40:1); marginally higher stall speeds; and, from 1961 on, a shorter, slightly lighter case. Turbine Drive was the sole transmission offered on full-size Buicks from 1961 through 1963.


Despite the ongoing development of Powerglide and Dynaflow, GM had no intention of abandoning Hydra-Matic, which was still used in substantial numbers by Pontiac, Oldsmobile, Cadillac, and several outside automakers. Aside from GM’s substantial capital investment in tooling and factory space, which the corporation wasn’t about to casually discard, the various users (and many of their customers) had strong feelings about the comparative advantages of Hydra-Matic and its assorted rivals.

1950 Oldsmobile 88 dashboard and shift quadrant © 2008 Aaron Severson
In the late forties and early fifties, Hydra-Matic was an expensive option on most of the cars that offered it, but it went into more than four-fifths of all Oldsmobiles and Pontiacs and nearly all postwar Cadillacs; Cadillac made it standard in 1952. This 1950 Oldsmobile Eighty-Eight has the earlier postwar Hydra-Matic, with only a single drive range; the Dual-Range version replaced it in 1952. (author photo)

In 1952, the Detroit Transmission Division embarked on a $35 million revamp of the four-speed Dual-Range Hydra-Matic. Walter B. Herndon, one of the engineers from Earl Thompson’s original transmission development group, filed a patent covering most of the fundamentals of the redesigned transmission (U.S. Patent No. 2,876,656) in November 1953, with most of the rest covered in a subsequent application by August H. Borman Jr., Forrest R. Cheek, and Milton H. Scheiter in December 1954 (U.S. Patent No. 3,048,055), but the second-generation Hydra-Matic didn’t actually go on sale until the 1956 model year. (We assume the destruction of the Hydra-Matic plant in Livonia in August 1953 was at least partly responsible for the delay.) Development of the production version, formally known as the Model 315 or Controlled Coupling Hydra-Matic, was credited to Detroit Transmission engineers P.J. Rhoads and Kenneth W. Gage; Gage subsequently moved to Buick, where he worked on later iterations of Dynaflow.

To understand the changes to the second-generation Hydra-Matic (called “Jetaway Hydra-Matic” by Oldsmobile, “Strato-Flight Hydra-Matic” and later “Super Hydra-Matic” by Pontiac), it’s helpful to first recap the major elements of the original version. As we’ve previously explained, the early Hydra-Matic had a single fluid coupling and three planetary gearsets controlled using two contracting band-type brakes, two multi-disc clutch packs, and (from 1951 on) a single cone clutch to provide four forward speeds and one reverse. The fluid coupling itself was driven indirectly: The torus cover, which was bolted to the engine flywheel, drove the annulus of the first planetary gearset, whose planet carrier drove a hollow intermediate shaft (surrounding and concentric with the transmission main shaft) that connected the fluid coupling impeller to the clutch assembly of the second gearset (which also partially bypassed the fluid coupling in third and fourth in order to reduce slippage). The fluid coupling’s turbine drove the transmission main shaft and the sun gear(s) of the second planetary gearset.

Color diagram of 1952-1956 Dual-Range Hydra-Matic © 2016–2017 Aaron Severson
A simplified (no, really), not-to-scale diagram of the late single-coupling Hydra-Matic, showing the reverse cone clutch added in 1951. (Earlier iterations had the reverse/parking pawl, but not the cone clutch.) Omitted, except for the oil pumps, are the complex hydraulic system and the control system changes made for 1952’s Dual-Range Hydra-Matic. (author diagram)

The following table shows the gear and band engagements for the 1952–1954 Dual-Range Hydra-Matic. (As noted in Part 1, earlier units had the same engagement sequence, but different ratios and (through 1950) used a pawl rather than a cone clutch for reverse. Some users retained the above ratios for 1955, but others (including Cadillac and Pontiac) adopted a revised front gearset with a ratio of 1.55:1, which made first gear 4.10:1.) In this table, “REL” and “ENG” abbreviate “RELEASED” and “ENGAGED” respectively.

1952–1954 Dual-Range Hydra-Matic Gearing Sequence
Front Planetary Rear Planetary Reverse Planetary
Gear Band Clutch Ratio Band Clutch Ratio Pawl Ratio Overall Ratio
1st ON REL 1.45 ON REL 2.63 OFF 3.82
2nd OFF ENG 1.00 ON REL 2.63 OFF 2.63
3rd ON REL 1.45 OFF ENG 1.00 OFF 1.45
4th OFF ENG 1.00 OFF ENG 1.00 OFF 1.00
Reverse ON REL 1.44 OFF REL ON -2.97† -4.30†

* In Neutral, the rear band is applied with the engine off, but released with the engine running.
† Negative signifies reverse.

The redesigned Hydra-Matic maintained the same general layout (although some components were repositioned), but replaced the front clutch pack with a second fluid coupling — the eponymous controlled coupling — located immediately behind the torus housing, between the first and second planetary gearsets. The second coupling was smaller than the main coupling and incorporated valves that allowed its oil supply to be completely drained or completely refilled in less than half a second. The rear clutch remained a multi-disc unit, although it was beefed up for greater torque capacity. (The design team considered adding a third fluid coupling to replace the rear clutch, but ultimately decided the benefits weren’t worth the substantial extra cost.)

The second fluid coupling had the same function as the multi-disc clutch it replaced: to put the front planetary gearset in direct drive by causing the annulus, the sun gear, and the planet carrier to rotate together at the same speed, or close to it. The main coupling torus cover drove both the front unit annulus and the impeller of the second coupling through its torus cover, which also drove the front oil pump. The second coupling’s turbine was connected (via a hollow sleeve shaft) to the front unit sun gear. If the coupling was empty, the impeller simply turned idly and the turbine remained stationary. Refilling the coupling would cause the impeller to drive the turbine — and thus the sun gear — at close to engine speed. (In technical terms, filling the second coupling split the engine’s torque between the annulus, which was driven mechanically, and the sun gear, which was driven hydraulically. The torque was then recombined by the planet carrier.)

Color diagram of 1956–1964 Controlled Coupling Hydra-Matic transmission © 2016–2017 Aaron Severson
The Controlled Coupling Hydra-Matic is so complex that even this simplified and abstracted diagram didn’t leave room for all of the captions. “C.C.” stands for “controlled coupling,” the engineering term for the drain-and-fill small fluid coupling. Note that the addition of the controlled coupling prompted the relocation of both the front planetary gearset (to a position inside the primary coupling’s torus cover) and the front oil pump (moved behind the small coupling and driven by its torus cover). This illustration omits the bell housing, which was now application-specific and fit over the torus covers. (author diagram)

The redesigned transmission also deleted the earlier Hydra-Matic’s front brake band, replacing it with a sprag-type one-way clutch that performed the same function: holding the front gearset sun gear in place whenever the front clutch was disengaged (or in this case empty). A similar sprag clutch was attached to the annulus of the second planetary gearset. Since the sprag clutches didn’t require any external engagement mechanisms, automatic shifts up or down could now be accomplished by controlling the front coupling and the rear clutch (as shown in the table below) rather than simultaneously coordinating clutch and brake engagements. The sprags also needed no routine adjustment.

The use of the sprag clutches necessitated an alternative means of obtaining neutral and reverse, which both required that the rear annulus be able to turn backward in some circumstances. In the earlier single-coupling Hydra-Matic, that was achieved by simultaneously releasing the rear clutch and the rear brake band, but the new transmission’s rear sprag clutch couldn’t be disengaged that way. Instead, the Controlled Coupling Hydra-Matic interposed a multi-disc neutral clutch between the rear sprag’s outer race and the transmission case. The neutral clutch was engaged in all forward gears, allowing the rear sprag to function normally. In neutral or reverse, with the neutral clutch disengaged, the sprags wouldn’t lock even if turned backward; reaction torque would just cause the neutral clutch hub to rotate backward along with the rear annulus. The front sprag, which had no such mechanism, remained locked in both neutral and reverse as long as the engine was running.

Hydraulic controls were similar in basic principle to those of the earlier Dual-Range Hydra-Matic, but the system was redesigned to include drain/fill valves for the front coupling and servo controls for the neutral clutch, overrun band, and overrun clutch.

Color diagram of 1956-1964 Controlled Coupling Hydra-Matic transmission fluid couplings and front sprag clutch © 2016–2017 Aaron Severson
We’ve taken some artistic liberties in depicting the interconnection (and scale) of the two torus covers in hopes of making their relationship a little clearer. The sun gear shaft (orange) served not only to connect the sun gear to the controlled coupling turbine, but also to the front sprag clutch (fuchsia with heavy black arrow) and the overrun clutch. The front sprag prevented the sun gear from turning backward. Engaging the overrun clutch also prevented the sun gear from turning forward, which kept the front planetary unit in reduction when coasting in third gear. The overrun clutch operated only in D3 (aka S or D-Right) ranges. (author diagram)

Another complication of the sprag clutches was that they would release on the overrun, so second or third gears provided no more engine braking than fourth and the transmission would freewheel when coasting in first. To compensate, the Controlled Coupling Hydra-Matic retained the rear brake band — now called the overrun band — and added a single-disc overrun clutch that could be engaged to lock the front unit sun gear sleeve shaft. The overrun clutch and overrun band served as auxiliary brakes, supplementing the sprag clutches in Low and D3 (aka S or D-Right) ranges. (The overrun clutch was also locked in reverse.) Neither the overrun clutch nor the overrun band was operative in D4 (aka D or D-Left) range, so there still wasn’t much engine braking in that range. Given the limitations of contemporary drum brakes, selecting D3 or Low for mountain driving or maneuvering on steep grades was prudent.

The following table summarizes the shift sequence for the Controlled Coupling Hydra-Matic. (Again, “REL” is short for “RELEASED” and “ENG” is short for “ENGAGED,” both abbreviated in the interests of space.)

1956–1964 Controlled Coupling Hydra-Matic Gearing Sequence
Front Planetary Rear Planetary Reverse Planetary
Gear Sprag Overrun
Coupling Ratio Neutral
Sprag Overrun
Clutch Ratio Cone
Overall Ratio
1st LOCK ENG* EMPTY 1.55 ENG LOCK ON* REL 2.55 REL 3.97
2nd FREE REL FULL 1.00 ENG LOCK ON* REL 2.55 REL 2.55

* In Low and D3/S ranges only.
† -2.41 from MY1958 on, making Reverse -3.74.

The redesigned Hydra-Matic now had a Park position on the selector, a first for the Hydra-Matic series. The parking pawl that position controlled wasn’t entirely new: Hydra-Matic had always incorporated a pawl to lock the annulus of the third planetary gearset, originally to provide reduction in reverse and, after the reverse cone clutch was added for 1951, later for use as a parking brake. The parking pawl on the Controlled Coupling Hydra-Matic now acted on the reverse planetary gearset’s planet carrier rather than the annulus and could be used in addition to or instead of a conventional emergency brake acting on the rear drums.

Since both fluid couplings were active in fourth gear, the Controlled Coupling Hydra-Matic also slipped a bit more at cruising speed than did its single-coupling predecessor. The torque split in third and fourth gears mitigated that somewhat, but the redesigned transmission nonetheless sacrificed some fuel efficiency. Interestingly, Herndon’s 1953 patent disclosure included provision for a mechanical lockup clutch to completely eliminate the second coupling’s additional slippage in fourth gear, but that feature was absent from the production transmission.

Shift quadrant of a 1958 Oldsmobile 98 (Ninety-Eight) Holiday hardtop © 2010 Aaron Severson
Like the earlier Dual-Range Hydra-Matic, the Controlled Coupling Hydra-Matic (which Oldsmobile called Jetaway) had dual driving ranges. D (labeled D4 or D-Left in other applications) provided normal shifting through all four speeds. S (which some users labeled D3 or D-Right) delayed the 3–4 shift in the same way as a wide-open throttle. In S range, the transmission would also automatically engage the overrun clutch to maintain engine braking. The “Safety Sentinel” panel above the quadrant of this 1958 Oldsmobile, not related to the transmission, operates a buzzer and warning light that automatically activate if you exceed a preset speed. (author photo)

The Controlled Coupling Hydra-Matic’s principal advantage was significantly smoother shifts than the single-coupling Hydra-Matic could muster. The rear clutch could still produce a mild thump on 2–3 or 3–2 shifts, but it was seldom objectionable and the 1–2 and 3–4 shifts were almost seamless. Shift quality was also more consistent than before — a distinct improvement over the single-coupling Hydra-Matic, which was very sensitive to proper adjustment of its bands and linkages. A bit of straight-line performance was inevitably sacrificed for that smoothness, but after 16 years of complaints about the endemic jerkiness of the single-coupling Hydra-Matic, that was a tradeoff many were prepared to accept.

Unfortunately, owners found that the new Hydra-Matic was somewhat less rugged than the single-coupling transmission it replaced. Particularly on early units, operation of the second coupling could be erratic in extreme temperatures, the aluminum torus cover was prone to cracks, and aggressive driving could damage the sprags of the one-way clutches. A litany of running changes progressively addressed most of those issues, but it’s interesting to note that GMC and Chevrolet trucks stuck with the older Dual-Range Hydra-Matic until the early sixties. (So did Rolls-Royce, which built the Dual-Range Hydra-Matic under license.)

The update also did nothing to reduce Hydra-Matic’s considerable weight, which now ran to some 225 to 240 lb (102 to 109 kg), or make it cheaper to build; it was undoubtedly one of the most costly, if not the costliest, of contemporary automatics. Consequently, there were fewer outside users than before. American Motors purchased some dual-coupling Hydra-Matics (which AMC dubbed “Flashaway”) for 1956–1957 Hudson and Nash models, but subsequently switched to less-expensive Borg-Warner (and later Chrysler) automatics. Even within GM, cost considerations would soon prompt Oldsmobile and Pontiac to adopt cheaper alternatives, although some Cadillac and Pontiac models would retain the Controlled Coupling Hydra-Matic through the 1964 model year.

1957 Oldsmobile 98 (Ninety-Eight) Holiday Coupe front 3q © 2008 Aaron Severson
The older Dual-Range Hydra-Matic remained optional on some Oldsmobile and Pontiac models in 1956, when the new dual-coupling transmission was first introduced. Most passenger car users dropped the older transmission by the end of the model year, but it was still used on some GMC and Chevrolet trucks into the early sixties. The new Hydra-Matic was standard on all Cadillacs and on senior Oldsmobiles like this 1957 Oldsmobile 98 Holiday Coupe. (author photo)


With the introduction of the dual-stator Variable Pitch Dynaflow, GM’s “pure” torque converter automatic had reached an advanced state of development. However, Oliver Kelley’s corporate transmission group was not yet satisfied and kept working on what was supposed to be the ultimate torque converter automatic: a triple-turbine transmission.

To be clear, there were actually two such transmissions: Chevrolet’s Turboglide, introduced as an option on 1957 Chevrolets with the 283 cu. in. (4,638 cc) V8 engine, followed a year later by Buick’s Flight Pitch Dynaflow, which was standard on the 1958 Buick Roadmaster and Limited and optional on other models. Although Turboglide and Flight Pitch Dynaflow (renamed Triple Turbine for 1959) differed in layout and in many details, both transmissions were based on a common set of ideas developed by Kelly’s team and were, like the original Dynaflow and Powerglide, essentially variations of the same design.

Color diagram of 1957 Chevrolet Turboglide transmission © 2016–2017 Aaron Severson
This diagram, again not to scale and omitting or simplifying many minor details, shows the original 1957 Chevrolet Turboglide transmission with its three cone clutches. A cone clutch, for those unfamiliar, operates in a manner not unlike stacking two disposable plastic or paper cups. One cone is stationary (at least in the longitudinal plane — depending on the layout, either or both cones may be able to rotate) while the other moves forward or backward to engage the stationary cone, locking them together. (author diagram)

The easiest way to conceptualize the triple-turbine transmission is as a Variable Pitch Dynaflow with an additional drive turbine rather than a second stator. The extra turbine was linked to its own set of planetary gears, the addition of which required moving both gearsets out of the converter hub and into the transmission case. Controlling those gearsets — which superseded Dynaflow’s familiar Ravigneaux gearbox — were no fewer than six clutches: two one-way clutches (not counting the stator clutch), a neutral clutch, a reverse clutch, a forward clutch, and a “hill retarder” or “grade retarder” clutch (the function of which we’ll explain shortly). Turboglide initially used cone-type neutral, reverse, and forward clutches with a multi-disc hill retarder clutch, but switched to a multi-disc neutral clutch for 1958 and adopted multi-disc reverse and forward clutches for 1959. Flight Pitch Dynaflow and Triple Turbine used only multi-disc clutches from the start.

The transmission’s two one-way clutches, which were linked to the reaction members of the two planetary gearsets — the front unit sun gear and rear unit annulus, as on Hydra-Matic — were cleverly interconnected, with the inner race of the front sun gear clutch forming the outer race of the rear annulus clutch. The forward clutch served to anchor both one-way clutches to the case, preventing either reaction member from turning backward. The rear annulus was free to rotate forward while the front sun gear remained locked, but the front sun gear could only turn forward if the rear annulus also did so. With the forward clutch released, reaction torque on the rear annulus would lock it against the front sun gear clutch, which caused both clutches to turn backward together, carrying their respective gears with them.

As in Twin-Turbine Dynaflow, the triple-turbine transmission’s first turbine was affixed to a support shell, within which were mounted the other two turbines. The support shell was splined to a central input shaft that caused the rear unit sun gear to rotate with the first turbine. The inner hub of the second turbine was attached to a hollow sleeve shaft that caused the second turbine and front unit annulus to rotate together. A third hollow shaft, located between the other two, connected the third turbine to the neutral clutch, which when engaged linked the third turbine to the planetary gearsets’ interconnected front and rear planet carriers. A flange at the trailing edge of the rear carrier allowed the carriers to drive the transmission output shaft.

The mechanics of the triple-turbine transmission were very similar to those of the twin-turbine units, but there were now three stages rather than two. At stall, most of the impeller’s torque (augmented as usual by the stator) was applied to the first turbine and thus the rear unit sun gear. This would exert reaction torque on the rear annulus, so if the forward clutch was engaged, both one-way clutches would lock, putting both gearsets in reduction. Oil exiting the first turbine would initially apply a small amount of positive torque to the second turbine and therefore to the front unit annulus. Once the turbines were moving, the oil stream exerted progressively less torque on the first turbine and progressively more on the vanes of the second. The torque exerted on each turbine was multiplied by their respective planetary gears and applied to the output shaft through the conjoined planet carriers. Turboglide’s gear ratios were 2.67:1 for the rear gearset and 1.60:1 for the front unit; the ratios for Flight Pitch Dynaflow/Triple Turbine were 2.86:1 and 1.55:1 respectively.

(We should emphasize here that while these transmissions technically had three geared ratios, they were NOT three-speed automatics. Over the years, some sources have incorrectly described them as such, which, while true in one sense, betrays a fundamental misunderstanding of how these transmissions actually function.)

Color diagram of 1958 Buick Flight-Pitch Dynaflow and 1959 Triple Turbine transmission © 2016–2017 Aaron Severson
This diagram (again, simplified for everyone’s sanity and definitely not to scale) illustrates the layout of the 1958 Buick Flight-Pitch Dynaflow/1959 Triple Turbine transmission. It looked and operated much like Turboglide, although some elements were in different places — for example, the interconnected one-way clutches were behind the second planetary unit rather than between the gearsets. Buick also eschewed the use of cone clutches for its triple-turbine automatics. (author diagram)

If you followed our explanation of Twin-Turbine and Variable Pitch Dynaflow earlier in this article, you may recall that in the single-stator versions of those transmissions, oil flow from the first turbine would initially oppose the rotation of the second, a problem rectified on later versions of Variable Pitch Dynaflow by the addition of the front stator. Since the triple-turbine transmissions lacked the additional stator, oil exiting the first and second turbines at or just above stall would similarly oppose the rotation of the third turbine, reducing the net torque on the output shaft. As torque shifted from the first turbine to the second, the oil flow from the second turbine began to exert positive torque on the third turbine. (The more aggressive the initial launch, the longer this took.)

Once the speed of the second turbine reached approximately 55–60% of the speed of the first turbine (the exact transition point depending on the comparative ratios of the front and rear gearsets), the front unit annulus would attempt to rotate its planet carrier faster than the rear carrier. Since the two carriers were connected, the rear carrier was obliged to rotate faster as well. This caused the carrier to overdrive the rear unit sun gear and the first turbine, which removed the reaction torque on the rear unit annulus and its one-way clutch. The first turbine would then freewheel idly, leaving the other two turbines to drive the output shaft. The second and third turbine would repeat this process once there was enough torque on the third turbine to drive it at more than about 60% of the speed of the second (again depending on the exact ratio of the front gearset), which left both the first and second turbines spinning idly. The stator continued to provide some torque multiplication until toroidal flow dropped off enough to release the stator’s one-way clutch.

