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

SIDEBAR: One-Way Clutches for Planetary Gearsets

As we explained briefly in Part 1 of this article, a planetary gearset consists of a central sun gear, one or more planet gears on a planet carrier, and an annulus (ring gear), all in constant mesh. For the gearset to provide any indirect ratio (that is, numerically higher or lower than 1:1), one of those elements must be held (becoming the reaction member) so that the other elements will rotate around it. In the early Hydra-Matic, Dynaflow, and Powerglide, this was accomplished by rigidly connecting the reaction member to a drum surrounded by a contracting band-type brake; engaging the band would prevent the drum from rotating and thus hold the reaction member in place.

The same effect can also be accomplished in another way: connecting the reaction member to a one-way sprag or roller clutch that prevents the reaction member from turning backward relative to engine rotation. Since several later GM transmissions used one-way clutches in this way, it’s worth pausing briefly to explain the mechanics.

Let’s say the flywheel of an automotive engine is connected to a simple two-speed planetary transmission, which is in turn connected to a propeller shaft, differential, axle, and drive wheels. The engine drives the transmission’s annulus and rotation of the planet carrier turns the propeller shaft. The sun gear is connected to the inner race of a one-way clutch while the clutch’s outer race (or cam, for cam-and-roller clutches) is anchored to the transmission case. Looking at a schematic of that layout, you would say that the annulus is the transmission’s driving member while the planet carrier is the driven member. However, in practice, it’s not quite that simple.

Any time you apply force to an object, that object’s inertia resists any change in momentum, in keeping with Newton’s First Law of Motion. Since in this case the driven member (the planet carrier) is connected to the propeller shaft, differential gears, axle shafts, wheel bearings, and so on, it has quite a bit of inertia. As the annulus begins to rotate, applying engine torque through the planet gears, the planet carrier responds by exerting an opposing force, or reverse torque. Since the gears in a planetary gearset are in constant mesh, the planet gears apply that reverse torque to the sun gear, which would normally turn the sun gear backward. (This is how the original Hydra-Matic obtained reverse; the rear planetary gearset’s annulus was driven backward by the sun gear and that reverse torque was then multiplied by the compounding of the rear and reverse gearsets.)

In this case, however, the reverse torque on the sun gear just causes the sprags or rollers of the one-way clutch to lock, holding the sun gear in place. The rotation of the annulus therefore causes the planet gears to orbit the stationary sun gear (which becomes the reaction member), driving the planet carrier forward at reduced speed and multiplying the input torque. (While this example assumes the annulus is the driving member, the same principles apply if the sun gear is driving and the annulus is attached to a one-way clutch.)

Unlike a brake band, a one-way clutch doesn’t require any external controls. As long as there is any significant load on the driven member, there will be reverse torque to lock the reaction member’s one-way clutch and the gearset will remain in reduction.

How then do you unlock the one-way clutch to put the gearset in direct drive? There are several possibilities:

  1. Lock the driving member to the reaction member. The locking mechanism (typically a plate clutch of some kind) will then absorb the reverse torque so the reaction member and driving member will turn together at the same speed and in the same direction, causing the driven member to do likewise.
  2. Lock the driving member to the planet carrier. The carrier will then be forced to rotate at the same speed and in the same direction as the driving member, forcing the reaction member to do likewise.
  3. Lock the reaction member to the planet carrier. Again, the locking mechanism will absorb the reverse torque, forcing the reaction member and the carrier to turn at the same speed and in the same direction as the driving member.
  4. Apply engine power directly to the reaction member as well as the driving member. The engine’s torque will then counter the reverse torque, driving the reaction member forward at the same or nearly the same speed as the driving member and causing the planet carrier to turn at that speed as well.
  5. Bypass the planet gears completely by disconnecting the engine from the driving member and instead allowing the engine to drive the output shaft directly.

As long as the reaction member and the engine flywheel are turning in the same direction, the one-way clutch won’t interfere with the reaction member’s rotation. That characteristic can be a double-edged sword because it also means the one-way clutch will disengage on the overrun, such as when the car descends a steep grade on a close throttle. If the carrier turns faster than the engine, the reaction member will rotate forward, allowing the one-way clutch to unlock.

To compensate, transmissions that use one-way clutches for their reaction members sometimes also include one or more auxiliary band or clutch-type brakes — typically called overrun brakes or coast brakes — to ensure that there will still be engine braking on the overrun, at least in certain gears or drive ranges.

TWIN-TURBINE DYNAFLOW

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.

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.)

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.

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.

CONTROLLED COUPLING HYDRA-MATIC

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.

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 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)

Chart of internal gearing and brake/clutch combinations for the single-coupling Dual-Range