Both Turboglide and Flight Pitch Dynaflow/Triple Turbine used variable-pitch stators, but of different designs. Turboglide had a two-position stator very similar to the one used in 1957 and later Variable Pitch Dynaflow/Twin Turbine transmissions, but Buick adopted a more sophisticated infinitely variable stator. As with the two-position unit, stator blade angle was controlled by the pivoting of an annular piston controlled by hydraulic pressure. However, rather than simply flipping back and forth between two discrete positions, the infinitely variable stator’s control piston was balanced between opposing converter and throttle valve pressures that could hold the piston at any position within its range of motion. In this way, the stator blades could continuously adjust their pitch based on load. A “kickdown” valve opened by flooring the accelerator would still force the blades to their highest possible angle, just as with the two-position stator.

Color diagram of 1958 Chevrolet Turboglide transmission © 2016–2017 Aaron Severson
No, you don’t have déjà vu — this is again the Chevrolet Turboglide transmission, here showing some of the changes made for the 1958 model year. “G.R.” stands for “Grade Retarder,” as the Hill Retarder position was renamed that year. All of Turboglide’s clutches were redesigned several times, but their functions remained the same: The reverse clutch locked the second turbine and front annulus; the neutral clutch linked the third turbine to the planet carriers; the forward clutch anchored the front one-way clutch; and the grade retarder clutch/hill clutch served to lock the rear annulus. (None are shown to scale.) (author diagram)

Reverse was an adaptation of the principle used in contemporary Hydra-Matics: allowing reaction torque on the reaction member of the rear gearset to provide reverse rotation and then compounding it with another gearset to provide reverse reduction. Since there were only two gearsets rather than three, the front unit now performed the latter chore. To accomplish all this, the neutral and reverse clutches were engaged, connecting the third turbine to the planet carriers and holding the front unit annulus in place, while the forward clutch was released so that the one-way clutches were no longer anchored to the case. The rotation of the first turbine (and thus the rear unit sun gear) therefore couldn’t apply any torque to the planet carrier, but their rotation would cause the rear unit annulus, both one-way clutches, and the front unit sun gear to all turn backward together. With the front unit annulus locked, the front planetary gearset would multiply this reverse torque and apply it to the planet carrier. Since the second turbine was connected to the front unit annulus, engaging the reverse clutch to lock the annulus also locked the turbine. This essentially transformed the second turbine into a stator, although its purpose was exactly the opposite of Variable Pitch Dynaflow’s forward stator, maximizing rather than removing the negative torque on the third turbine so that torque would be added to the reverse torque the front unit exerted on the output shaft.

The last major element of the triple-turbine transmission was the hill retarder or grade retarder clutch. As we previously mentioned, Twin-Turbine Dynaflow provided little engine braking in Drive and the triple-turbine automatics suffered the same problem. To compensate, both triple-turbine transmissions could be shifted to HR/GR, which engaged the hill clutch — locking the rear annulus — while releasing both the forward clutch and the neutral clutch to disconnect the one-way clutches from the case and the third turbine from the planet carriers. In that condition, only the first turbine could transmit any torque to the output shaft and the rear planetary unit would remain in reduction until the driver shifted to a different range.

In principle, this mode could be used as a low range, although in practice, doing so created too much slippage to have any performance advantage. The real purpose was to provide engine braking: The hill clutch would not unlock even on the overrun, so coasting would cause the rear planetary unit to act as an overdrive, causing the first turbine to attempt to overdrive the engine. This created a strong braking effect, but the rear unit gear ratios were so short — comparable to first gear in many contemporary manual transmissions — that using it at higher speeds was dangerous. (Causing the first turbine to abruptly turn more than twice as fast as the impeller would certainly slow the car, but could overheat the transmission.)

Color diagram of 1959 Chevrolet Turboglide transmission © 2016–2017 Aaron Severson
The late (1959–1961) Turboglide began to resemble its Buick cousin with the adoption of additional multi-disc clutches. Chevrolet never used Buick’s continuously variable stator, however, opting for a simpler two-position type. Again, simplified, not to scale, etc. (author diagram)

As with most of GM’s early automatics, the triple-turbine triple turbines had front and rear oil pumps, the latter used for push-starting and cruising. These transmissions also adopted Dynaflow’s hydraulic accumulators and Powerglide’s vacuum modulator, adjusting operating and engagement pressures based on load and selector position. The layout of the hydraulic control system, which in complexity now fell somewhere in between Dynaflow and Powerglide, required a new shift pattern: PRNDHR (or PRNDGR) rather than the GM’s previously obligatory PNDLR pattern.

Another unusual and somewhat radical move, at least for the late fifties, was the use of die cast aluminum for the transmission case and the tail housing; cast iron was used only for the hydraulic valve body. This was more expensive and posed some significant manufacturing challenges, but it saved quite a bit of weight. In fact, Chevrolet claimed that Turboglide weighed a substantial 88 lb (40 kg) less than Powerglide, which at that point still had an iron case.

Turboglide quadrant on a 1958 Chevrolet Impala convertible © 2010 Aaron Severson
Although Turboglide and Flight Pitch Dynaflow/Triple Turbine were similar in their basic operating principles, they had different ratios: Turboglide’s mechanical ratios were 2.67:1 for the rear gearset, 1.60:1 for the front gearset, with a maximum converter multiplication of 1.60:1 at stall, giving a ratio spread of 4.30:1 to 1.00:1. The Buick’s ratios were 2.86:1 and 1.54:1, with a maximum converter multiplication of 1.66:1, giving a ratio spread of 4.50:1 to 1.00:1 (4.70:1 to 1.00:1 in 1959). That ratio spread, incidentally, is quite similar to that of modern continuously variable transmissions. (author photo)


The point of all this complexity is easy enough to see. Both triple-turbine automatics were what we would now call continuously variable transmissions, offering a highly respectable amount of torque multiplication over a broader range of speeds than any previous automotive torque converter. With its stator blades at their low angle, Turboglide provided a stall ratio of 3.8:1 at a nominal 1,700 rpm, better than the dual-turbine Variable Pitch Dynaflow could manage at full throttle. With the throttle floored to shift the stator blades to high angle, Turboglide’s stall ratio rose to 4.3:1 at a nominal 2,700 rpm, better than Powerglide could offer even in Low. Since Flight Pitch Dynaflow’s stator blades were infinitely variable, Buick quoted only a single ratio: 4.5:1 at a nominal 3,200 rpm in 1958, rising to 4.7:1 for the 1959 Triple Turbine, which had revised impeller and second turbine blades.

On paper, at least, it appeared that GM had finally created the ideal automatic transmission: lightweight and perfectly smooth, with ample torque multiplication. Being (marginally) less complex than some rivals, it also promised to be more reliable. Unfortunately, the reality fell short of the sales pitch.

It should be said that at least part of the problem was one of perception. The triple-turbine transmissions’ torque multiplication depended on keeping the turbine speeds (and thus the speed of output shaft) well behind the speed of the impeller for as long as possible. While that was also true of Twin-Turbine/Variable Pitch Dynaflow, the triple-turbine units’ shorter gearing made the gap between engine speed and output shaft speed more pronounced and thus more noticeable. With an aggressive launch, the speed of the third turbine and output shaft might not approach the speed of the engine until the car was moving more than 50 mph (80 km/h), which could leave the uninitiated driver fearing that the transmission was about to self-destruct. Since the lag in rotational speeds did not directly reflect the transmission’s mechanical efficiency, the slippage wasn’t as nearly dire as it seemed, but it was disconcerting, if nothing else.

1958 Chevrolet Impala hardtop front 3q © 2010 Aaron Severson
Chevrolet advertising for 1958 promoted both Turboglide and the new “Turbo-Thrust” 348 cu. in. (5,694 cc) engine; this Impala Sport Coupe has both. Even with the big engine, it’s not an outstandingly fast car — in January 1958, Car Life magazine clocked a similar Impala with the 250 hp (186 kW) engine and Turboglide from 0-60 mph (0-97 km/h) in a tick over 10 seconds. (author photo)

As with the dual-turbine Dynaflow, the nonlinearity posed a bigger problem when it came to passing response. Unless output shaft speed fell below about 60% of engine speed, the stator was the sole source of torque multiplication for passing. That was often marginal unless the stator blades were at their highest angle, which even with Buick’s infinitely variable stator was only obtainable with the accelerator floored. Compared to the convenience of Hydra-Matic’s part-throttle kickdowns, this was frustrating, making it seem that the transmission had to be constantly thrashed to provide adequate performance. Naturally, this style of driving did nothing good for overall fuel consumption, although steady-speed economy wasn’t terrible for this era. (Buick nonetheless hedged its bets for 1959 by numerically lowering the standard axle ratio for Triple Turbine cars to 2.78, compared to 3.07 for Twin Turbine or manual shift, which improved fuel economy at further cost in performance.)

Exacerbating this exasperation was the fact the triple-turbine transmissions had no Low range. If the 1.82:1 ratio of Dynaflow’s Low gear was less than ideal, it nonetheless provided immediate relief for any shortage of midrange punch and, with typical late fifties axle ratios, it could be used up to about 60 mph (97 km/h). Turboglide and Flight Pitch/Triple Turbine had only the hill retarder/grade retarder, which was similar to Dynaflow and Powerglide’s Low range only in its position on the selector and was intended for slowing down, not for accelerating. Anyone who shifted from Drive to GR thinking it would improve passing or hill-climbing power was quickly disabused of that notion. (The owner’s manual cautioned against engaging the hill clutch at more than 40 mph (64 km/h), lest you overheat the torque converter.)

As for reliability, it was initially quite poor for both Turboglide and Flight Pitch Dynaflow. One problem was the aluminum case; although aluminum transmission cases would become very common just a few years later, aluminum die castings of this size and complexity were still at the bleeding edge of GM’s metallurgical capabilities (a problem that also dogged the early Buick/Oldsmobile aluminum V8s). On early units, it was not uncommon for the case to crack or split, particularly if the transmission was overheated. It also appears that Chevrolet, at least, underestimated the demands on the clutches — particularly in the area of heat dissipation, which was the primary rationale for the subsequent switch from cone to multi-disc clutches. Even then, the clutches had to be beefed up several times and their engagement pressures increased (among various other changes). Many of the early issues had been addressed by 1959, but neither transmission ever lived down its checkered reputation.

1958 Buick Limited Riviera four-door sedan front 3q © 2007 clicks_1000 (used with permission)
Buick’s Flight Pitch Dynaflow, introduced in 1958, was standard on Roadmaster and Limited, optional on lesser Buicks. The Limited, a name Buick had used in 1941–1942, was revived in 1958 for a new top-of-the-line model. Fewer than 7,500 were sold, most of them Riviera four-door hardtops like this one. (Photo: “Classic 1958 Buick Limited 4 door Hardtop” © 2007 clicks_1000; used with permission)

Even if the triple-turbine automatics had been 100% reliable, we suspect that many buyers would have had difficulty seeing the point. That a great many American new car buyers of the time preferred automatic transmission is beyond question, but the need for multiple automatic transmission options was a good deal less obvious. Both Powerglide and Variable Pitch Dynaflow/Twin Turbine certainly had their flaws, but by the late fifties they were well-proven and worked well enough for many customers. The operating principles of Turboglide and Flight Pitch/Triple Turbine are complex enough to mystify even many automotive writers, so it’s easy to imagine the befuddlement of contemporary buyers trying to decide whether the triple-turbine transmissions were worth the attendant price premium. Turboglide’s continuously variable smoothness was a relative novelty for Chevrolet, but for Buick buyers, the dual-turbine Dynaflow, which was also functionally a CVT, was just as smooth. Therefore, the pricier transmission’s notional advantages were probably lost on all but the most technically savvy shoppers.

The upshot of all this was that most buyers shied away, which made both triple-turbine automatics costly failures. Since they shared very little with other Chevrolet and Buick transmissions (although Chevrolet later borrowed some Turboglide pieces for Powerglide), the tooling bill was immense — Buick alone spent a reported $86 million (around $730 million in 2016 dollars) — and warranty costs were high. The extensive changes necessary to address the various reliability problems can’t have helped; we don’t suppose that repeatedly redesigning Turboglide’s clutches was cheap.

1959 Buick Electra 225 convertible front © 2009 Aaron Severson
Buick’s 1958 sales were so dire that the division abandoned most of its previous model names — and the Dynaflow name — for 1959. The Variable Pitch Dynaflow became Twin Turbine while the Flight Pitch Dynaflow was renamed Triple Turbine. The latter cost $295.63 on LeSabres, $75.25 on Invictas and Electra 225s (on which Twin Turbine was standard). Although $75 doesn’t sound like a lot, it’s the equivalent of more than $600 today. (author photo)

Chevrolet, at least, was better able to absorb that expense. For Buick, the failure of Flight Pitch Dynaflow/Triple Turbine was yet another in a long list of calamities to befall the division during this period, doing serious damage to both sales and market share. The new transmission was certainly not the primary culprit — bigger issues included a newly recessionary economy, unpopular styling, and an assortment of assembly woes — but it added yet more red ink to the ledger at a time when Buick could least afford it.

The triple-turbine transmissions also marked an inauspicious period in the career of O.K. Kelley, who had left the Engineering Staff to become Buick’s chief engineer in August 1957. Less than two years later, Buick’s financial woes led to a major shakeup of the division’s upper management, beginning with the replacement of general manager Ed Ragsdale with Edward D. Rollert that April. Kelley departed about seven months later to a new post as chief technical adviser for GM’s Defense Systems Division. Even before he left, Buick terminated production of the Triple Turbine transmission, which vanished at the end of the 1959 model year.

Chevrolet continued to offer Turboglide through the 1961 model year, perhaps in the vain hope of getting their money’s worth. Experience with Turboglide did help Chevrolet engineers develop the Corvair Powerglide and the successful aluminum-case Powerglide (introduced in 1962–63), so it wasn’t a total loss, but all in all, it was not a particularly successful experiment. Looking back on it now, it seems like an intriguing idea that was under-developed and over-sold.

GM’s experience with the triple-turbine automatics was unhappy enough that these transmissions had no direct successors as such. (Kelley also designed a quadruple-turbine transmission, but nothing came of it.) However, some of their design elements did find their way into subsequent GM automatic transmission designs, as we’ll see in the next section.

1959 Buick Electra 225 convertible dashboard © 2009 Aaron Severson
Although the Triple Turbine was much improved in 1959, most buyers opted for the cheaper and better-regarded Twin Turbine. The easiest way to tell the difference is by the shift quadrant: Twin Turbines had a PNDLR pattern, Triple Turbines PRNDG. (author photo)


As we mentioned previously, the late fifties were not a particularly good time for radical or elaborate new designs. The new car market, which had boomed in 1955, slumped badly for 1956. By 1957, a national recession had buyers searching for smaller, cheaper, more economical cars.

Chevrolet general manager Ed Cole used this opportunity to push through his radical rear-engine, air-cooled Corvair, but did nothing to help GM’s mid-priced divisions, which had been hit hard by the recession. Senior corporate management responded with the X-100 project, a collaborative program to give Pontiac, Oldsmobile, and Buick their own small (or at least smaller) cars for the 1961 model year, a year after the debut of the Corvair.

Although the X-100 cars were intended to have a high degree of commonality so as to share the substantial costs of the new models, the final products ended up considerably less alike than the corporation originally hoped. The so-called “senior compacts” — the Buick Special/Skylark, Oldsmobile F-85/Cutlass, and Pontiac Tempest/Le Mans — did share the same unitized Y-body shell (an enlarged version of the Corvair body) and various minor components, but there were significant differences in their mechanical layouts and powertrains, including three completely different automatic transmissions.

Ironically, the most conceptually exotic of the trio, the Pontiac Tempest’s rear-mounted automatic transaxle, was probably the cheapest of the three to develop and tool. Dubbed “TempesTorque,” it was a variation of the Corvair’s optional Powerglide transaxle. As in the Corvair Powerglide, TempesTorque’s torque converter was at the back of the transaxle. Since the Tempest had a front-mounted engine, TempesTorque used the Corvair transmission’s front oil pump driveshaft as an input shaft, driving the torque converter impeller through the torus cover. That input shaft was also splined to the hub of the direct drive clutch, just like in a conventional RWD Powerglide. In high, input torque was therefore split approximately 45/55 between the front sun gear, which was driven by input shaft through the direct drive clutch, and the rear sun gear, which was driven by the torque converter. As with Hydra-Matic, this “split torque” layout served to reduce slippage at cruising speed in high gear.

Color diagram of 1961–1962 Pontiac TempesTorque transaxle © 2016–2017 Aaron Severson
More déjà vu: The early (1961–1962) Pontiac TempesTorque — again NOT to scale — bore a strong, non-coincidental resemblance to Corvair Powerglide. The famous flexible driveshaft connected the engine (in front, not pictured) to the central input shaft, a clever re-purposing of the Corvair transaxle’s front oil pump drive. Aside from the location of the engine, the major difference between the Tempest and Corvair transmissions was that in TempesTorque, the direct drive clutch hub was splined to the input shaft rather than the main shaft. As a result, in high gear, the front sun gear turns at engine speed while the rear sun gear (driven by the main shaft) turns at the speed of the torque converter turbine; the output shaft therefore turns faster than the turbine. (author diagram)

The 1961–1962 TempesTorque had a lower converter stall ratio than did the Corvair Powerglide (2.00:1 rather than 2.60:1), but the indirect ratios were the same (+/-1.82:1 for low and reverse). As with the Corvair transmission, there was no parking pawl.

Pontiac made a variety of changes to TempesTorque for 1963, the “rope-drive” cars’ final year. Torque capacity was increased to accommodate the Tempest’s newly optional 326 cu. in. (5,340 cc) V8 engine while new planetary gears, borrowed from the latest heavy-duty Powerglide, gave indirect ratios of +/-1.765:1. A new direct drive clutch deleted the previous high-gear torque-splitting feature and the torque converter was redesigned to provide higher stall ratios (2.40:1 for four-cylinder cars, 2.20:1 for the V-8). TempesTorque and the rope-drive Tempest/Le Mans disappeared for good after the 1963 model year.

Color diagram of a 1963 Pontiac TempesTorque transaxle © 2016–2017 Aaron Severson
The most obvious difference between the 1963 TempesTorque and its immediate predecessors was that the direct drive clutch hub was now driven off an extension of the main shaft (as in the Corvair Powerglide) rather than by the input shaft. Gearing was also revised. (author diagram)


The two-speed torque converter automatic transmission offered on Buick’s Y-body compacts, dubbed Dual-Path Turbine Drive, was quite different from TempesTorque and for that matter the twin-turbine transmission used in contemporary full-size Buicks. Today, the Dual-Path Turbine Drive is one of the most obscure and poorly understood of GM’s early automatics, in part because it was used only on the 1961–1963 Buick Special and Skylark. In a sense, it was Buick’s first true automatic transmission, since it was the first to actually include provision for automatic shifts between two distinct stepped ratios in Drive, something the designers had taken pains to avoid with the Dynaflow family.

The Dual-Path transmission seems to represent a merger of two distinct conceptual threads within GM’s corporate transmission group. One was the use of a split-torque clutch to provide direct drive, a concept dating back to the original Hydra-Matic (and which was essayed in somewhat simpler form around 1957 by Oliver Kelley’s colleagues Robert M. Tuck and James J. Mooney, Jr. — see U.S. Patent No. 2,929,270). The other, developed by Kelley and Gilbert K. Hause, was a three-element torque converter with a stator that could do double duty as a drive turbine. Judging by the earliest relevant patent disclosures (U.S. Patents 2,957,370 and 3,030,823), the latter was conceived as a streamlined and simplified evolution of the Twin-Turbine Dynaflow, applying some concepts from the triple-turbine transmissions to allow the deletion of Dynaflow’s separate planetary gearbox.

It appears that there was some consideration of using the Dual-Path transmission in the other X-100 cars. The patent outlining most of the major mechanical details was actually filed by John DeLorean, then the head of Pontiac’s advanced engineering section, although Hause led the development of (and patented) the hydraulic control system. Interestingly, a subsequent patent filed by Kelley and Hause described several possible rear transaxle versions, although the production Dual-Path transmission was only for front-engine/rear-drive applications. The only significant element TempesTorque ended up sharing was the torque-splitting feature, which Pontiac implemented differently.

Like the contemporary Powerglide, Dual-Path Turbine Drive had a three-element torque converter and a single planetary gearset with dual sun gears, but the mechanical similarities ended there. As in the dual-turbine Dynaflow series, the actual planetary gears were nestled in the center of the torque converter torus, with the turbine hub driving the annulus and the planet carrier driving the transmission main shaft. Unlike previous Dynaflow transmissions (and most other automatic transmissions), the impeller was mounted on the flywheel side of the torus housing, facing backward (i.e., toward the rear axle) while the turbine faced forward, toward the engine, and was connected to its hub by a series of narrow struts.

Color diagram of 1961-1963 Buick Dual-Path Turbine Drive transmission © 2016 Aaron Severson
Dual-Path Turbine Drive’s torque converter (again, simplified/abstracted and not to scale) was ‘backward,’ with the impeller (red) facing toward the rear axle. Several narrow struts connect the turbine (blue) to its hub, which contains the annulus. Note that both one-way clutches (fuchsia with heavy black arrows) shared a common cam, which was anchored to the case by the forward clutch. (author diagram)

Unlike Powerglide and Dynaflow, Dual-Path Turbine Drive used no brake bands. Instead, it was controlled by four multi-disc clutches (the direct drive/converter clutch, reverse clutch, forward clutch, and coast clutch) and two one-way clutches (one for the stator, the other for the planetary gearset’s rear sun gear). The converter clutch was mounted in the hub of the torque converter impeller, allowing the torus cover to be locked to the planetary gearset’s front sun gear. The other five clutches occupied most of Dual-Path’s cast aluminum transmission case.

The main shaft was surrounded by four concentric sleeve shafts of varying lengths. The outermost sleeve allowed the torus cover to drive the transmission’s single oil pump, which was mounted in the front of the transmission case, like Dynaflow’s front pump. Within that shaft was a sleeve shaft connecting the converter turbine to the hub of the reverse clutch. The two innermost shafts connected the stator and rear sun gear to the inner races of their respective one-way clutches. Those clutches, which were of the cam-and-roller type, shared a common cam, which was connected to the forward clutch and coast clutch. The forward clutch, like the neutral clutch of the Controlled Coupling Hydra-Matic or Triple Turbine transmissions, allowed both one-way clutches to be selectively neutralized (i.e., allowed to turn freely without locking) by disconnecting the cam from the transmission case. The coast clutch, meanwhile, allowed the cam to be locked to the inner race of the rear sun gear so that all three would turn together on the sun gear sleeve shaft.

Color diagram of 1961-1963 Buick Dual-Path Turbine Drive transmission torque converter and planetary gearset © 2016 Aaron Severson
Simplified detail (still not to scale) of Dual-Path Turbine Drive’s converter and the planetary gearset in its hub. (author diagram)

All this sounds very complex, but Dual-Path’s operation was reasonably straightforward. Selecting either D (Drive) or L (Low) on the selector (which had a PNDLR pattern) would engage the forward clutch. This enabled the one-way clutches for the stator and the rear sun gear, preventing them from turning backward. The hub of the torque converter turbine then drove the annulus of the planetary gearset, with the stator providing additional torque multiplication in the customary fashion; stall ratio was 2.50:1. Reverse torque on the planet carrier locked the rear sun gear’s one-way clutch, putting the planetary gearset in first gear and providing a mechanical gear reduction of 1.58:1.

Selecting L would engage the coast clutch as well as the forward clutch. This allowed the stator to function normally, but prevented the rear sun gear from turning in either direction, ensuring that the planetary gearset would remain in reduction on the overrun. Dual-Path’s hydraulic controls included no provision for automatically disengaging the coast clutch, so in Low, the transmission could not shift out of first gear.

With the selector in D, however, the hydraulic control system would shift automatically between first and second. Upshifts were executed by engaging the converter clutch, which established a mechanical connection between the torus cover and the front sun gear. That drove both sun gears forward, which caused the rear sun gear’s one-way clutch to automatically unlock so that the planetary gearset was no longer in reduction. Engine torque was then split 36.6/63.4 between the front sun gear (which turned at impeller/engine speed) and the annulus (which turned at turbine speed), reducing converter slippage. Downshifts were executed by simply releasing the converter clutch, which caused the rear sun gear to automatically re-lock and put the transmission back in first. Shift points were determined by a combination of throttle setting and car speed. Dual-Path was the first Buick automatic to be equipped with a centrifugal governor, previous Dynaflow and Turbine Drive control units having had no need to measure road speed.

Color diagram of 1961-1963 Buick Dual-Path Turbine Drive transmission one-way, forward, and coast clutches © 2016 Aaron Severson
Detail (not to scale) of the Buick Dual-Path Turbine Drive’s interconnected one-way clutches. With the forward clutch engaged, the clutch cam was stationary. Each race could rotate with the engine (as indicated by the heavy black arrows), but would lock against the cam if turned backward. The coast clutch locked the two races to one another. (author diagram)

Dual-Path obtained reverse in much the same way as the earlier triple-turbine automatics, although the arrangement was slightly simpler because Dual-Path had only one turbine and one planetary gearset. Moving the selector to R released the forward clutch and engaged both the reverse and coast clutches. The reverse clutch then held the turbine stationary, which caused the turbine and stator to effectively swap roles. With the forward clutch disengaged, releasing the one-way clutch race from the case, reaction torque on the stator caused the stator, its sleeve shaft, and the clutch race to spin backward. Since the coast clutch was also engaged, the reverse rotation of the clutch cam drove the rear sun gear backward. The annulus, which was held in place along with the turbine, then acted as a reaction member, allowing the sun gear to drive the planet carrier in reverse reduction. Mechanical gear ratio in reverse was 2.73:1; the reaction torque created by the stationary turbine would provide additional torque multiplication of 1.50:1, giving a maximum stall ratio in reverse of 4.10:1.

The point of this unusual arrangement was to minimize the number of components, keeping the transmission as compact and as light as possible. With its air-cooled aluminum case, Dual-Path Turbine Drive was one of the lightest automatic transmissions ever developed by a U.S. automaker, weighing only 95 lb (43 kg) with fluid. That was 10 lb (4.5 kg) less than the Buick Special’s standard three-speed Warner Gear T-85 manual transmission and less than half as much as the full-size Turbine Drive or four-speed Hydra-Matic. The transmission tunnel intruded into cabin space only slightly more than that of the rope-drive Tempest, with its rear transaxle.

1962 Buick Special convertible front 3q © 2010 Aaron Severson
Dual-Path Turbine Drive was used only in 1961–1963 Buick Specials and Skylarks. It was nominally a $189 option, but our guess is that the automatic went into something like 85–90% of all Y-body Buicks. That would bring the three-year production total to around 330,000–350,000 — a respectable total by any standard except GM’s. Specials with automatic had a 3.08:1 axle ratio, but Skylarks with the hotter optional V8s typically had a shorter 3.36:1 axle. (author photo)

From a performance standpoint, a two-speed automatic linked to an assortment of modestly powered V8 and V6 engines doesn’t sound promising, but contemporary testers found that cars equipped with Dual-Path were unexpectedly spry. Although Dual-Path’s first gear was quite tall — it was only slightly shorter than second gear in the T-85 three-speed manual or, for that matter, third gear in the Controlled Coupling Hydra-Matic — the torque converter provided a starting ratio comparable to first gear in the four-speed Hydra-Matic. With a 3.08:1 axle, standard on automatic Specials, first could be held to 63–64 mph (101–103 km/h), so Low was useful in mountain driving that would be uncomfortably buzzy with many contemporary two-speeds. In all, performance was really not bad, if still somewhat inferior to the available manual transmissions. The closeness of the ratios and the use of one-way clutches rather than brake bands also made Dual-Path’s shifts impressively smooth.

One minor sacrifice was the capacity for push-starting, something allowed by most earlier GM earlier automatics. Hause’s patent (U.S. No. 3,108,493) included an auxiliary oil pump to be used solely for that purpose, mounted at the rear of the transmission just ahead of the governor. However, the production transmission had only one pump, presumably in the interests of minimizing cost and weight.


Since Oldsmobile’s Y-body compact, the F-85/Cutlass, shared the same basic V8 engine block as the Special (albeit with different cylinder heads and air cleaner), the same three-speed manual transmission, and even the same driveshaft, it would have made sense for the two cars to also share the same automatic transmission. Instead, Oldsmobile opted for a scaled-down version of Detroit Transmission Division’s latest, third-generation Hydra-Matic.

For the sake of clarity, we’ll describe the third-generation Hydra-Matic as “Roto Hydra-Matic,” which is what Pontiac called the transmission in 1963 and 1964; most users simply called it “Hydra-Matic.” (Confusingly, Oldsmobile used the trademark “Roto-Matic” for power steering!) There were actually two different versions of the new transmission: The standard Model 375 (aka Type 61-10) unit was used in full-size Oldsmobiles and some full-size Pontiacs. The light-duty Model 240 (aka Type 61-5) was optional on the Y-body Oldsmobile F-85/Cutlass and on GM’s senior Australian, German, and English cars: the EK (and later EJ) Holden Special, the Opel Kapitän L, and Vauxhall Cresta. The Model 375 was 29 lb (13 kg) heavier than the smaller version, had greater torque capacity, and used fractionally taller (lower numerical) indirect ratios.

1961 Oldsmobile Super 88 hardtop front 3q © 2008 Aaron Severson
While Pontiac retained the earlier dual-coupling four-speed Hydra-Matic (now dubbed “Super Hydra-Matic”) for the division’s biggest cars, Oldsmobile switched entirely to the new three-speed units for 1961. Hydra-Matic was nominally optional on Oldsmobile Super 88s like this two-door Holiday coupe, but very, very few contemporary Oldsmobiles were built with manual gearboxes. (author photo)

Judging by the relevant patent disclosures (U.S. Patents 3,141,354 and 3,132,535), Roto Hydra-Matic was developed by some of the same engineers responsible for the four-speed Controlled Coupling Hydra-Matic, including Walter B. Herndon (with Howard E. Olsen) and August Borman, Jr. (with Charles W. Cline and Carl E. Shellman). The production transmission is typically credited to Detroit Transmission’s assistant chief engineer, Jack W. Qualman, and his boss, Jack R. Doidge. In any case, the new transmission’s conceptual relationship to earlier Hydra-Matics remained evident, although it borrowed a few concepts from the triple-turbine transmissions as well.

Compared to its immediate predecessor, Roto Hydra-Matic was lighter, more compact, and mechanically simpler. There were now only three forward speeds rather than four; two planetary gearsets rather than the previous three; and a single three-element torque converter rather than two fluid couplings. The front overrun clutch and sprag brake were deleted, as was the rear oil pump. The previous neutral clutch was retained, as were the rear overrun band and the reverse cone clutch, although the latter was now part of the front gearset. There was also a new multi-disc front clutch, located between the front unit annulus and the torus cover.

Color diagram of 1961–1964 Roto Hydra-Matic transmission © 2016–2017 Aaron Severson
The diagram above illustrates the major mechanical components of the three-speed Roto Hydra-Matic. The torque converter of this transmission was based on the smaller second coupling of the four-speed Controlled Coupling Hydra-Matic and could be emptied and filled in the same manner. The converter was empty in second gear (whether in Drive or Super/D-Right) and full in all other gears, including neutral and reverse. This diagram again is not to scale (proportionally, the converter is even smaller than this) and has been simplified in the increasingly vain hope of visual coherency. Note that the oil pump is mounted inside the torus housing, driven directly off the engine flywheel. (author diagram)
Color diagram of 1961–1963 Model 5 (light-duty) Roto Hydra-Matic transmission © 2016–2022 Aaron Severson
The light-duty (Model 5) version of the Roto Hydra-Matic transmission, used from 1961 to 1963, was similar to the larger units found in full-size cars, but it used a brake band rather than a sprag clutch and forward clutch, which was cheaper, lighter, and more compact. (author diagram)

Roto Hydra-Matic’s two planetary gearsets were interconnected by three concentric shafts. The main shaft, innermost of the three, connected the torque converter turbine to the rear gearset sun gear through a vibration damper, a steel-jacketed, splined rubber cushion designed to damp torsional vibrations and keep the main shaft splines from rattling. Around the main shaft was the carrier shaft, which connected the planet carriers of both gearsets to the torque converter’s reaction member and the transmission output shaft. Surrounding the main shaft was a hollow sleeve shaft that linked the reaction members of the two gearsets — the front sun gear and rear annulus — to a single centrally mounted sprag clutch that would hold both elements stationary in first and second gears. The overrun band, which surrounded the the rear annulus, could be engaged to do the same thing. (On the light-duty 61-5 model, both the sprag clutch and overrun band were replaced by a double-wrap brake band, which was engaged in first and second gear and released in third; eliminating the sprag clutch also allowed the deletion of the neutral clutch, since the brake band was completely released in reverse and neutral.)

Interconnecting the two gearsets in this manner meant that their ratios couldn’t be compounded as in earlier Hydra-Matics, which is why Roto Hydra-Matic had only two indirect ratios rather than three. (In fact, the interconnection of the planet carriers meant that putting one gearset in reduction effectively put the other in overdrive, although the overdriven member simply spun idly.) Power flowed through the rear gearset in first and the front gearset in second.

Even more unusual was the torque converter. Derived from the Controlled Coupling Hydra-Matic’s smaller second coupling, it was similar in size — diameter was only 8 inches (203 mm) — and retained the earlier coupling’s dump-and-fill capacity and straight impeller and turbine blades. Nestled within a cutout section of those blades around the converter hub was the converter’s third element: a 22-vane torque multiplier that Oldsmobile marketing pithily dubbed the “Accel-A-Rotor.” The Accel-A-Rotor was not a stator in the customary sense; since it was splined to the carrier shaft, it always rotated at the same speed as the driveshaft and could turn in either direction.

Color diagram of 1961–1964 Roto Hydra-Matic transmission reaction members and clutches © 2016 Aaron Severson
Another diagram of the three-speed Roto Hydra-Matic, highlighting the way the torque multiplier (green) is rigidly attached (splined) to both planet carriers, which in turn are attached to the output shaft. This layout provided a slight improvement in torque multiplication in reverse, but greatly limited the torque converter’s ability to provide useful torque multiplication in any forward gear. In September 1963, Jack Qualman applied for a patent (issued as U.S. Patent No. 3,270,584) on a revised layout that would have given the torque multiplier a conventional one-way clutch while divorcing it from the carrier shaft, but that solution was never implemented in production. (author diagram)

To avoid impairing converter efficiency at cruising speeds, the torque multiplier provided a nominal stall ratio of only 1.30:1. In practice, torque multiplication was both more and less than that modest figure. As explained on page 2, during torque multiplication, oil leaving the turbine exerts reaction torque on the stator. Unlike a conventional stator, Roto Hydra-Matic’s torque multiplier applied that reaction torque directly to the carrier shaft and would actually turn backward if the car was moving in reverse. In principle, that allowed the torque multiplier to function as an auxiliary turbine, although the practical effect was just a small amount of extra leverage in reverse that increased the effective stall ratio to 1.42:1 in that gear. In first, however, the reaction torque on the Accel-A-Rotor resisted the carrier shaft’s forward rotation, reducing the converter’s effective stall ratio to a meager 1.20:1.

Unlike earlier Hydra-Matics, the impeller of Roto Hydra-Matic’s torque converter was driven by the torus cover in more or less conventional fashion and therefore always rotated at engine speed. The converter housing was always full in Park, neutral, first gear, and reverse, enabling the engine to idle without stalling and providing extra torque multiplication when starting. When idling in any forward drive range, the neutral clutch was engaged and the front clutch was disengaged, so Roto Hydra-Matic would always start in first. On the Model 10, moving the selector to Low or S/D-Right, the overrun band would also engage to keep the reaction members locked when coasting; the band wasn’t used at all in normal D/D-Left range. (The Model 5, which didn’t have a sprag clutch, simply left the brake band engaged in those ranges.)

For the 1–2 shift, the torque converter’s oil supply was rapidly emptied; all three elements continued to rotate, but with no working fluid to move, they had no effect. As the converter drained, the front clutch engaged, allowing the torus cover to simultaneously drive the impeller and the annulus of the front gearset. (With the selector in Low, the transmission could not shift into second.) In a panic stop, cut-off valves in the hydraulic control system would quickly refill the converter and disengage the front clutch so the engine wouldn’t stall when the car came to a halt.

For the 2–3 shift, the torque converter was refilled, reestablishing the hydraulic connection between the turbine and the rear sun gear, but this time the front clutch remained engaged. That unlocked the sprag clutch and allowed both gearsets to turn together in direct drive (or near enough). (In S/D-Right range, the shift to third would also automatically release the overrun band; on the lighter 61-5 transmission, which had no sprag clutch, the brake band released on the shift to third.) In third, torque was split three ways: through the front clutch to the front annulus; through the converter turbine to the rear sun gear; and through the torque multiplier to the carrier shaft.

Color diagram of 1961–1964 Roto Hydra-Matic transmission reaction members and clutches © 2016–2017 Aaron Severson
Yet another diagram of the three-speed Roto Hydra-Matic. As in the earlier Controlled Coupling Hydra-Matic, Roto Hydra-Matic’s neutral clutch, located between the two planetary gearsets, served to anchor the outer race of a one-way sprag clutch to the transmission case. However, in the three-speed Hydra-Matic, the front sun gear and rear annulus (orange) were permanently interconnected, allowing both gearsets to share a single sprag clutch and a single overrun brake. The light-duty version of this transmission used a double-wrapped brake band rather than the sprag clutch and did not have (or need) the neutral clutch. (author diagram)

Discounting the unusual behavior of the torque multiplier, reverse functioned much the same way as in earlier Hydra-Matics. Moving the selector to Reverse disengaged both the front clutch and the neutral clutch (or brake band) while engaging the reverse cone clutch to lock the front annulus. The torque converter drove the rear sun gear, just as in first, but with the neutral clutch now released to neutralize the sprag clutch (or, on light-duty models, with the brake band released), the rear sun gear drove the rear annulus — and with it the front sun gear — backward. The stationary front annulus served as a reaction member, causing the driven planet carrier — and thus the carrier shaft and driveshaft — to rotate backward in reduction.

The following table summarizes the shift sequence for the full-size Roto Hydra-Matic. (Again, “REL” = “RELEASED” and “ENG” = “ENGAGED”; you can probably guess that “Torque Conv.” = “Torque Converter.”)

1961–1964 Roto Hydra-Matic Gearing Sequence
Front Planetary Rear Planetary
Gear Torque Conv. Front Clutch Reverse
Sprag Overrun

† In Low and S/D-Right ranges only; always off in Drive/D-Left.
‡ Plus torque multiplier effect at stall.

Like its predecessors, Roto Hydra-Matic placed Reverse at the far end of the shift pattern, adjacent to Low, and allowed the car to be “rocked” by moving the selector back and forth between Low and Reverse. A reverse blocker (theoretically) prevented the transmission from going into reverse if the car was moving faster than a crawl. However, as with Dual-Path Turbine Drive, there was no longer any provision for push-starting. The single oil pump was now driven directly off the engine flywheel, so neither could be driven by the propeller shaft with the engine off.

1961 Oldsmobile Dynamic 88 convertible dashboard © 2009 Aaron Severson
Oldsmobile adopted the three-speed Hydra-Matic for both the compact F-85 and for 1961-1964 Eighty-Eight and Ninety-Eight models, like this 1961 Dynamic Eighty-Eight. Pontiac used Roto Hydra-Matic for the Catalina, Ventura, and Grand Prix, but for some reason opted to retain the dual-coupling four-speed automatic (now called Super Hydra-Matic) for the big Star Chief and Bonneville through 1964. Cadillac never used Roto Hydra-Matic, staying with the four-speed Hydra-Matic until switching to Turbo Hydra-Matic in 1964–1965. (author photo)

Roto Hydra-Matic was even smoother than the four-speed Controlled Coupling Hydra-Matic, but a certain amount of performance was sacrificed in the process. In fact, many contemporary reviewers judged the three-speed Hydra-Matic in the Oldsmobile F-85 inferior to the two-speed Dual-Path Turbine Drive used in the Buick Special or even Powerglide in both performance and shift quality. Part of the problem was that Roto Hydra-Matic’s shifts were now quite slow. The adoption for 1962 of a new hydraulic pressure control system allowed shift speed and firmness to vary with engine torque, which helped some, but the assertive shift quality that was once a Hydra-Matic hallmark was now long gone.

A bigger issue, so far as performance was concerned, was that the three-speed transmission’s ratios (listed in the table below) were far from ideal. Despite the torque multiplier and a rather short first gear, starting ratios were still taller than the four-speed unit’s. That wouldn’t have been so bad, but Roto Hydra-Matic’s second and third gears were closer to third and fourth in the dual-coupling Hydra-Matic, leaving a big gap between first and second that the torque multiplier (which was ineffective once the car was in motion) could not plug. The annoyance of the ratio gap was compounded by the hydraulic control system’s frustrating tendency to vacillate between second and third.

1961–1964 Roto Hydra-Matic Gear Ratios
Full-Size Oldsmobile/Pontiac Holden/Opel/Vauxhall and Y-Body Oldsmobile
Gear Ratio At Stall* Ratio At Stall*
1st 2.97 3.56 3.03 3.64
2nd 1.56 N/A 1.58 N/A
3rd 1.00 N/A 1.00 N/A
Reverse -2.49 -3.53 -2.52 -3.57

* The torque multiplier was effective only in 1st and Reverse, and only when starting from rest.

Another unhappy peculiarity was a penchant for oil leaks. We don’t know all the factors that may have contributed to that problem, although we wonder if it was partly related to Roto Hydra-Matic’s operating pressures, which were generally higher than with its four-speed predecessor and may have tested the integrity of the seals. Particularly noteworthy is the fact that converter charging pressure was quadrupled (to 180 psi/12.41 bars) to make up for the torque capacity sacrificed to the torque converter’s diminutive size. We assume the rationale for the small diameter was, as before, to facilitate rapid drainage and refilling. The dilemma, of course, was that the dump-and-fill coupling in the earlier Controlled Coupling Hydra-Matic never had to bear more than 40% of input torque; Roto Hydra-Matic’s torque converter had to bear the full engine output in first gear.

The good news was that the new layout, along with a switch from cast iron to aluminum for the transmission case, made Roto Hydra-Matic — soon nicknamed “Slim Jim” — more compact and some 75 to 95 lb (34 to 43 kg) lighter than the dual-coupling Hydra-Matic (which remained in production for Cadillac and some Pontiacs). It was also cheaper to build, if not to buy.

(To the latter point, we should note that while the list prices of automatic transmissions had crept steadily upward since the forties, that inflation had been at a somewhat slower rate than the inflation in new car prices. Thus, while automatic transmissions weren’t getting any cheaper, the price of the option as a percentage of the cost of a new car had actually decreased.)


By the mid-sixties, the autonomy GM had long allowed its individual automotive divisions was beginning to give way to a new emphasis on inter-divisional commonality. We don’t know if the Y-body compacts represented some kind of breaking point in that regard, but we wouldn’t be surprised. Their development and manufacturing costs had been high — higher, we have little doubt, than most of GM’s contemporary full-size cars, and largely concentrated in areas that the average buyer wouldn’t even notice — and sales had been disappointing, which was a recipe for lackluster profits.

During this period, GM began a belated move toward standardized transmissions. Having multiple automatic transmissions probably seemed reasonable when Buick was selling more cars than Plymouth and half the industry used Hydra-Matic, but the market downturn and various missteps of the late fifties and early sixties made the proliferation of sui generis transmissions seem like economic folly. The three-year production total for Dual-Path Turbine Drive, for example, was well short of the average annual volume of the early-fifties Hydra-Matic. Numbers like that made it harder to justify the R&D and tooling costs of multiple transmission designs.

GM initially opted for a two-pronged approach: a new two-speed automatic for Buick, Oldsmobile, and Pontiac A-body intermediates, which replaced the Y-body compacts for 1964, and a new three-speed transmission to replace the Roto Hydra-Matic and Controlled Coupling Hydra-Matic in bigger cars. Chevrolet, whose annual production generally exceeded the combined totals of the other four automotive divisions, continued to build and use its own two-speed Powerglide.

The new transmissions were developed by engineers from the corporate transmission group and Detroit Transmission Division, which was formally renamed Hydra-Matic Division on October 1, 1963. The two-speed, which Buick called Super Turbine 300 (ST-300) and Oldsmobile called Jetaway, was mechanically very similar to the aluminum-case Powerglide, using a Ravigneaux gearset to provide indirect ratios of +/-1.765:1. The three-speed unit was the Turbo Hydra-Matic 400 (TH-400), which Buick called Super Turbine 400 (ST-400), an all-new design using a licensed version of Howard W. Simpson’s patented “Simpson gearset“: two planetary gearsets sharing a single common sun gear. Both transmissions had three-element torque converters and used a new type of vacuum modulation.

Some sources — including contemporary Buick publicity and marketing material — suggest a lineal connection between these transmissions and the earlier Dynaflow/Turbine Drive, Dual-Path, and Hydra-Matic units they replaced, which was really only true in certain broad or incidental ways. Gone for good were the multiple turbines, dump-and-fill couplings, and split torque clutches (although Turbo Hydra-Matic would eventually add a lockup torque converter clutch in the pursuit of better fuel economy). The one exception was that some 1964–1967 ST-300/Jetaway and 1965–1967 ST-400/TH-400 transmissions used a two-position variable-pitch stator, similar in principle to the one Dynaflow had first adopted back in 1955. However, the pitch angles were different and the stator servo control valve was now operated by a solenoid triggered by the kickdown switch. Pontiac and Chevrolet never used the “switch-pitch” stator, nor did Series Seventy-Five Cadillacs; other users deleted the feature after the 1967 model year.

The new two-speed automatic was first offered on the 1964 A-body Buick Special/Skylark, Oldsmobile F-85/Cutlass, and Pontiac Tempest/Le Mans/GTO and the B-body Buick LeSabre and Oldsmobile Jetstar 88. At the same time, Turbo Hydra-Matic replaced Turbine Drive on full-size Buicks (including the Riviera) and superseded the four-speed Hydra-Matic on the Cadillac DeVille, Sixty Special, and Eldorado. All remaining U.S. users of both earlier Hydra-Matics switched to TH400 for the 1965 model year. In mid-1965, Chevrolet also began offering Turbo Hydra-Matic as an option for full-size cars equipped with the new 396 cu. in. (6,488 cc) “Turbo Jet” engine. Turbo Hydra-Matic became available on certain A-body intermediates for 1967 and on the Corvette for 1968.

By the late sixties, two-speed automatics were becoming increasingly anachronistic, so the ST-300/Jetaway was relatively short-lived. Starting in 1969, both ST-300/Jetaway and Powerglide were phased out in favor of scaled-down, medium- and later light-duty versions of Turbo Hydra-Matic. Two-speed automatics had disappeared from all of GM’s North American cars by the 1974 model year.

1963 Buick Riviera front 3q © 2008 Aaron Severson
The 1963 Buick Riviera used Buick’s older twin-turbine transmission — now called simply Turbine Drive — but the 1964 model was one of the first users of the new Turbo Hydra-Matic three-speed transmission, which Buick called Super Turbine 400. (author photo)

The mechanics and further development of Turbo Hydra-Matic (sometimes styled “Turbo Hydra-matic” or “Turbo-Hydramatic”) are beyond the scope of this article, but suffice to say it was a very successful and generally well-regarded line. Like the old four-speed Hydra-Matic, the TH400 was also used by a variety of outside automakers, including Rolls-Royce and Bentley, Jaguar, and even Ferrari.

In 1983, GM chairman Roger Smith ordered the consolidation of all the corporation’s transmission plants under the control of Hydra-Matic Division, eliminating the last vestiges of the old divisional rivalry. In the early nineties, GM created GM Powertrain by combining Hydra-Matic Division with GM Engine and later the Central Foundry Division and the Advanced Engineering Staff, the heirs of the group that originally developed Hydra-Matic and Dynaflow.

Since 2010, the GM Powertrain group has been part of the larger Global Products Operations organization, although the Hydra-Matic name is still in use — and of course remains a registered trademark of General Motors. Modern Hydra-Matic transmissions, however, bear only a faint resemblance to their pioneering and sometimes peculiar forebears.



The author would like to offer special thanks to reader Dave Ostroska for generously providing us with a copy of the factory service manual for the Buick Dual-Path Turbine Drive, which is now quite hard to find.


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Kelley, assignor to General Motors Corporation, “Combination Fluid Turbo Clutch and Variable Speed Gearing,” U.S. Patent No. 2,176,138, applied 5 February 1937, issued 17 October 1939; “Fluid Flywheel Gearing Arrangement,” U.S. Patent No. 2,211,233, applied 10 April 1939, issued 13 August 1940; and “Transmission Drive,” U.S. Patent No. 2,377,696, filed 15 December 1941, issued 5 June 1945; “Compound Power Transmission,” U.S. Patent No. 2,433,052, filed 6 September 1943, issued 23 December 1947; “Combined Transmission,” U.S. Patent No. 2,606,460, filed 29 November 1944, issued 12 August 1952; “Tank Cross Drive for Steering by Variable-Speed Ratio Driving Means,” U.S. Patent No. 2,585,790, filed 16 April 1945, issued 12 February 1952; “Fluid Drive and Controls,” U.S. Patent No. 2,625,056, filed 14 September 1946, issued 13 January 1953; “Rotary Hydraulic Torque Converter,” U.S. Patent No. 2,687,616, filed 11 January 1949, issued 31 August 1954; “3-Phase Turbine Drive,” U.S. Patent No. 2,737,061, filed 19 November 1949, issued 6 March 1956; “Fluid Control for Rotary Turbine Type Hydraulic Torque Converters,” U.S. Patent No. 2,638,746, filed 30 November 1949, issued 19 May 1953; “Dual Range Plural Turbine Gear Drive,” U.S. Patent No. 2,766,641, filed 8 November 1950, issued 16 October 1956; “Multiple Rotor Converter Having Plural Impellers,” U.S. Patent No. 2,727,360, filed 23 November 1951, issued 20 December 1951; “Hydrodynamic Torque Converter and Gearing,” U.S. Patent No. 2,782,659, filed 18 June 1952, issued 26 February 1957, reissued 14 March 1961; “Four Phase Converter Drive,” U.S. Patent No. 2,803,974, filed 5 August 1953, issued 27 August 1957; “Four Phase Converter Drive,” U.S. Patent No. 2,981,124, filed 5 August 1953, divided 25 April 1957, issued 25 April 1961; “Hydrodynamic Torque Converters and Controls Therefor,” U.S. Patent No. 2,999,400, filed 13 January 1954, issued 12 September 1961; “Hydrodynamic Torque Converters,” U.S. Patent No. 2,910,832, filed 22 July 1954, issued 3 November 1959; “Transmission,” U.S. Patent No. 2,821,095, filed 19 October 1955, issued 28 January 1958; “Hydraulic Torque Converter,” U.S. Patent No. 2,882,684, filed 17 July 1956, divided 31 July 1957, issued 21 April 1959; “Transmission,” U.S. Patent No. 2,882,751, filed 17 July 1956, divided 31 July 1957, issued 21 April 1959; “Hydrodynamic Torque Converters,” U.S. Patent No. 3,025,720, filed 26 March 1958, issued 20 March 1962; “Transmission,” U.S. Patent No. 3,030,823, filed 11 July 1957, issued 24 April 1962; and “Transmission,” U.S. Patent No. 3,242,677, filed 29 September 1955, issued 29 March 1966; Oliver K. Kelley and Gilbert K. Hause, assignors to General Motors Corporation, “Multi-Phase Torque Converter,” U.S. Patent No. 2,957,370, filed 11 July 1957, issued 25 October 1960; Oliver K. Kelley and John D. Lindsay, assignors to General Motors Corporation, “Multiple Stator Torque Converter,” U.S. Patent No. 3,025,719, filed 28 December 1954, issued 20 March 1962; Oliver K. Kelley and Robert S. Plexico, assignors to General Motors Corporation, “Transmission Control System,” U.S. Patent No. 2,865,227, filed 4 June 1952, issued 23 December 1958; Oliver K. Kelley and Robert M. Schaefer, assignors to General Motors Corporation, “Composite Fluid and Gear Drive,” U.S. Patent No. 2,782,658, filed 18 January 1951, issued 26 February 1957; Oliver K. Kelley and William S. Wolfram, assignors to General Motors Corporation, “Rotary Hydraulic Torque Converter with Dynamic Braking,” U.S. Patent No. 2,651,918, filed 30 July 1949, issued 15 September 1953; Carroll K. Lenning, assignor to General Motors Corporation, “Transmission Drive Cooling System,” U.S. Patent No. 2,270,536, filed 17 February 1940, issued 20 January 1942; Jim Lodge, “’55 Buick Roadmaster Special,” Motor Trend Vol. 7, No. 7 (July 1955), reprinted in Buick Performance Portfolio 1947-1962, pp. 50–53, 71 “’56 Buick Special and Century,” Motor Trend Vol. 8, No. 6 (June 1956), reprinted in ibid, pp. 56-59, and ; Joseph Lowrey, “Dynaflow Drive in the Alps,” The Motor 20 April 1949, reprinted in ibid, pp. 14-18; “M18 Hellcat Tank Destroyer,” n.d, m18hellcat. com/m18hellcat/ Home.html, accessed 20 September 2015; L.H. Nagler, “How Your Car Shifts for Itself,” Popular Mechanics Vol. 89, No. 5 (May 1948), pp. 102–106, 264, 268, 272; Chuck Nerpel, “Buick Invicta,” Motor Trend Vol. 12, No. 6 (June 1960), reprinted in Buick Performance Portfolio 1947-1962, pp. 100–104; “New Dynaflow Buicks,” The Motor 8 December 1948, reprinted in Buick Performance Portfolio 1947-1962, pp. 10–13; Paul Niedermeyer, “Powerglide: A GM’s Greatest Hit or Deadly Sin?” Curbside Classic, 30 March 2012, www.curbsideclassic. com/automotive-histories /powerglide-gms-greatest-hit-or-deadly-sin/, last accessed 4 January 2016; “1951 Chevrolet,” Special Interest Autos #17 (June-July 1973), reprinted in The Hemmings Book of Postwar Chevrolets, pp. 12–17; the Old Car Brochures website (oldcarbrochures.org); the Old Car Manual Project (www.oldcarmanualproject. com); “Packard’s Ultramatic Drive,” Product Engineering July 1949, reprinted in Packard Gold Portfolio 1946-1958, pp. 22–24; Robert S. Plexico and Russell E. Kaufman, “Chevrolet’s Automatic Transmission, The SAE Journal Vol. 58, No. 3 (March 1950), pp. 27–33; Jim Potter, “’54 Buick Special,” Motor Trend Vol. 6, No. 10 (October 1954), reprinted in Buick Performance Portfolio 1947-1962, pp. 42–43; “Road & Track Road Test: Buick Special V-6,” Road & Track Vol. 11, No. 3 (November 1961), reprinted in Buick Performance Portfolio 1947-1962, pp. 124-127; “Road Test: Buick Century,” Motor Life March 1955, reprinted in ibid, pp. 48–49, 63; “Road Test: The Buick V-8,” Motor World 22 May 1953, Buick Performance Portfolio 1947-1962, pp. 37–38; “Road Test: The Invicta and the Special,” Motor Life January 1961, reprinted in ibid, pp. 112-119; “Road Test – The 1956 Buick Century,” Motor Life May 1956, reprinted in ibid, pp. 60–61; Maurice S. Rosenberger, “Transmission Control System,” U.S. Patent No. 2,766,639, filed 8 November 1952, issued 16 October 1956, reissued 5 June 1962; Christian Seabaugh, “1944 Buick M18 Hellcat Tank Destroyer First Drive: Seek, Strike, Destroy!” Truck Trend 28 October 2013, www.trucktrend. com, accessed 20 September 2015; Wilbur Shaw, “Buick Hooks New V-8 to Dynaflow+Gears,” Popular Science Vol. 162, No. 2 (February 1953), pp. 159–162, 248; Don Sherman, “Reviews: Buick Hellcat Tank,” Automobile February 2005, www.automobilemag. com, accessed 20 May 2010; Alfred P. Sloan with John McDonald, My Years with General Motors (Garden City, NY: Doubleday, 1964); Edwin Storm’s Free Car Brochures website at the Old Car Manual Project (storm.oldcarmanualproject. com); Robert Temple, “Transmissions and Drive Lines (Know Your Car Part Two),” Motor Trend Vol. 15, No. 1 (January 1963), pp. 54-59; “The Autocar road tests 1814: Buick Special,” The Autocar 31 March 1961: 494–497; “The New Buicks: Slicker, Smoother,” Popular Science Vol. 153, No. 6 (December 1948), pp. 106–107; “The New Buick: With Hydraulic Torque Converter and Two-speed Epicyclic Gear,” The Motor 21 January 1948, reprinted in Buick Performance Portfolio 1947-1962, pp. 6-9; Wayne Thoms, “Road Trial: Buick Special,” Motor Trend Vol. 12, No. 12 (December 1960), pp. 22–27; Robert M. Tuck and James J. Mooney, Jr., assignors to General Motors Corporation, “Transmission,” U.S. Patent No. 2,929,270, filed 7 March 1957, issued 22 March 1960; U.S. Department of the Army, Principles of Automotive Vehicles (Department of the Army Technical Manual TM 9-2700) (Washington, DC: U.S. Government Printing Office, November 1947); Jim Whipple, “Buick Special’s performance-plus economy,” Popular Mechanics Vol. 115, No. 3 (March 1961), pp. 122–125, 286–291; and “PM’s 1000-Mile Road Test of Buick’s New V-6,” Popular Mechanics Vol. 116, No. 4 (October 1961), pp. 108–111, 258–260; ‘woodbox’ and ‘Buickspec6231,’ “1963 Skylark Dynaflow trans help required,” Team Buick, 20 November 2011, www.teambuick. com/ forums/ showthread.php?20995-1963-Skylark-Dynaflow-trans-help-required/, accessed 21 February 2016; and Jim Wright, “A Wildcat from Buick,” Motor Trend Vol. 14, No. 8 (August 1962), reprinted in Buick Performance Portfolio 1947-1962, pp. 136-140, and “Chevrolet Impala SS 250 H.P. 340 H.P.,” Motor Trend Vol. 15, No. 3 (March 1963), reprinted in Impala & SS Muscle Portfolio 1958–1972, pp. 48–53.

Additional information on the Controlled Coupling Hydra-Matic came from August H. Borman, Jr.; Forrest R. Cheek; and Milton H. Scheiter, assignors to General Motors Corporation, “Controlled Coupling Automatic Transmissions,” U.S. Patent No. 3,048,055, filed 27 December 1954, issued 7 August 1962; Ray Brock, “Olds … ’59 Class Leader,” Hot Rod June 1959, reprinted in Oldsmobile Automobiles 1955–1963, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1989), pp. 44–48, 60; “Olds 88 for ’60,” Hot Rod January 1960, reprinted in ibid, pp. 51–55; and “Pontiac – 3000 Mile Road Test,” Hot Rod December 1958, reprinted in Pontiac Limited Edition: 1949-1960, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1999), pp. 74–79, 89; Arch Brown, “1957 Nash Ambassador: Twilight of the Dinosaurs,” Special Interest Autos #115 (January-February 1990), reprinted in The Hemmings Book of Nashes: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard A. Lentinello (Bennington, VT: Hemmings Motor News, 2002), pp. 110–117; “1959 Cadillac Eldorado Biarritz: Nothing Succeeds Like Excess,” Special Interest Autos #88 (August 1985), reprinted in Cadillac Automobiles 1949–1959, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1990), pp. 92–100; the Cadillac & LaSalle Club Modified Chapter website (www.modifiedcadillac.org); Cadillac Motor Car Division of General Motors Corporation, “Cadillac data book ’56,” September 1955; “Car Life Consumer Analysis: 1956 Cadillac,” Car Life Vol. 4, No. 5 (June 1956), reprinted in Cadillac Automobiles 1949-1959, pp. 52–53; “Car Life Consumer Analysis: 1956 Pontiac,” Car Life Vol. 4, No. 2 (March 1956), reprinted in Pontiac Limited Edition: 1949-1960, pp. 46–47; Floyd Clymer, “Owners Praise Cadillac’s Performance But Complain of Transmission Troubles,” Popular Mechanics Vol. 105, No. 4 (April 1956), pp. 105–108, 250–258; “Controlled Coupling Hydra-Matic Fundamentals,” Chilton’s Auto Repair Manual (Philadelphia, PA: Chilton Book Company, 1958), pp. 79-132; Jack R. Doidge and Victor C. Moore, assignors to General Motors Corporation, “Transmission,” U.S. Patent No. 2,947,199, filed 26 November 1957, issued 2 August 1960; Walter B. Herndon, assignor to General Motors Corporation, “Controlled Coupling Multistep Automatic Transmissions,” U.S. Patent No. 2,876,656, filed 23 November 1953, issued 10 March 1959; and “Variable Capacity Pressure System for Transmissions,” U.S. Patent No. 2,875,699, filed 19 July 1954, issued 3 March 1959; Bill Holland, “Bill Holland Tests … The Cadillac 60 Special,” Motorsport May 1956, reprinted in Cadillac Automobiles 1949-1959, pp. 54–56; “Hydra-Matic Dampens That Thump,” Popular Science Vol. 167, No. 5 (November 1955), pp. 119–121, 260; John F. Katz, “1956 Oldsmobile Super 88 Convertible,” Special Interest Autos #145 (January-February 1995), reprinted in The Hemmings Book of Oldsmobiles: driveReports from Hemmings Special Interest Autos magazine, ed. Terry Ehrich (Bennington, VT: Hemmings Motor News, 2001), pp. 65-74; and “1960 Pontiac Bonneville Vista,” Special Interest Autos #172 (July-August 1999), reprinted in The Hemmings Motor News Book of Pontiacs: driveReports from Hemmings Special Interest Autos magazine, ed. Terry Ehrich (Bennington, VT: Hemmings Motor News, 2001), p. 68-77; Oliver K. Kelley, assignor to General Motors Corporation, “Combination Fluid Turbo Clutch and Variable Speed Gearing,” U.S. Patent No. 2,176,138, applied 5 February 1937, issued 17 October 1939; “Fluid Flywheel Gearing Arrangement,” U.S. Patent No. 2,211,233, applied 10 April 1939, issued 13 August 1940; and “Transmission Drive,” U.S. Patent No. 2,377,696, filed 15 December 1941, issued 5 June 1945; Dale Kelly, “An Engineer Analyzes the 1957 Oldsmobile,” Popular Mechanics Vol. 108, No. 1 (July 1957), reprinted in Oldsmobile Automobiles 1955–1963, pp. 25–26; Al Kidd, “drivescription: ’56 Oldsmobile,” Motor Trend Vol. 7, No. 12 (December 1955) and “’56 Oldsmobile Road Test, “Motor Trend Vol. 8, No. 4 (April 1956), reprinted in ibid, pp. 12-13 and 16-19; Ed Mobley, “Controlled Coupling Hydramatic/Jetaway Automatic Rebuild,” Edscars, 2006, www.photopaige. com/ edscars/ 60caddy/ CaddyWebSitev2_files/ TrannyRebuild2.htm, accessed 29 May 2010; Victor C. Moore, assignor to General Motors Corporation, “Transmission,” U.S. Patent No. 2,919,607, filed 30 November 1956, issued 5 January 1960; “New Cars Described: 1956 Pontiacs Have Latest Transmission,” The Autocar 18 November 1955, reprinted in Pontiac Limited Edition: 1949-1960, p. 45; “1960 Pontiac Tempest,” Hot Rod May 1960, reprinted in Pontiac Limited Edition: 1949-1960, pp. 86–89; Oldsmobile Division, General Motors Corporation, “’58 Oldsmobile” [brochure, ca. November 1957]; “Oldsmobile” [1956 brochure, ca. October 1955]; and “Oldsmobile’s New Jetaway Hydra-Matic” [brochure], 1955; Pontiac Motor Division of General Motors Corporation, “Answers That Sell: 1964 New Product Facts” [dealer literature], 30 August 1963; “Facts About the New ’56 Pontiac: Star Chief, 870 and 860 series” [dealer literature], September 1955; “Introducing Your 1957 Pontiac” (Pontiac 1957 Owner’s Guide S-5701), January 1957; 1957 Hydra-Matic Manual (with 1956 Appendix) (Pontiac, MI: Pontiac Motor Division, General Motors Corporation, March 1957); and “Pontiac ’58” [brochure, ca. October 1957]; “Oldsmobile: not the rocket it used to be,” Motor Life March 1960, reprinted in Oldsmobile Automobiles 1955–1963, pp. 56–57; “Oldsmobile Road Test,” Motor Life February 1959, reprinted in ibid, pp. 42–43; “Power Is Oldsmobile’s Top Feature, Say Owners from Coast to Coast,” Popular Mechanics Vol. 108, No. 1 (July 1957), reprinted in ibid, pp. 24–26; “The 1956 Cadillac,” Motor Life December 1955, reprinted in Cadillac Automobiles 1949-1959, p. 49; William K. Toboldt and Larry Johnson, Goodheart-Willcox Automotive Encyclopedia (South Holland, IL: The Goodheart-Willcox Company, Inc., 1975); Johnny Tolan, “Johnny Tolan Tests the ’57 Oldsmobile,” Speed Age March 1957, reprinted in Oldsmobile Automobiles 1955–1963, pp. 20–23; United Motors Service Division, The Hydra-Matic Transmission 1946-1955: On-the-Car Adjustment Service Manual (Detroit, MI: United Motors Service Division of General Motors Corporation, 1956), and Hydra-Matic Controlled Coupling Transmission Service Manual (Bulletin A-3755) (Detroit, MI: United Motors Service Division of General Motors Corporation, 1 November 1957); U.S. War Department, Ordnance Maintenance: Hydra-Matic Transmission and Propeller Shafts for Light Tanks M5, M5A1, and 75-MM Howitzer Carriage (War Department Technical Manual TM 9-1727C (Washington, DC: U.S. Government Printing Office, 5 February 1943); Joe H. Wherry, “’58 Oldsmobile on trial,” Motor Trend Vol. 10, No. 3 (March 1958), reprinted in Oldsmobile Automobiles 1955–1963, pp. 30–35; and Otto Zipper, “Road Test: Two Pontiacs,” Motor Trend Vol. 9, No. 3 (March 1957), reprinted in Pontiac Limited Edition: 1949-1960, pp. 54–57, 59. John D. Kelly later helped us to sort out some technical points about the original single-coupling unit in emails to the author, 7 to 8 March 2017.

Additional information on the triple-turbine automatics came from Al Berger, “’59 Chevrolet Has Fins, Will Travel,” Speed Age December 1958, reprinted in Impala & SS Muscle Portfolio 1958–1972, pp. 12–15; Terry Boyce, “Paragon of Excess: 1958 Buick Limited,” Special Interest Autos #53 (September-October 1979), reprinted in The Hemmings Book of Buicks, pp. 65-71; Johnny Boyd, “Johnny Boyd Tests the ’57 Buick,” Speed Age June 1957, reprinted in Buick Performance Portfolio 1947-1962, pp. 68–71; Arch Brown, “1957 Chevrolet Bel Air: The Really Hot One,” Special Interest Autos #96 (November-December 1986), reprinted in The Hemmings Book of Postwar Chevrolets, pp. 54-69; “Buick Builds a Better One,” Hot Rod March 1959, reprinted in Buick Performance Portfolio 1947-1962, pp. 92–95, 104; “Buick 1960,” Motor Trend Vol. 11, No. 11 (November 1959), reprinted in ibid, pp. 96–97; Jim Carroll, “’59 Buick on Trial,” Motor Trend Vol. 10, No. 10 (October 1958), reprinted in ibid, pp. 87-91; Charles S. Chapman, Jr., and Kenneth W. Gage, assignors to General Motors Corporation, “Transmission,” U.S. Patent No. 2,912,876, filed 20 May 1957, issued 17 November 1959; Chevrolet Engineering Center, Engineering Product Information Department, 1957 Chevrolet Engineering Achievements: Passenger Car Features (Detroit, MI: October 1956); Chevrolet Motor Division of General Motors Corporation, “Chevrolet 1957” [brochure, ca. October 1956]; “Chevrolet 1958: It Goes Big…With Spectacular New Shape!” [brochure, ca. October 1957]; 1958-1960 Chevrolet Turboglide Transmission: Construction and Operation (Detroit, MI: Chevrolet Motor Division, General Motors Corporation, May 1960); Gilbert K. Hause, assignor to General Motors Corporation, “Transmission,” U.S. Patent No. 2,919,608, filed 2 August 1956, issued 5 January 1960; Vincent Douglas, “1961 Impala: Big-Block Chevy, Family Style,” Special Interest Autos #147 (May-June 1995), reprinted in The Hemmings Book of Postwar Chevrolets, pp. 78–85; Tim Howley, “1959 Buick Electra 225 Convertible: Flash and Fins,” Special Interest Autos #126 (November-December 1991), reprinted in The Hemmings Book of Buicks, pp. 72-77; and “SIA comparisonReport: ’58 vs. ’59 Chevrolet Impala: What a Difference a Year Makes!” Special Interest Autos #140 (March-April 1994), reprinted in The Hemmings Book of Postwar Chevrolets, pp. 70–77; Oliver K. Kelley, assignor to General Motors Corporation, “Hydraulic Torque Converter,” U.S. Patent No. 2,882,684, filed 17 July 1956, divided 31 July 1957, issued 21 April 1959; and “Transmission,” U.S. Patent No. 2,964,976, filed 13 January 1958, issued 20 December 1960; Oliver K. Kelley and Gilbert K. Hause, assignors to General Motors Corporation, “Triple Turbine Bus and Truck Transmissions,” U.S. Patent No. 3,021,727, filed 13 October 1958, issued 20 February 1962; Oliver K. Kelley, Gilbert K. Hause, and Frank A. Swindell, assignors to General Motors Corporation, “Reactor Blade Pitch Control of a Hydro-Dynamic Torque Converter,” filed 6 March 1957, issued 10 November 1959; Richard M. Langworth, “Something Ventured, Nothing Gained: The Story of the 1957-58 Buick,” Collectible Automobile Vol. 17, No. 5 (February 2001), pp. 8–21; Mike Mueller and Anthony Young, Classic Chevy Hot Ones: 1955–1957 2nd ed. (Ann Arbor, MI: Lowe & B. Hould Publishers, 2002); “1958 Chevrolet Impala Road Test,” Motor Life January 1958, reprinted in Impala & SS Muscle Portfolio 1958–1972, pp. 5-7; Tom Sidoti, “1959 Buick Triple Turbine Transmission,” 1959 Buick Electra 225 Convertible, 20 October 2009, 1fine59. com/?paged=2, accessed 17 November 2015; “Testing the 60’s: Chevrolet V-8: Plushness…with a Price,” Motor Life February 1960, reprinted in Impala & SS Muscle Portfolio 1958–1972, pp. 21-22; “The 1959 Buick,” Motor Life November 1958, reprinted in Buick Performance Portfolio 1947-1962, p. 80–83; Jim Whipple, “Car Life 1958 Consumer Analysis: Buick,” Car Life Vol. 6, No. 3 (April 1958), reprinted in ibid, pp. 72–75; Frank J. Winchell and Oliver K. Kelley, assignors to General Motors Corporation, “Transmission,” U.S. Patent No. 3,008,349, filed 25 February 1957, issued 14 November 1961; and Walt Woron, “Chevrolet ’57,” Motor Trend Vol. 8, No. 12 (December 1956), reprinted in Chevrolet 1955-1957, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1988), pp. 64-68.

Additional information on Roto Hydra-Matic came from “Autocar Road Test 1908: Vauxhall Cresta Hydra-Matic 2,651 c.c.,” Autocar 11 January 1963, pp. 58–62; Terry Bebbington, “EJ-EH Holden History and Information,” Australian Classic Car December 2003; August H. Borman, Jr.; Charles W. Cline; and Carl E. Shellman, assignors to General Motors Corporation, “Transmission,” U.S. Patent No. 3,132,535, filed 20 September 1960, issued 12 May 1964; “Car Life Road Test: Oldsmobile F-85,” Car Life Vol. 9, No. 4 (May 1961), reprinted in Oldsmobile Automobiles 1955-1963, pp. 66-70; “Car Life Road Test: Oldsmobile 98 Holiday Sports Sedan,” Car Life Vol. 10, No. 3 (April 1962), reprinted in ibid, pp. 74-78; “EJ Holden,” “EK Holden,” and “Holden History,” Unique Cars and Parts [Australia], n.d., www.uniquecarsandparts. com.au, accessed 12 November 2015; Ken Fermoyle, “Buick, Olds, Pontiac Go Compact,” Popular Science Vol. 177, No. 4 (October 1960), pp. 72–76, 244–246; General Motors Continental, “Kapitän / Kapitän L” [Dutch brochure, ca. 1961]; General Motors-Holden Ltd., “Holden: Australia’s Own Car” [EK Holden brochure, 1961]; Walter B. Herndon, “GM develops light-weight, compact, Hydra-Matic transmissions,” The SAE Journal Vol. 69, No. 3 (March 1961), pp. 46–48; Walter B. Herndon and Howard E. Olsen, assignors to General Motors Corporations, “Transmission,” U.S. Patent No. 3,141,354, filed 8 March 1962, issued 21 July 1964; “Hydra-Matic 61-05 Transmission, Parts I and II,” Automobile Engineer Vol. 50, No. 15 (December 1960), pp. 524–31, and Vol. 51, No. 1 (January 1961), pp. 2–8; J.L. Spoormaker N.V., “Opel” [Dutch brochure], 1961; Oliver K. Kelley, Stanley L. Buckay, and Paul J. King, assignors to General Motors Corporation, “Balanced Inertia Plural Step-Ratio Transmissions,” filed 29 April 1955, issued 6 March 1962; “Olds F-85: Another Rocket Hits the Road,” Popular Mechanics Vol. 114, No. 4 (October 1960), p. 100–102, 310; Oldsmobile Division, General Motors Corporation, “F-85 by Oldsmobile” [brochure], February 1961; “Oldsmobile for ’64: Where the Action Is!” [brochure], September 1963; “’61 Olds” [brochure], October 1960; “’62 Oldsmobile” [brochure], September 1961; “’63 Oldsmobile” [brochure], September 1962; and “’64 Oldsmobile: Models • Equipment • Prices” [dealer literature], February 1964; “Oldsmobile Dynamic 88 Celebrity Sedan,” Car Life Vol. 10, No. 7 (August 1962), reprinted in Oldsmobile Automobiles 1955-1963, pp. 84-87; “Oldsmobile F-85,” Car and Driver Vol. 6, No. 11 (May 1961), reprinted in ibid, pp. 71-73, 100; “Oldsmobile F-85,” Motor Trend Vol. 13, No. 2 (February 1961), reprinted in ibid, pp. 61-65; Oldsmobile Mail List Server Community, “Transmissions,” Olds FAQ, 1996–2000, www.442. com/oldsfaq/ oftrn.htm, last accessed 15 February 2016; Pontiac Motor Division of General Motors Corporation, “Answers That Sell: 1964 New Product Facts” [dealer literature], 30 August 1963; “1961 Pontiac” [brochure, ca. September 1960]; “Come see our ’63 Pontiacs” [brochure, ca. October 1962]; and “Wide-Track Pontiac ’62” [brochure, ca. October 1962]; “Transmissions,” Popular Mechanics Vol. 115, No. 1 (January 1961), pp. 157–158; and Jim Whipple, “PM Owners Report: Nimble Olds F-85 Pleases Owners; Mileage, Transmission Draw Fire,” Popular Mechanics Vol. 120, No. 1 (July 1963), pp. 76–79, 196–197.

Additional information on the Corvair Powerglide and Pontiac TempesTorque came from Bill Carroll, “Inside Pontiac’s Terrific Tempest!” Sports Cars Illustrated Vol. 6, No. 4 (October 1960)) and “Pontiac Tempest Road Research Report,” Sports Cars Illustrated Vol. 6, No. 9 (March 1961), both reprinted in Car and Driver on Pontiac 1961–1975, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1986), pp. 5-16; Chevrolet Motor Division of General Motors Corporation, “Corvair by Chevrolet: The Prestige Car in Its Class” [1960 brochure], 1959; “Corvair Automatic Transmission (Road & Track Road Test 235),” Road & Track Vol. 11, No. 6 (February 1960), reprinted in Corvair Performance Portfolio 1959-1969, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1998), pp. 22–23; Ken Fermoyle, “Pontiac Tempest: Radical New Compact,” Popular Science Vol. 177, No. 3 (September 1960), pp. 53–58; Wick Humble, “1961 Pontiac Tempest: But cars aren’t supposed to have curved driveshafts,” Special Interest Autos #48 (November-December 1978), reprinted in The Hemmings Motor News Book of Pontiacs, pp. 74–86; Oliver K. Kelley, Kenneth W. Gage, and Richard W. Craig, assignors to General Motors Corporation, “Transmission and Swinging Drive Axles Including Torque Converters,” U.S. Patent No. 3,170,534, filed 7 January 1959, issued 23 February 1965; Karl Ludvigsen, “SCI Analyzes Ed Cole’s CORVAIR,” Sports Cars Illustrated Vol. 5, No. 5 (November 1959), reprinted in Corvair Performance Portfolio 1959-1969, pp. 5–13, 17; Jan P. Norbye and Jim Dunne, Pontiac 1946-1978: The Classic Postwar Years (Osceola, WI: Motorbooks International Publishers & Wholesalers, 1979); Pontiac Motor Division of General Motors Corporation, “1962 Tempest by Pontiac” [brochure, ca. October 1961]; “’63 Pontiac Tempest” [brochure, ca. October 1962]; and “Tempest: Quality Newcomer from Pontiac!” [brochure, ca. November 1960]; and Wayne Thoms, “Tempest Le Mans,” Motor Trend Vol. 15, No. 2 (February 1963), pp. 54–59.

Other background information came from Robert Ackerson, “1950 Packard DeLuxe Eight: The Last of Packard’s Postwar Pachyderms,” Special Interest Autos #64 (July-August 1981), reprinted in The Hemmings Motor News Book of Packards: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor New, 2001), pp. 58–65; Allison Transmission’s History-Heritage page at www.allisontransmission. com, accessed 13 October 2015; Oscar H. Banker, “Change Speed Planetary Transmission,” United States Patent No. 2,077,387, applied 16 July 1934, renewed 22 March 1935, issued 20 April 1937; Oscar H. Banker, “Transmission Mechanism,” U.S. Patent No. 1,795,465, filed 26 November 1928, issued 10 March 1931; Oscar H. Banker, assignor to Continental Illinois Bank and Trust Company, “Transmission,” U.S. Patent No. 1,795,464, filed 21 October 1927, issued 10 March 1931; “Transmission,” U.S. Patent No. 2,003,963, filed 21 March 1930, issued 4 June 1935; “Automatic Transmission,” U.S. Patent No. 1,843,193, filed 9 April 1930, issued 2 February 1932; “Automatic Change Speed Transmission,” U.S. Patent No. 1,843,195, filed 12 February 1931, issued 2 February 1932; “Automatic Clutch,” U.S. Patent No. 1,851,146, filed 20 March 1930, issued 29 March 1932; “Automatic Change Speed Transmission,” U.S. Patent No. 1,943,293, filed 24 July 1931, issued 16 January 1934; Oscar H. Banker, assignor to New Products Corporation, “Variable Speed Transmission,” U.S. Patent No. 1,937,503, filed 3 September 1931, issued 5 December 1933; “Clutch Mechanism,” U.S. Patent No. 2,042,454, filed 19 March 1932, issued 2 June 1936; “Automatic Change Speed Transmission,” U.S. Patent No. 1,996,790, filed 3 November 1932, issued 9 April 1935; “Change Speed Transmission,” U.S. Patent No. 1,985,884, filed 14 December 1932, issued 1 January 1935; “Planetary Transmission Mechanism,” U.S. Patent No. 2,005,726, filed 29 June 1933, issued 25 June 1935; “Change Speed Transmission,” U.S. Patent No. 2,077,387, filed 16 July 1934, issued 20 April 1937; “Clutch Mechanism,” U.S. Patent No. 2,104,014, filed 16 July 1934, issued 4 January 1938; “Automatic Transmission,” U.S. Patent No. 2,199,095, filed 13 October 1934, issued 30 April 1940; “Change Speed Transmission,” U.S. Patent No. 2,140,502, filed 30 November 1934, issued 20 December 1938; “Automatic Transmission,” U.S. Patent No. 2,171,534, filed 29 May 1935, issued 5 September 1939; “Automatic Transmission,” U.S. Patent No. 2,262,747, filed 18 September 1936, issued 18 November 1941, reissued 18 May 1943; and “Automatic Transmission,” U.S. Patent No. 2,237,297, filed 15 September 1937, issued 8 April 1941; Oscar H. 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Clarke (Cobham, England: Brooklands Books Ltd., ca. 1992), p. 78; Cadillac Motor Car Division, General Motors Corporation, “1966 Cadillac: New Elegance…New Excellence…New Excitement” [brochure], 1966; and “1968 Cadillac” [brochure], 1968; “Cadillac Series 60,” Car Life Vol. 11, No. 10 (November 1963), reprinted in Cadillac Automobiles 1960–1969, pp. 48–49; “Car Life Road Test: Buick LeSabre 400,” Car Life Vol. 12, No. 12 (January 1965), reprinted in Buick Muscle Portfolio 1963-1973, ed. R.M. 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Clarke (Cobham, England: Brooklands Books Ltd., ca. 1999), pp. 11–17; “Car Life Road Test: Oldsmobile Delta 88,” Car Life Vol. 13, No. 3 (April 1965), reprinted in ibid, pp. 33–37; Chevrolet Motor Division of General Motors Corporation, “Chevrolet Camaro” [brochure D-78776 R-1], 1969; “Chevy’s New Little Car Is Open for Business” [brochure 1102], ca. September 1970; “Discover all the facts and features about the beautiful full-size Chevrolet ’66” [brochure], 1965; “1940 Chevrolet: Special Deluxe, Master Deluxe, Master 85,” [brochure, ca. September 1939]; “1954 Chevrolet Advance-Design Trucks: For Loads of Value: [brochure 1,000 M], October 1953; “1971 Nova Coupe/Sedan/SS” [brochure 1144 R-1], January 1971; “’72 Nova. How to see less of your mechanic and more of America.” [brochure 1618], September 1971; “’74 Nova: Building a better way to see the U.S.A.” [brochure 2676], September 1973; “’74 Vega” [brochure 2677-Rev], January 1974; and “The Little Car That Does Everything Well” [Vega brochure 1619], September 1971; Allan Coats, “Rotary Power Transmission Mechanism,” U.S. Patent No. 1,760,480, filed 4 December 1925, issued 27 May 1930; and “Rotary Mechanism for Transmission of Power,” U.S. Patent No. 1,760,397, filed 18 November 1927, issued 27 May 1930; John Etheridge, “It’s White Tie & Tails for Chevy Caprice ‘396,’” Motor Trend Vol. 17, No. 6 (June 1965), pp. 48–53; Hermann Föttinger, “Flüssigkeitsgetriebe mit einem oder mehreren treibenden und einem oder mehreren getriebenen Turbinenräder zur Arbeitsübertragung zwischen benachbarten Wellen,” DRP Nr. 221422, filed 24 June 1905, issued 25 April 1910; Hermann Föttinger, assignor to Stettiner Maschinenbau AG ‘Vulcan,’ “Hydraulic Device for Transmitting Power,” U.S. Patent No. 1,199,359, filed 19 June 1906, issued 26 September 1916; “Transmission Device,” U.S. Patent No. 1,199,360, filed 26 January 1910, issued 26 September 1916; and “Transmission Device,” U.S. Patent No. 1,199,361, filed 26 January 1910, issued 26 September 1916; General Motors Corporation, “GM Powertrain: Past, Present, Future,” www.gm. com/experience/ technology/ gmpowertrain/ about/powertrain_history.jsp [now www.gmpowertrain. com], accessed 28 May 2010; “Golden Anniversary Packard Models,” The Motor 6 July 1949, reprinted in Packard Gold Portfolio 1946-1958, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1988), pp. 19–21; Tony Hogg, “Audi 5000 Turbo,” Road & Track Vol. 31, No. 9 (May 1980): pp. 62–64; Ari Holopainen, “Planetary Gears,” LUGNET News, 2005, www.lugnet. com/~3813/epicyclic, accessed 9 June 2017; Roger Huntington, “The Great Transmission Controversy: Coupling vs. Converter,” Car Life Vol. 10, No. 2 (March 1963), pp. 18–21, and “Turnpike Cruiser: Oldsmobile Designs a Long-Legged, Strong-Willed Gas Miser,” Car Life Vol. 14, No. 3 (April 1967), reprinted in Cutlass and 4-4-2 Muscle Portfolio 1964–1974, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1998), pp. 34–39; “Invicta Black,” Motorstown, n.d., www.motorstown. com/news/ 1091-invicta-black.html, accessed 15 October 2015; “Invicta Black Prince,” A to Z of Cars, Classic & Sports Car, 29 March 2011, www.classicandsportscar. com, accessed 15 October 2015; Invicta Car Development Co., “Invicta” [brochure, 1947]; Robert W. Irwin, “GM’s New 3-Speed Automatic Transmission,” Motor Trend Vol. 20, No. 9 (September 1968), pp. 44–45; Achim Leutz’s Hermann Föttinger website, hermann-foettinger.de, last updated 2014, accessed 24 September 2015; Patrick Lindemann, Markus Steinberger, Thorsten Krause, iTC, “Innovative Solutions for Torque Converters Pave the Way into the Future,” Schaeffler Symposium 2014 (Schweinfurt, Germany: Schaeffler Technologies AG & Co. KG), pp. 280–301; “Lost Marques: Invicta,” Unique Cars and Parts, www.uniquecarsandparts. com.au/ lost_marques_invicta.php, accessed 15 October 2015; Bob McVay, “Road Testing the Oldsmobile Jetstar 88,” Motor Trend Vol. 16, No. 4 (April 1964), reprinted in Oldsmobile Muscle Portfolio 1964–1971, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1999), pp. 5–10; Donald E. Meyer, “A Brief Outline of the First Century of GMC Truck History,” Generations of GM, GM Heritage Center, 8 March–14 April 2008, history.gmheritagecenter. com/ wiki/ index.php/ A_Brief_Outline_of_the_ First_Century_of_GMC_Truck_History, accessed 8 October 2015; Ralph Nader, Unsafe at Any Speed: The Designed-in Dangers of the American Automobile (New York: Grossman Publishers, 1965); Paul Niedermeyer, “Curbside Classic: GMC TDH-5105 Old Look Transit Bus – GM’s Greatest Hit #9, Despite Being the Agent of a GM Deadly Sin,” Curbside Classic, 3 April 2012, www.curbsideclassic. com/ curbside-classics-american/ curbside-classic-gmc-tdh-5105- old-look-transit-bus-gms-greatest-hit-9- despite-being-the-agent-of-a-gm-deadly-sin/, accessed 8 October 2015; Eric Nielssen, “Six Luxury Cars: a view from the Automotive Engineering Side,” Car and Driver Vol. 11, No. 1 (July 1965), 26–31, 62–65, 75; Oldsmobile Motor Division, General Motors Corporation, “1969 Oldsmobile: Salesmen’s Prices/Equipment, Colors and Trim/Specifications” [dealer literature] October 1968; “1970 Olds Escape Machine power teams” [brochure], 1970; and “The 1970 Escape Machines: Oldsmobile” [brochure], October 1969; Pontiac Motor Division of General Motors, “A device for shrinking time and distance: Pontiac GTO” [ad insert], Motor Trend Vol. 16, No. 4 (April 1964), nn; “Low-priced-car buyers rejoice! You’ve got a new choice. 1964 Wide-Track Pontiac Tempest.” [brochure, ca. October 1963]; “Nineteen Sixty Five Pontiac” [brochure, ca. October 1964]; “1972 Pontiac” [brochure DM 26627], August 1971; “Pontiac 1973” [brochure], August 1972; “Pontiac ’69” [brochure], October 1968; “Pontiac’s 70’s” [brochure DM 27229], September 1969; “Pure Pontiac” [brochure DM 29103], August 1970; Arthur Pound, The Turning Wheel: The Story of General Motors Through Twenty-Five Years 1908–1933 (Garden City, NY: Doubleday, Doran & Co., Inc., 1934); “Q&A: dangling definitions,” Motor Trend Vol. 20, No. 7 (July 1968), p. 102; Carlton R. Radcliffe, “Hydraulic Power Transmitting Apparatus,” U.S. Patent No. 1,576,996, filed 3 August 1920, issued 16 March 1926, and “Hydraulic Power Transmitting Apparatus for Motor Vehicles,” U.S. Patent No. 1,667,565, filed 24 April 1917, issued 24 April 1928; Pol Ravigneaux, “Speed Changing Device,” U.S. Patent No. 2,220,174, filed 22 August 1936, issued 5 November 1940; “Speed Changing Device,” U.S. Patent No. 2,194,954, filed 10 October 1937, issued 26 March 1940; “Gear Box With Free Wheeling Gear,” U.S. Patent No. 2,195,783, filed 29 December 1937, issued 2 April 1940; “Variable Speed Gear,” U.S. Patent No. 2,239,973, filed 7 July 1938, issued 29 April 1941; “Epicyclic Change-Speed Gear,” U.S. Patent No. 2,631,476, filed 22 July 1950, issued 17 March 1953; and “Epicyclic Change-Speed Gear,” U.S. Patent No. 2,761,333, filed 24 March 1952, issued 4 September 1956; Bill Sanders, “Now You Can Have It Too: Econoperforleration* (*Economy, Performance, Acceleration): Oldsmobile Has Added It for 1968,” Motor Trend Vol. 20, No. 7 (July 1968), pp. 94–97; Steve Simpson, “Corvair Among Coconuts,” Modern Motor September 1961, reprinted in Corvair Performance Portfolio 1959-1969, pp. 36–38, 48; R.P. Stevenson, “British Cars – Mechanical Marvels,” Popular Science Vol. 150, No. 6 (June 1947), pp. 158–162; Summit Racing, “Torque Converter Selection – Summit Racing Quick Flicks,” YouTube, 6 April 2012, https://www.youtu.be/j5JFMs8gdbs, accessed 23 September 2014; “The Hydraulic Torque Converter,” Rail Motor Society, n.d., www.railmotorsociety. org.au/ rm_trans_htc_page.htm, accessed 13 October 2015; “The Motor Road Test No. 16/56 (Continental): The Packard Clipper,” The Motor 27 June 1956, reprinted in Packard Gold Portfolio 1946-1958, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1988), pp. 142–145; Mark Theobald, “Yellow Coach,” Coachbuilt, 2004, www.coachbuilt. com, accessed 8 October 2015; Vincent Tocco, Jr., “Fluid Drive History,” American Blower, n.d., americanblowercorp. com, accessed 15 May 2010; Hans Tore Tangerud’s Autoblog website (www.lov2xlr8.no); U.S. War Department, Ordnance Maintenance: Ordnance Engine Model 975-C4 (Continental) (War Department Technical Manual 9-1725) (Washington, DC: U.S. Government Printing Office, 27 January 1944), and Ordinance Maintenance: Ford Tank Engines (Models GAA, GAF, and GAN) (War Department Technical Manual TM 9-1731B) (Washington, DC: U.S. Government Printing Office, June 1945); Vulcan Werke Hamburg, Stettiner Maschinenbau Actiengesellschaft, “Flüssigkeitsgetriebe zur Arbeitsübertragung zwischen benachbarten Wellen mittels treibender und getriebener Turbinenräder” DRP Nr. 238804, filed 24 June 1905, issued 30 September 1911; Ron Wakefield, “1970 Camaro & Firebird: Chevrolet & Pontiac versions of a new American GT, plus a facelifted Corvette for 1970,” Road & Track Vol. 21, No. 7 (March 1970), reprinted in Firebird and Trans-Am Muscle Portfolio 1967–1972, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1998), pp. 92–95; Alan Wenbourne, “Ravigneaux Planetary Transmission,” South East London Meccano Club, 2006, www.selmec. org.uk/ article_0001_ ravigneaux_planetary_transmission.aspx, accessed 11 October 2015; “Wilson, Charles E.,” Generations of GM History, GM Heritage Center, n.d., history.gmheritagecenter. com/ wiki/ index.php/ Wilson,_Charles_E., accessed 8 November 2015; and Frank J. Winchell, Jerry R. Mrlik, John E. Mahoney, Jack W. Qualman, Thomas R. Zimmer, and August H. Borman, assignors to General Motors Corporation, “Transmission and Control System,” U.S. Patent No. 3,321,056, filed 12 December 1963, issued 23 May 1967.

The typeface used in this article’s author-created tables, diagrams, and other graphics (and for the English-language captions we added to kamasko’s 2013 torque converter diagram in the sidebar on torque converters) is Liberation Sans, one of the Liberation Fonts (version 2.00.1 or later), which are copyright © 2012 Red Hat, Inc., used under the SIL Open Font License, Version 1.1. Liberation is a trademark of Red Hat, Inc. registered in U.S. Patent and Trademark Office and certain other jurisdictions. Red Hat is a trademark of Red Hat, Inc., registered in the United States and other countries.

Our inflation estimates came from the U.S. Bureau of Labor Statistics Inflation Calculator at data.bls.gov/cgi-bin/cpicalc.pl. Please note that inflation estimates are provided solely for readers’ general information; this is an automotive history, not a treatise on the historical value of money, and nothing in this article should be taken as financial advice of any kind!



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  1. Hey,how come you can yack all day long about this ones gearset setup,or that ones turbine combination,but no illustrations???
    Just because you can picture the entire mechanical world with words doesn’t mean the rest of humanity can.
    Pictures Please!!!!

    1. Um, no “Thank you for an awesome article and site?”

      There is an illustration of a Turboglide and it’s hardly fair to expect Aaron to write an great article about the development of the automatic AND delve into all the technical details. He does to a degree, but that’s not the overwhelming emphasis of the site, as far as I understand it.

      How about Googling “Turboglide,” “Dynaflow” or “Powerglide?”

    2. (ETA May 30, 2016): Very late, but there are now diagrams! I’m not a technical illustrator by any stretch of the imagination, but you can at least get a sense of how these things were laid out.

      1. There’s a site here that has a diagram of an overhaul of the controlled coupling hydra-matic. I can really see why GM wanted to get way from this design. Although today’s ZF 8 and 9 speeds are probably worse, but then half of the world industry is sharing the development costs for these.

        1. …And yet, they were damn near indestructible. We had a ’58 Pontiac that took a lot of punishment in the snow, yet worked without any issues, other than a small oil leak, until I had to sell it in late 1964.If I remember correctly, it was cast iron and weighed around 225 lbs.

          1. The ’58 edition weighed about 240 lb. GM was able to trim about 10-11 lb for 1960 by slimming down the case a bit.

          2. I had a friend that bought a 58 Pontiac in about 1968. With five people in it, I being one, saw that thing do over 90 MPH in a true quarter mile from standing start (on the speedometer) and he offered one guy with a 63 Impala 327-300 to race for titles one night and the guy with the Chevy declined. That old 4 Speed hydro would flat get with the program. It had a big Rochester 2 barrel on it, but to this day I don’t believe that thing was an old 370 CI. It had a lopey cam and idled about 900 or 1,000 RPM. He’d hold the brake down and rev the engine slipping his foot of the brake and lurching forward. That thing was doing 50 MPH in a flash. My dad had a 62 Catalina with that stupid Roto Hydro in it and it woudn’t even get on the bus with that old 58. Looking back on all that it pisses me off now. Pontiac had a known entity and they cheated people by putting that dud transmission in. Most people never knew the difference but gear heads did. I think many people bought Pontiacs thinking they were getting that good ol’ hydramatic and they got instead a dog.

    3. If you read up about these transmissions, you would know exactly what he’s talking about. Don’t blame him SMH.

  2. In the photo of the Hydra-Matic shift quadrant in the ’50 Olds 88, is that an aftermarket turn signal unit? If so, it’s a reminder of how times have changed! I understand that at that time, a heater was an option on many cars.

    1. I believe turn signals were standard on Oldsmobiles by 1949, at least on DeLuxe models. I’d need to find somebody with an Olds dealer book from that period to know for sure, but my information suggests they were standard fit.

      Pretty much everything [i]else[/i] was at least technically optional at that point, including oil filters, wheel covers, hood ornaments, windshield washers, and (at least until after the war) reversing lamps. Heaters didn’t become standard even on Cadillacs until almost the mid-fifties, and they weren’t standard on cheap cars for another decade after that. Very few cars were built without a lot of these items, but they weren’t included in the list price for many years.

    2. I have a 62 Buick special with a v6,, what other transmission can I replace the 2 speed turbine dual path with? If you know please let me know, thanks

      1. None that I’m aware of. The 61-63 V8/V6 had their own unique bellhousing flange

      2. Speed Gems makes an adapter kit for the buicks

  3. At least they did not charge extra for chrome after the war.

    I remember seeing a ’50s car ad that mentioned the [i]reverse[/i] gear was an optional extra. On the other hand many cars (particularly British) came with leather seats only because it was cheaper than vinal.

    1. I don’t know of any cars that late that didn’t come with a reverse [i]gear[/i], although reversing [i]lamps[/i] were still extra on many inexpensive cars at that point. Turn signals, as well.

    2. Just as well they didn’t charge extra for chrome.
      The ’58 Buicks & Olds would have cost a small country to buy.

      Back on topic, thank you once again for an
      entertaining read.


      1. Well, in essence, they did charge extra for the chrome, though fortunately not by the pound. On most cars of that era the amount of brightwork was tied to the trim level, and naturally the higher the trim level, the higher the price. Beyond that, there were often extra-cost dress-up packages (either factory- or dealer-installed) that primarily consisted of additional chrome trim. Such things didn’t really disappear from American options lists until the rise of Japanese-style tiered equipment packaging quite a few years later.

    3. Ahh! Those were the days! Everything from a Roller (that’s Rolls Royce to you Yanks) to a Moggy (Morris Minor) with a leather interior. I remember the smell well as a small child in the early ‘sixties.

      Unfortunately British manufacturers did make the switch to vinyl during that decade for economy reasons and every non-luxury car came with a ghastly black vinyl interior that was composed of shiny paper-thin crap. On hot days (mercifully few and far between in the UK), first degree burns to your back and ass were the minimum you could expect. No wonder parts counters did a roaring trade in textile seat covers — they may have been ugly, especially the furry ones, but sure beat the OEM’s one and only offering of black vinyl by the acre.

      I owned a 1966 Pontiac Bonneville 4-door for a short while in 1979-80 (I sold the engine and transmission to a local drag racer and scrapped the body because it was too rusty to repair). It was white with a turquoise interior (even the steering wheel was see-through turquoise perspex). The upholstery was Morrokide and that was a revelation to me. It just shouted quality and put into stark perspective just how short-changed we Europeans were when it came to cars, forced to pay over the odds for inferior rubbish. The only way to go lower was to buy something from the Soviet Block — not that a Lada or a Yugo could possibly be worse than a Hillman Avenger (Plymouth Cricket in the US). [Aside: Thanks a bunch Chrysler. You took over the Rootes Group, at the time manufacturers of the Sunbeam Tiger, and turned them to manufacturing the most embarrassing pile of dross in automotive history. Shite is shite regardless of whether you brand it as Hillman or Chrysler or Talbot, as happened to the Avenger over its lifespan.]

      Did things get better in the ’70s and ’80s? Not unless you consider flimsy Dralon “better”. As I recall, you purchased a car new paying extra for the “luxury” option and well before it got to five years old the upholstery was torn and stained and looked like a pigsty. I still get nostalgic for that old Pontiac — The body may have been a rust bucket but the interior was palatial.

  4. Thanks for a great website and particularly for the GM transmissions articles. Every article I’ve read has been complete, accurate, and very interesting.

  5. Thank you for the automatic transmission article(s) on GM. Finally, someone has accurately chronicled the myriad development story for us.
    Your site is a valuable and entertaining resource – keep up the great work!

  6. This brought back some memories – I remember when I first got my license driving my Dad’s ’65 Olds F-85 with Jetaway and those 1-2 shifts at about 70mph if you held your foot in it. I have a question – I have an childhood memory of an early 50’s vehicle ( think it was a Chevy ) with a “Torque-Glide” logo on the trunk lid instead of “Power-Glide”, but that can’t be right, can it?

    1. Chrysler had a number of semi-automatics in that period with a variety of bizarre names: Gyro-Torque, Fluid Torque Drive, Fluid-matic, Fluid-Drive, and Plymouth’s Hy-Drive. Maybe it was one of those?

    2. Actually, from 1965 up, the F-85, Buick Skylark, and Pontiac Tempest all utilized the newly available Turbo-Hydramatic 300, which in essence was the same thing as a Powerglide, but with non-interchangeable parts. Early versions had variable pitch and a rear pump. It was with the advent of these new automatics that the shift indicators from that time forward would read P R N D L.

      1. The latter point is correct, but the rest is not. As the text explains, the two-speed transmission used on 1964-on B-O-P A-bodies is not Powerglide, although they’re similar in many respects. Although the two-speed (which Buick called Super Turbine 300) was manufactured by Hydra-Matic Division, it was not called Hydra-Matic. (I know the source you’re looking at, and it’s incorrect.) The three-speed Turbo Hydra-Matic became optional in 1967 with the big engines only and was later supplemented by the medium-duty TH350. The two-speed remained available on low-end models into the early seventies.

        1. You are wrong the turbo 400 was built by the Buick division of GM in1964 and all divisions but Cheyenne used them in full size cars. I have a GM delve that is 3 inches thick telling how to rebuild every automatic transmission they used from 1956 to 1964 with service bullion so from Buick staring in
          1964 I used for 45 years in the transmission business

          1. At least some early TH400s and later TH350s were indeed built by Buick rather than Hydra-Matic Division, that’s true. (My assumption is that it was in part a retooling issue, since Hydra-Matic was still building substantial numbers of other designs, including Roto Hydra-Matic and limited numbers of the four-speed dual-coupling unit.) And some non-Buick users did indeed switch to TH400 for some models in 1964, although not all and not as widely as in 1965. (I assume by “Cheyenne” you mean “Chevrolet,” which first offered TH400 on B-body cars with the Turbo-Jet big blocks in mid-1965.)

            I’m familiar with the type of service manual you’re describing; I may even have referred to the same one you have. While manuals like that are handy from a technical standpoint, they aren’t ideal historical sources, which of course isn’t their function. Their technical information may be more or less correct at the time it was originally written (although it’s not altogether uncommon to find errors in that as well), but manuals like that often don’t do a great job of reflecting running production changes and the intricacies of what was offered on what model/in what combination and when are beyond their scope.

  7. I have a 62 Buick,Skylark,with the dualpath Tranny.the trans is in direct drive,only goes foward,no neautral,park orreverce,is thier a fix for this.

    1. Can some one HELP.
      I have a 1962 Olds Cutlass F 85, Auto Hydro Matic floor shift.
      I had the transmission rebuilt 3 times already.
      and the problem is that when the car warms to operating temp
      it starts to jerk and gos into neutral. it clears once i accelerate.
      RPMs Are normal. trans just dosnt stay in low gear when moving at 10mpg or at a stop. Thanks- Robbe California

      1. @Robert: I’m afraid I’m not at all qualified to offer repair or troubleshooting advice — sorry!

  8. this article was great. It answered my question as to why the 52 Super I just inherited doesn’t shift….that would be because it isn’t made to shift automatically….I read a blog online saying
    1952 Buick – the slowest car I ever loved….so true!

  9. Are the dyno-flow and power glides enter change able? With other motor?

    1. I’m not able to provide any kind of modification or repair advice, but there’s an old saying to the effect that you can make anything fit if you have a big enough hammer. I honestly don’t know how much trouble would be involved in interchanging them, but since they were never designed to be used behind the same engines or in the same cars, I imagine it would take some work.

      At one time, Buick Nailhead engines were popular with drag racers, so if you were asking this question in, say, 1964, there might have been aftermarket kits to mate an older Buick V-8 with a beefed-up Powerglide. (Some drag racers used Powerglide because it consumed relatively little power and they didn’t need a lower first gear.) Today, I suspect you’d have more luck finding some way to put in a Turbo Hydra-Matic. I’ve never looked, though.

      This is a question that would probably be best put to a performance transmission manufacturer or a shop that specializes in parts for older transmissions.

    2. No the dynaflow and the powerglide are not interchangeable. the dynaflow is about three times heavier and will not fit up to any engine that was made for the powerglide. The powerglide came in two models first being the cast iron model that was used through 1954 then the aluminum powerglide after that. both very good transmission, and easily rebuildable.

      1. The earliest Powerglide is very similar to the early Dynaflow, although I doubt they’re easily interchangeable. As the revised text explains, Powerglide had several phases: the early dual-impeller variety, used through 1952; the later iron-case version with a three-element converter, used, with various evolutionary changes, from 1953 to 1962–1963; and the late aluminum-case version. The aluminum Powerglide (for RWD cars — all Corvair Powerglide units had an aluminum case) was introduced for some models in 1962 and for others in 1963.

        1. Back in the day my buddy had a 1950 Chev with the “no shift” Powerglide, it felt just like the DynaFlow but much slower, so slow that my Salsbury motor scooter with a belt CVT drive would beat him off the line for about a block. A later 2-speed PG made the car driveable.

          1. Not surprising — the Chevrolet six had something like 90 net horsepower on Powerglide cars, and with early Powerglide transmissions, it was like starting in second gear while also running something substantial with a power takeoff belt!

          2. hey was always wondering if my buddys 51 chevy pg was supposed to start out in 1st gear. he seemed to have to manually shift it into low. But, due to its constant state of malfunction, due to the way it was hot rodded,I never was sure.

          3. If it was the original transmission, the answer was “no”: selecting Drive on a ’51 Powerglide would engage the high clutch and you’d start in direct drive. However, if at some point your friend replaced the original transmission with a Powerglide from a ’53 or later Chevrolet, then it was supposed to start in 1st. (Whether it did or not is another matter, of course!)

  10. chevy had 2 auto transmissions in 61and62 1 was a turbo glide the other was –glide that changed by fluid. there was no gears in the trans. on the gear selector was P R D G G was for grade as going up a hill. what was the name of that trans?

    1. The two transmissions were Powerglide and Turboglide. Powerglide was the familiar two-speed-plus-torque-converter Chevrolet automatic, while the transmission you’re thinking of was Turboglide, which is described in the text.

      The G position was for Grade Retarder. It was intended not for climbing hills, but for descending them; it was supposed to mimic the effect of engine braking, of which the Turboglide otherwise didn’t allow very much. The Grade Retarder was not useful for acceleration or hill climbing, although some people had problems because they assumed it worked like the Low position on Powerglide, which was definitely not the case!

    2. I understand chevy had a semiautomatic pg available at least in chevy 11 153 cubic inch 4 cylinder cars

      1. Yup — it’s mentioned briefly in the text. It was called Torque-Drive, offered on the Chevy II, Nova, Camaro, and (briefly) Vega. It was essentially an aluminum Powerglide with a much simplified valve body and no vacuum modulator, governor, throttle valve, or kickdown switch.

  11. Re read this as a refresher on the development of the automatic. Thank you again. Your site is an invaluable resource and I cannot thank you enough for doing what you do.

  12. Thank you for your clear and concise explanation of Dynaflow, and how it differs from the other two GM automatics. As we were a “Buick family,” the innate superiority of Dynaflow was never a question; it was an article of faith. I remember the feelings of incredulity and betrayal I felt when I was told for the first time that Dynaflow was “Just Powerglide with a different name,” and that Hydramatic was obviously better, because Olds and Cadillac used it. You have restored my faith in Dynaflow.

  13. We have recently inherited a 53 Roadmaster. I think it is an early model serial #26854377 because the 322 nailhead has a weighted pully instead of a rubber loaded harmonic balancer. The Dynaflow is now in the transmission shop and we are finding puzzles. According to the shop manuals the 53 should be the new twin turbine with only 1 pump and one stator. This trans has the words “twin turbine” cast into the bellhousing. But inside it has 2 pumps and 2 stators. Do we have a transitional factory job or a trans shop hybrid? Was the change made to save money (fewer parts) or to improve performance? Will our new Roady rise and fly?

  14. Fascinating info.

  15. Just wanted to say this is a great article. I started out looking to find the difference between the hydra-matic dual range and the strato-flight and wound up learning a lot more.

  16. The article refers to the Hydramatic’s jerkiness. Actually, many Hydramatics were so smooth that you could not even feel the shift; you could just hear the drop in engine speed. I remember in 1959 riding in a 1949 Lincoln with Hydramatic; it accelerated quickly and so smoothly that I could not feel the shifts. The same was true with some other cars with Hydramatic in which I rode, including a 1950 Pontiac, and those were all before GM introduced the Hydramatic with the second (controlled) fluid clutch in 1956. On the other hand, I rode in a 1953 Cadillac with had very firm shifts.

    The downshift resulting from flooring the accelerator were another matter; they were always accompanied by a mechanical clunk.

    1. The issue with the original Hydra-Matic was that because its shifts were mechanically complex (particularly between second and third, which was the most complicated sequence), its smoothness depended a great deal on how well the bands were adjusted, the condition of the transmission fluid, and other maintenance- and condition-related factors. If everything was perfectly adjusted, it would be quite acceptably smooth (particularly by the fifties, by which time GM had made a lot of minor refinements). If not, it would throw off the shift timing just enough to make the shift jerky, albeit not necessarily enough to really impair the transmission’s function. I suspect a lot of owners who complained to their dealers or mechanics were told, “Ehh, they all do that.”

      Even some of the engineers who originally designed the Hydra-Matic thought it was too complicated for its own good, which is why they subsequently got into the torque converter automatics, which didn’t shift at all. The original Dynaflow was very much the antithesis of the Hydra-Matic in a lot of these respects.

    2. My experience with Hydromatic cars was that they were fairly smooth in shifting. PowerGlide cars had a very pronounced jerk when shifting. When my city purchased GM buses in the sixties, the Hydromatic was very rough when shifting with an easily heard lowering in engine sound as speed increased.

      1. The difficulty with making blanket statements in this area is that each of these transmissions was around for a long time in several quite distinct versions, not all of which felt or acted the same.

        As the text explains, early Powerglide cars did not provide any automatic shifting in Drive, relying on torque converter multiplication exclusively. Powerglide was revised in 1953 to start in first and shift automatically to second. So, early Powerglides (or Dynaflow) were smoother than even a well-adjusted early Hydra-Matic, albeit not especially quick or efficient. After that, there were early (iron-case) and later (aluminum-case) Powerglide transmissions, tuned in different ways for different engines.

        Similarly, the early (1940 to 1955) and late (1956-1964 dual-coupling) Hydra-Matics were significantly different mechanically — albeit still related — and felt quite different.

        So, while it may sound pedantic, it’s important to qualify statements like, “X was smoother/rougher than Y.”

      2. Those GM buses had a 1 speed automatic Allison transmission. Great roaring noises as the variable torque converter changed pitch and allowed the bus to gradually accelerate to 25 mph, then an almighty clonk as the torque converter was locked-up with a mmm-uhh-mmm vibration that gradually settled down as the engine bounced up and down on its mounts. Crude or what! Engine note and speed decreased at point of lockup.

        I blame those buses, their braying, outlandishly noisy two-stroke GM diesels and the pathetic transmission for ruining the quiet of our city at night when introduced. Went to London for grad work in 1969, and it was obvious that a AEC 4 stroke diesel packing all of 120 hp and four speed preselector gearbox not only got a double-decker bus going from stop much quicker than a GM bus, it was at least 10 times quieter doing it.

        Speaking from my point-of-view as a mechanical engineer. In those days as a student I had to ride buses and had a keen interest as to why the GM was so unrefined and the engine so noisy. No domestic competition would be my guess.

        1. Noisy or not, I loved those old roaring GM buses, when in “hydraulic drive” mode. That mode would seem to be not very fuel-efficient; a 4-speed pre-selector as you mention, should indeed have been more fuel-efficient (as well as quicker, as you mention). I have read that a later version of this Allison transmission arrangement actually had a second gear, making for a true two-speed, plus lockup in high. I cannot confirm that, though.

  17. I’ve heard a story about the Hydra-Matic, as follows:

    Supposedly Rolls-Royce acquired a Hydra-Matic for evaluation. They liked it but thought one particular part had too rough a finish. When they fabricated a smoother-finished version of the part and incorporated it into the reassembled Hydra-Matic, the transmission didn’t work. True, or urban legend?

    1. I’ve heard that story in regard to the Turbo Hydramatic (not the original), which Rolls-Royce also built. The way I’ve heard it is more that they tightened up the tolerances, which didn’t necessarily work out well. I don’t know if it’s true or not, but it’s not implausible. There’s an analogy to be made with pistols, where getting everything “tuned” to tight tolerances improves accuracy, but makes the action less tolerant of dirt or debris. (This is why police and military sidearms are not built like target pistols.)

      1. I am reasonably certain that while Rolls Royce licensed & built in England the original HydraMatic, it imported the Turbo HydraMatic 400 from GM in the states.

        1. You’re correct; my previous comment was based on a point I was only half-remembering. They did import them, but asked for higher-than-standard tolerances.

      2. as in ak 47 s they are unstoppable but not very accurate

        1. Well, the essential lesson of automatic transmissions until fairly recently was “just because something or someone does something for you doesn’t mean they do it well.”

  18. Thank you for this very complete summary. I have been curious about these transmissions for quite some time, and this is quite helpful. Your research is impressive, as is the writing.

  19. The main problem with reliability of the Slim Jim was the weakness of the front oil pump cover; they cracked. An improved pump with webbing on the cover was designed to replace failed units. RHM 375 Model 10’s made at Willow Run ceased in 1962. The THM 350 signalled the beginning of a long slide toward mediocrity by GM.

    1. I have to wonder if the Roto Hydra-Matic’s various weaknesses, including the propensity for leaks and the issue you describe, were exacerbated by the very high operating pressures. As mentioned, the RHM’s operating pressures were substantially higher than the earlier dual-coupling HM’s, which is a lot of added stress to put on what was still fundamentally an adaptation of the earlier transmission.

      I’m not sure how your last statement follows. The THM350, which didn’t arrive until five years or so after the RHM expired, was effectively a replacement for the Powerglide and Super Turbine two-speed automatics, and in that sense were an improvement in most respects. (There have been some harsh criticisms of the later TH200, but that’s a different story.) Since most rivals had long since offered three-speed automatics for most engines, the TH350 was also arguably overdue. It wasn’t quite as heavy-duty as the TH400, but it wasn’t designed to be, trading off some torque capacity for lighter internals and lower power consumption.

    2. I would disagree; I had very good luck with the THM350 in my 1973 Nova 350; it reached 185,000 miles, with no issues other than some fluid leakage. Shifting was still quick and firm. I have not heard of a lot of issues with this tranny.

      1. The lighter TH200 has gotten a pretty bad rep, but I’ve never heard anything particularly bad about the TH350.

        1. thm 350 s are excellent, but the best ive seen are 4l60e s 1 of which ive driven 362,000 miles in my 1994 chevy astro with NO hickups

          1. The TH350, TH400, and their immediate descendants were quite good, at least with a V-8 or a big six. That was really GM’s sweet spot in terms of powertrain refinement: a transmission well-matched to an engine with lots of torque and modest revs, giving a sense of effortless response. Unfortunately, it didn’t translate so well to smaller engines with narrower power bands, and the light-duty TH200 gave away too much beef in the interests of lightness.

  20. I had a 1949 buick super with dynaflow, four door. It averaged about 8 mpg. It took everything I earned as a super market clerk to keep the transmission running, most repairs were $300 to $400.

  21. Studebaker developed their own automatic and introduced it in 1950. Ford wanted to license it, but Studebaker turned them down. Studebaker started using the Borg Warner later, when manufacturing costs of theirs got too expensive. If I recall, a European manufacturer bought the tooling, and used it in their own cars?

    1. I believe the Studebaker automatic became the basis of the Borg-Warner DG, which was used on a number of British and European cars of the ’50s.

    2. I see this is an old posting, but I thought some clarity would be helpful.
      Studebaker did not develop its own automatic transmission. The automatic Studebaker announced and offered in 1950 was engineered for Studebaker by the Detroit Gear division of Borg Warner, hend the model designation DG-200. Around the same time, Ford had contracted with Warner Gear, another division of Borg Warner, to engineer, manufacture and license Ford to manufacture several models of automatic transmissions. Both divisions developed 3-speed planetary geared units and employed a torque converter coupling. The DG design was the more advanced of the two but was also more expensive to produce. When Ford became aware of this competing design, they inquired about switching… for several legal, commercial, and logistical reasons that was not possible. In the mid 50s, Jaguar acquired rights to the DG design and manufacture. The Warner Gear design became the core of Ford’s AT portfolio in the 50s, evolved into the Ford FMX trans of the 60s and 70s, and was the starting point for Ford’s first 4-speed overdrive automatic, AOD (later AOD-E and 4R70W) in the 80s and 90s.

      1. The Borg-Warner DG series is discussed in greater detail in the article about lockup torque converters and split-torque transmissions, since the original iteration had a fully mechanical lockup in direct drive. That article also talks about the Ford AOD.

  22. Thanks so much for the great overview.

  23. Great job like the article ? would you have any info on the olds roto hydromatic . I have a 62 any m having some small issues
    Thank you Mike

    1. I’m not able to help with any kind of troubleshooting or repairs, sorry!

  24. Thanks again for a great resource. I find myself returning to it for a periodic refresher when a relevant vehicle appears. (Today’s is a 1961 Buick.)

    1. Thanks, Ed! I’m actually in the process of updating this article as I recently did with the Hydra-Matic story, to fix some minor factual glitches, clarify the technical details (which is a major project, let me tell you), and add some new info.

  25. Try this… as good an explanation of your problem as I’ve ever understood: https://www.youtube.com/watch?v=rLDgQg6bq7o

    1. He talks about your differential girdle spring at starting at ~1:10. It’s supposed to be hooked onto the upend of the gramys.

  26. All this effort and expense just so drivers don’t have to clutch and shift? Turns out major beneficiaries of automatic transmissions are texters. Who cause many of the accidents on the road now!

    1. Given the timeframes of the respective inventions, I would said that definitely constitutes an unanticipated side benefit…

  27. I believe that the first automotive use of planetary gears was in the Model T. As I recall, you would press down on one pedal to get the car going (1st gear), then move the gear lever and let the pedal up for high gear. It wouldn’t have taken much to use a servo to make these motions and a combination speed and throttle position sensor to determine when to make them. That could have been an early two speed automatic. The original Hydra-Matic is just a more sophisticated, four-speed version with a fluid coupling, isn’t it?

    1. That is how a Model T transmission worked, although it was not the first automotive application for epicyclic transmissions; a number of other cars, including Cadillac, used planetary gears before the Model T was introduced. (I’m always leery of pointing to anything as The First just because it’s often wrong unless you add a lot of qualifiers — a surprising number of innovations were tried or at least considered decades earlier than you might expect, even if manufacturing or machining technology wasn’t up to making it work.)

      It is certainly true that Henry Ford remained a stubborn proponent of planetary gears, which he continued developing for tractor use even after he was persuaded to allow a conventional gearbox in the Model A. (One of the engineers who worked closely with him in that, Howard Simpson, went on to design and patent the “Simpson gearset,” licensed by many other manufacturers including GM and Mercedes-Benz.) However, the Model T certainly wasn’t automatic and it would have needed some other control mechanism to execute shifts without driver intervention.

      As Part 1 of the Hydra-Matic article touches on, there were various efforts to do that, many of which used planetary gears because the brakes and clutches could be controlled hydraulically, electromagnetically, or by some other remote mechanism. So, there is a parallel, but it only goes so far and there were a lot of steps in between.

      1. Once more, a comment years after the initial conversation was posted.
        This year, after 50 years of driving, I learned to drive a Ford Model T which I now do three days a week at the Henry Ford / Greenfield Village in Dearborn, MI. Your comment that Cadillac employed a planetary transmission design before Ford deserves a footnote… Henry Ford’s second unsuccessful attempt to start a car company was recapitalized by Henry Leyland in 1902 as the Cadillac Motor Car company. Cadillac’s planetary trans reflects the preference of its first chief engineer, Henry Ford!

        The Model T’s transmission is driver operated, however the shift from low to high (direct) gear does not involve any selector lever. Release of the left pedal enables a spring operated clutch to engage a 1:1 high gear. While this appears primative today, the T’s transmission was MUCH more user friendly than early sliding gear manual gearboxes that required double-clutching, and were prone to accidental damage by unskilled drivers. The advent of constant mesh gearing and synchronizers made the familiar manual trans an acceotable “standard” of the industry.

        Development of torque converters and hydraulic control systems gave planetary gearing a new lease on life that continues to this day. Subsequent development of electronic controls and computer genenerated geartrain combinations enable ratios spreads and counts unimagined decades ago.

        Ironically, after a day driving a manual planetary equipped 100 year old “T” I drive home in a car with the only 6-speed parallel-shaft-geared automatic I know of… a 10 year old V6 Honda. Interesting that it was recently replaced by the world’s first FWD 10-speed PLANETARY automatic!

  28. Minor glitches: The TH 400 was used by Buick AND CADILLAC in 1964. The variable-pitch stator was not used on the TH 400 in ’64, but was available on some Olds, Buick, and I guess Cadillac vehicles from ’65–’67. Ironically, the variable-stator design was used on the “big” engines in the more-expensive cars; the small-blocks and six-poppers needed the torque boost more than the big-blocks.

    For the record, the ’64 TH 400 uses a substantially-different valve body and in-case fluid channels than the ’65-newer TH 400. The valve body of the front-wheel-drive version (the TH 425) uses the ’64-style system. Therefore, a “shift kit” for a 65-newer TH 400 won’t fit a ’64 TH 400 or the TH 425, but a shift kit for a TH 425 will work in a ’64 TH 400.

    The TH 350 was actually a joint development of Chevrolet and Buick engineers, both divisions looking for replacement of the two-speed transmissions they were currently using (Powerglide and Super-Turbine 300) with the resulting “350” produced by the Hydra-Matic Division.

    1. Thanks for the notes — I’m aware of both of the errors you note and they’ll be fixed in the extensive revamp of this article on which I’m currently working. (See the most recent post for details.) I won’t be getting into a detailed discussion of Turbo Hydra-Matic in the revised version, which is already monstrously long and has been eating my brain for months.

      TH400 wasn’t offered on all 1964 Cadillacs, incidentally; it was initially available only on De Ville, Eldorado, and Fleetwood Series Sixty. I wasn’t aware that the TH425 used the original valve body pattern, though. (I know generally how the TH425 is laid out, but I can’t say I’ve ever looked at its hydraulic control layout.)

      1. Okay, the revision is now complete and those corrections are now reflected in the text.

  29. Great, great job Aaron! That was awesome, and I was glad to help

  30. I think I can appreciate how big an undertaking revising this article has been. Hats off to you Aaron, for possibly the best explanation of early GM automatics expressed in laymans terms.
    GM didn’t swallow its pride and licence the Simpson system and tried to develop practical cost effective alternatives in its various divisions until the ’60s. Seems a classic case of corporate wilful blindness until we remember hindsight is the only exact science.
    In 1966 “Motoring Which?” the UK’s equivalent to “Consumer Reports” published a test of three 1.5 liter automatic British sedans, a Ford, a Hillman, and a Vauxhall. Vauxhall is the UK subsidiary of GM. The Vauxhall had a GM two speed transmission, the others both used a Borg Warner 35 three speed. They noted that they all had slightly worse performance and fuel economy than their stick versions, but the Vauxhall also had a big gap in its performance between 35-50 mph just when it was most needed. It was likened to driving a stick four speed using only second and top gears. The article also mentioned “Consumer Reports” had harsh words for GM cars using two speed transmissions, I’m guessing Ford, Chrysler, and AMC had all switched to three speed transmissions by then?.

    1. By 1966, I think Ford’s two-speed Fordomatic may still have been available for the cheapest U.S. Falcon models — I would have to double-check, as it may have been dropped after 1965 — but otherwise the other U.S. automakers all had smaller three-speed units for their low-end cars by then. (The light-duty TorqueFlite was one of the big pluses of Chrysler’s compact Plymouth Valiant and Dodge Dart, in my view.)

      The general attitude of GM engineers in this era was that a two-speed torque converter automatic was a perfectly reasonable substitute for a three-speed manual transmission while being simpler, lighter, and cheaper than a three- or four-speed automatic. The latter was of course perfectly true and the former was at least a supportable position. I also suspect some of the transmission engineers were soured a bit by experience with the small three-speed Hydra-Matic, which was little better than a decent two-speed automatic. (The transition from the smaller three-speed unit in the 1961–1963 Y-body Oldsmobile F-85 to the two-speed Super Turbine 300/Jetaway in the 1964+ A-body equivalent was certainly no great loss and probably an improvement in some respects.) On the other hand, by the mid-sixties, very, very few Americans still bought three-speed manual transmissions and it was certainly clear that a good three-speed torque converter automatic was considerably better than the best two-speed. It was also a bigger deal for non-U.S. cars and the later U.S. ventures into the “subcompact” [sic] realm, since having 3 or more liters’ displacement to fall back on masks an assortment of deficiencies.

      I don’t think GM was willfully blind so much as having a fair bit of (understandable) inertia. As this article should hopefully make very clear, GM had invested an absolutely staggering amount of money in automatic transmission development and engineering, accumulating a towering stack of basic patents. The tooling alone was a king’s ransom — in the early fifties, Detroit Transmission built more Hydra-Matics each year than the entire contemporary British auto industry built cars, and that wasn’t even GM’s only automatic! So, a reluctance to completely reinvent the wheel or to unnecessarily license outside technology isn’t difficult to understand. (To be clear, what GM licensed from Simpson and Simpson’s estate was a specific arrangement of planetary gears, not a complete transmission. Part of the reason that arrangement ended up being so widely licensed was that Simpson, like Pol Ravigneaux a decade or so before, had patented many different variations that there was no getting around them.)

      1. I’d forgotten three speed manual transmissions were still commonplace in the USA in the timeframe we are discussing. A two speed automatic makes a lot more sense then.
        I wasn’t suggesting GM was willfully blind, but had missed a trick in not adopting the Wilson system (or at least parts of it).
        As you say, GM spent vast amounts developing their transmissions. I wonder how much it cost Chrysler Corp to licence and develop their transmissions, which I think were superior to any other automatic transmission available at the time.

        1. To be clear, what’s commonly called a “Simpson gearset” really just refers to any compound planetary unit sharing a single sun gear, just as a Ravigneaux gearset is a compound planetary unit sharing a planet carrier and at least one planet gear. There were actually multiple variations of each, most of which Howard Simpson and Pol Ravigneaux dutifully also patented. While each of those layouts has certain advantages, particularly as regards packaging and cost, the invention, as was, didn’t encompass how the gears were selected and chosen. In fact, while there were a bunch of automatic transmissions that used these gear layouts, including Chrysler’s TorqueFlite and GM’s Turbo Hydra-Matic, each was quite a bit different. So, the credit for the functional effectiveness of TorqueFlite or Turbo Hydra-Matic really goes to the Chrysler and GM engineers who developed them. I’ve never seen anything to suggest how much any of the companies paid to license Simpson’s gearset patents, although there were so many users that if there was any kind of per-transmission royalty, Simpson and his estate would have made out quite handsomely.

          Developing an automatic transmission was a very costly business in general, I have no doubt, but in Chrysler’s case, they developed fewer of them — the original PowerFlite two-speed torque converter automatic, the early iron case TorqueFlite, and then lighter aluminum TorqueFlite units with a variety of evolutionary changes — and used them across all the automotive models. GM, by contrast, had three distinct transmission families (Hydra-Matic, Dynaflow, and Powerglide) that each went through several generations and iterations, each notably different, but with a lot of what a software designer might call legacy features. (The outliers there were Turboglide and Flight Pitch Dynaflow, which were not “clean-sheet” designs in a conceptual sense, but shared little with Powerglide and earlier Dynaflow transmissions mechanically and later contributed various ideas and some components to subsequent versions.)

          The three-speed manual transmission occupied a very peculiar space in the American automotive firmament in the sixties and seventies, being simultaneously ubiquitous and rather uncommon. It was notionally standard on a great many cars into the late seventies, but you’d hardly ever see one. The real rationale for its existence, so far as I can tell, was to allow a greater retail markup on the automatic transmissions (or four-speed manual transmissions) most people actually bought. By this point, no one pretended that Cadillac or Imperial buyers would have a manual gearbox, even the carriage-trade versions, but the three-speed was still nominally standard equipment on some quite improbable big sedans.

          1. Was there anything that could have been described as “patent squatting” obstructing legitimate engineering advances in automatic transmission development?

          2. That’s really a loaded question, to be honest — and I say this as one with strong negative feelings about the modern proliferation of “patent trolls” and the abuse of IP law to try to block people from repairing or modifying their own cars.

            The purpose of patent law is to promote technological development by providing a legal incentive for inventors to publicize their inventions. The whole point is that it encourages others to find improvements or alternative methods; if a competitor can come up with a better solution that doesn’t infringe on the claims of the original patent, the idea is that the public ultimately benefits. Patents are intended to avoid the problem where inventors feel compelled to hide their discoveries for fear that their ideas will simply be poached by opportunistic rivals with greater resources, although in practice that ends up happening anyway, especially with independent inventors who don’t have the money for prolonged litigation against a major corporation with its own legal department. It’s not at all uncommon that an invention is created by someone who doesn’t have the resources to manufacture the invention, but hopes to interest some larger player in the merits of licensing and producing the design. The engineers of a big corporation may see independents like Oscar Banker as nuisances or squatters, especially if the independent is someone they don’t want to deal with or who wants more than they’re willing to pay, but that’s a really subjective judgment. Assigning some special degree of legitimacy to engineers with greater production capacity or distribution ability or whatever would encourage monopolies, which is something that is seldom in the public interest and would have a variety of ugly consequences.

            Now, there are certainly cases of patents that really shouldn’t have been granted — that are over-broad, that fail to take into account the prior art, or both — and there are areas I don’t think should be eligible for patent protection. (We would be better off, in my view, if the U.S. did not allow software patents or patents on living things.) But the fault there is in the patent office examiners rather than in the inventors per se. (There are inventors who are over-ambitious, but in principle, the examination process is supposed to be a check on that!)

  31. Great job Aaron, you’ve outdone yourself. I enjoy coming to this site to expand my knowledge. It’s a fantastic resource indeed. I also enjoy your clarifications on “Curb Side Classics” and can faithfully know that any input you offer will be well reasoned and researched. You offer a great service to like minded Auto Industry nuts.

  32. Wow! My brain has tech-overload.I’m going to have to re-read the article in sections to have any hope of absorbing all the new information. Fantastic job on the revision, Aaron, it was well worth the wait. Thanks for the monumental effort!

  33. Great article! One point of contention is some of the THM-400 transmissions fitted to Chevrolets did have the “switch-the-pitch” feature I remember working on a 67 Impala station wagon, with the 327″ engine and THM 400 which had the pitch angle switch on the throttle linkage. This was in the early 1970’s and this appeared to be an O.E. Installation on a stock automobile.

    1. Hmm. To be honest, I had thought until this afternoon that TH400 wasn’t offered with the 327 at all — a number of vintage car magazines complained about that, in fact — but I found one brochure that indicated the 327/THM combination was indeed optional on the ’67 Impala and Caprice. (It may have been a midyear or late introduction.) I’ve never seen any indication that the TH400 fitted to the big Turbo-Jet engines (396/427) had the variable-pitch stator, but it’s possible the ones used with the 327 did. If so, it was likely short-lived, as the switch-pitch stator was dropped for 1968. However, a 327 with switch-pitch THM actually sounds like a pretty nice combination. It would be much more flexible than Powerglide, that’s for sure!

      (I tried very hard not to get sucked into a more involved discussion of Turbo Hydra-Matic in this article for what I imagine will be obvious reasons, but I wanted to mention the variable-pitch stator because it was really one of the only Dynaflow/Twin Turbine/Turbine Drive features to survive into the later era.)

  34. I was under the impression that Chevrolet division never used the variable-pitch stator design, but regarding the 327/THM combo for big Chevrolets – it seems likely. Olds offered the THM 400 as an option on it’s small-block (330/350) powered 88 models for sure in ’67 & ’68, not positive about ’65-66. Both my ’67 Delmont 88 330 and my ’68 Delmont 88 350 came with THM400’s rather than the usual Jetaway 2-speed (ST300). The ’67 is a variable-pitch model, the ’68 is fixed. In normal operation, I don’t really see a pronounced performance advantage to the variable-pitch stator.

    1. The other divisions’ experience isn’t necessarily suggestive regarding TH400 availability. Buick, for example, offered it on the smaller-engine LeSabre (with the 300 cu. in. engine) as early as 1964, whereas the loosely comparable Oldsmobile Jetstar 88 was available only with the two-speed in ’64 and you could still get Jetaway on a base-engine Delta 88 until 1969. Chevrolet didn’t offer Turbo Hydra-Matic at all until mid-1965 and until 1967, it was only available on full-size cars with the 396 or 427. I think part of the rationale was that TH400 was bulkier and consumed more power than Powerglide (hence the later TH350), although the 327 obviously could have benefited from an extra gear.

      When Oldsmobile dropped the variable-pitch stator for 1968, they also gave both Jetaway and TH400 higher-ratio torque converters, so there really isn’t much difference in all-out performance. The point of the variable-pitch stator vanes was to keep the converter “tight” in gentle driving while still providing extra multiplication for fast starts or quick bursts of acceleration, even if you were over the maximum kickdown speed. With the kind used on Turbo Hydra-Matic and Jetaway/Super Turbine 300, it also limited creep on a closed throttle. (The old Buick and Turboglide stators variable couldn’t do that because the stator servo valve was triggered by throttle movement rather than electrically.) So, it was about flexibility more than anything else.

  35. Terrific article with this latest revision!

    The first car I can remember was a ’56 Oldsmobile and by the time I was 8 years old or so my dad had described to me how the “fill and flush” coupling worked in cushioning the shifts. Anytime we were driving I kept track of which was in use. Walking to school I would hum to myself as I walked, imitating the engine speed ramping up in each gear, pretending to be a car with Hydramatic.
    The Oldsmobile was replaced by a Buick LeSabre. We ended up buying the “400” version in order to avoid the two speed automatic. The “switch the pitch” stator was what got Dad’s attention in this car (even if its actual operation wasn’t very noticeable).
    Stuff like this is what motivated me to become a mechanical engineer.

    Thanks for all of your work. It brings back good memories.

    1. Thanks, Chris. I can see that the Controlled Coupling Hydra-Matic would be sort of a crash course in mechanical engineering, since it has a little of just about everything. Bands! Couplings! All kinds of clutches — disc, multi-disc, cone, and sprag! If it had a torque converter and a lockup clutch, it would be a veritable omnibus of early automatic transmission ideas. (If they’d used Walter Herndon’s lockup clutch concept, it wouldn’t have been a complete lockup in the sense of a modern torque converter; it would just have locked out the smaller coupling.)

      What I love — and GM accountants presumably did not love — about the second-generation Hydra-Matic is that it incorporated a bunch of changes that make its basic operation smoother and mechanically simpler, but each change then required a bunch of belts-and-braces stuff to make up for the minor drawbacks created by the simplification, such the need to still use separate overrun brakes so as to not end up freewheeling down every steep hill. It’s a useful reminder that just because something is cleverer doesn’t necessarily mean it’s better.

    2. And here I thought I was the only kid (early ’50s) to imitate a Hydramatic!

  36. Great information.
    Drawing on personal experiences from cars my friends and I owned when we were young men two speed automatics, mostly powerglides, were something we wanted to get rid of if we could afford it. I had a ’65 Pontiac Laurentian (283-2 speed) ’64 Chev Impala SS (283-2 speed) and a “68 Camaro ( 327-2 speed).
    I put a Turbo 350 in the Camaro later and it was a nice addition.
    I know the racecar guys like them but we had full size ’60’s sedans with 283’s and 235’s, not 800-2000 horsepower racecars.
    To this day ( I’m 60) I would rather have a manual than automatic transmision I think because of powerglides.
    In the late ’80’s I learned about Variable Pitch converters some Turbo 400s had, bought the pieces from Kenne-Bell, converted my ’80 GMC ( 350, later 454) heavy half and ’74 Olds (455) Delta 88 convertible over to them. I also added the 2.75 low first gear kit to the Olds also because I’m married to the 2.73 rear gears ( 9 3/8 ring gear diamter) so I’m looking for mutiplication wherever I can get it.
    With the warmer than stock cam (268 Comp Cams) It gives me way better traffic drivability than I had before, particularly when towing a trailer on holidays.
    According to a book I once had, it claimed the fixed pitch 400 converter stator angle is 24 degrees if my memory serves me correctly. I think the Variable Pitch swings between around 18-26 degrees. I have to get another book to be more accurate. I have a variable pitch stator and if you put it through its motions you can see how it would give different stall angles, all you have to do is compare it to boat or aircraft propellers.
    According to my information a fixed pitch 400 converter gives up to 2:00-1 multiplication and variable pitch goes up to 2.5:-1. That helps in a heavy car with tall rear gears.
    Over the years i’ve been to a few “burger stand or shopping mall car shows” and described the variable pitch converter system the guy has on his car and he generally has no idea what i’m talking about.
    Some years of Oldsmobiles (the ones I’m most familiar with) had a switch in the speedometer cable and was in high pitch until a certain speed and some had it in their throttle linkage.
    If one is not careful when they have their transmission rebuilt the variable pitch stuff is not put back in and fixed pitch stuff substituted.
    Transmission repairmen, if not familiar with it tend to think it’s an earlier fluid coupling and primitive garbage from the days before “real” transmissions were made. They”re usually pushing a modified TH700R4 which, in my humble opinion, is not designed for a big motor in a heavy vehicle.
    That being said, the decendant, the 4L60E, is doing just fine in my stock ’96 Impala SS and that 700 would have been a huge improvement in our old ’60’s cars.
    However, some know exactly what that VP is, and if the owner has no idea what he has and someone they know wants one, it’s gone.
    This happens to the factory low first gear kits that are in motorhomes and heavier trucks too.
    The variable pitch really shines when you run more cam or a turbocharger, in high stall they let the engine get above 2500 rpm before they stall and let the engine wind up, making more power.
    In high stall it’s too high to have all the time and in low stall it would leave you wanting more in stop and go traffic, particularly when towing something, but together a nice blend.
    They, along with 2.75 or 3.00:-1 low first gear and overdrive kits were the darlings of the motorhome crowd until heavy versions on the overdrive automatics came along. Those in the know had them, guess where some of them pieces came from. Not everyone in this world has scruples.
    GM made two sizes, the mid size “A bodies” had 10 inch and full size sedans had 12 1/2 inch.
    The big fixed and variable pitch torque converters were the same size and the stators interchanged but the varibable pitch ones were referred to as 12 1/2 inch and the fixed pitch ones as 13 inch.
    I was told the reason was that’s how GM differentiated between the two.
    Several things that I have read over the years described the phasing out of the variable pitch according to GM was it was “a feature that only engineering types seemed to understand plus some customers complained about the whirring noise they made”. And,” with the new large displacement engines coming out it is unecessary”.
    Why spend money on a feature something very few people understand?
    With an overdrive kit, and a variable pitch a TH400 becomes a 12 speed. Not a cheap proposition though.
    Thank you and enjoy.

    1. Thanks for your thoughts, Wayne. The pitch angles of the TH400 variable-pitch stator were 32° and 51°, at least as GM measured them. Fixed-pitch TH400 converters actually varied quite a bit in stall ratio depending on the application, from 2.00 to about 2.50:1 for street applications, whereas the standard variable-pitch units were 1.8/2.2. The switch-pitch stator didn’t necessarily mean greater maximum multiplication. As you note, the main advantage is that you have the higher stall speed when you need it and aren’t stuck with high-stall converter blues the rest of the time.

  37. A monumental amount of work involved in this revision. A labor of love really. Congratulations on unraveling the details in all these GM transmissions, and presenting the results so clearly.

    My further kudos in your even responses to comments where old wives’ tales and “my friend the transmission overhauler tells me you’re wrong” comments seem intent to belittle you. Haven’t seen anyone conclusively prove you incorrect, possibly because you know about 10 times more than they do, and I’m speaking as a retired mechanical engineer who’s had people who just don’t understand that they don’t understand try to sell me a line of magic dreamed up in their heads! It’s how myths and legends are born. AWD systems seem to be completely misunderstood by just about everyone but the engineers who designed them, for example. Especially that particular group of people known as Marketing and their adjunct advertising copywriters.

    If one goes back a bit further to the brief time interval between synchromesh and the first Hydramatic, my speculation for the real reason an automatic transmission was needed was because so little effort was ever applied to designing a half-decent shift linkage and low clutch effort. That’s why people hated driving those clunkers – they were awkward to say the least. Try a ’49 Pontiac three-on-the-tree. Blech.

    So when we youngsters got to drive Austins and euro Fords in the 1950s and heaven! the first Volvo 4 speed manual, the ease of use was outstanding compared to the US stuff. No longer was shifting a chore, it was fun, column or floor shifter. I mean Chev thought the Powerglide more important to introduce than replacing the oil dippers on their six cylinder engine and giving it proper full pressure lubrication, so designing an ergonomic manual shifter was obviously beyond them. Strange attitude to me other than dreams of golden showers of dollars raining upon them for presenting no-shift motoring at a premium.

    Even early to mid ’60s 4 speeders needed a manly-man to shift their obdurate levers. No snickety-snick there. The Corvair 4 speed was an outright laugh compared to the Volvo, but in those days the scorn heaped on “tiny” foreign cars meant Americans in general somehow believed that foreign ideas came from the dark ages and were no good. Same in Canada where I live and lived through endless Ford versus Chev arguments in both high school and college where nothing was ever settled.

    All that personal reflection aside, I must reiterate you’ve put forward a first class effort here and deserve much praise. It’ll probably become a reference work.

    1. Thanks, Bill. It’s certainly true that the shift linkages of domestic cars had a lot to do with the preference for automatic. Three-on-the-tree is mildly amusing to the modern driver as a novelty, but a regular dose of it — particularly with a non-synchronized (or indifferently synchronized) transmission — would be a strong argument for Powerglide. As for the sixties four-speeds, I assume part of the problem was that they were intended primarily for racing homologation or drag racing, rather than something your average consumer might buy (a thesis strongly supported by the fact that a four-speed typically cost as much as or more than automatic).

      On the other hand, there’s a strong argument to be made that automatic transmission is a natural evolutionary development of automotive technology, just like, say, automatic spark advance (another development that was still fairly recent when Hydra-Matic first came on the scene). Even with excellent modern five- and six-speed gearboxes, effective synchros, and low-effort clutches, it’s hard for me to argue that manual shifting is a lot of work of a kind many drivers are perfectly happy not bothering with. The strongest arguments for it, aside from it being a moderately entertaining diversion, are that it makes the most out of smaller engines without a lot of torque and that it spares you the exasperation of delegating a complicated chore to an automated subordinate of often questionable judgment, both of which have become progressively weaker as engine and transmission technology improve. (I say this, mind, as someone who has never owned a car with automatic transmission and who had to learn to drive on a manual gearbox.) So, I can understand, though not really defend, why Detroit engineers treated manual transmissions as a legacy system only being (grudgingly) retained for buyers too cheap to pony up the extra $200-ish.

      (What’s harder to understand, frankly, is that GM let O.K. Kelley and his guys keep churning out different automatic transmission designs of several very different flavors for an astonishingly long time before they finally decided to consolidate around yet another, mostly unrelated design!)

    (1961 to 1963). I cannot find any information on this. I need to know if it can be done and if so what ports on the transmission do I use?

    1. I’m afraid I’m not qualified to advise you on modifying your transmission, especially not in the way you describe. Sorry!

  39. One problem with the original Hydramatic transmissions made prior to the Dual-range Hydramatics, which came out in 1952, is how drivers were able to get engine braking when there was no way to downshift a hydramatic car from fourth to third gears. So if you going down a long, steep hill at about 55 mph there was no way to get engine braking with the original Hydramatic. My father had a 1954 Buick with Dynaflow and you could manually downshift from High to Low at 55 mph or slower and get engine braking going down a hill. The Dynaflow was a super smooth transmission and with torque converter technology I think was a very underrated transmission compared to the Hydramatic. It was also very reliable and we had the 54 Buick for 13 years with no problems with the Dynaflow.

    1. Dynaflow versus Hydra-Matic was really a fascinating philosophical debate, especially when you consider that they were conceived by many of the same people. Hydra-Matic was efficient and, at least in Dual-Range and later forms, versatile, but it was also complicated and fussy. Dynaflow was seamless, but, especially with the earlier ones, you really had to use Low a lot to get the best out of it. It’s very interesting to me that GM spent so much developing these two wildly different transmission concepts.

  40. hello i am changing cooler on my dads 58 olds holiday hydra-matic jetaway “slimjim” and would like to know what size threads are in cooler boss on tranny

    1. Derik,

      As I keep saying, I am not able to advise people on repairs, modifications, or restoration — you will need to find a shop manual for that information. The good news is that you may be able to find one online (try the Old Car Manual Project) or at your local public library.

      To avoid confusion, I will note that the 1958 Oldsmobile Hydra-Matic is NOT the transmission popularly known as “Slim Jim,” but the earlier and considerably bulkier dual-coupling four-speed unit, also known as the Controlled Coupling Hydra-Matic.

  41. Hello Mr Severson, could you tell me more about the engineer Gilbert Kenneth Hause? Thank you so much.

    1. I’m afraid I have no biographical information on him other than that he was part of Kelley’s corporate Engineering Staff transmission group in the late ’50s and was involved in the development of the triple turbine automatics and Dual-Path Turbine Drive, as well as some variations of those transmissions that never made it to production.

      1. Thank you for your response. Unfortunately, I’ve also asked to the GM Heritage center and I didn’t obtain more informations.

        1. What you might try is doing a patent search on his name. While that won’t provide you with much in the way of biographical detail, patent disclosures include the inventor’s city and state of residence at the time of filing. Reviewing an engineer or inventor’s patent history can sometimes give you a decent idea of their career progression. For instance, if Hause left GM to work for another company, the assignee data for subsequent patent filings might tell you where he went and provide a general timeframe.

          1. Thanks for your advise! In particular, I wanted to know the early years of his career. Apparently, he worked to GM in late thirties. He was specialized in the engineering relative to the hydraulic systems including the disk brakes, pumps and and indeed the transmissions.

  42. My comment is merely “Thanks”.

    I know more about the triple turbine in my long-gone ’58 Caballero Estate Wagon than I did when I owned 40, no 50 years ago. The context of “people in industry” was/is particularly interesting.

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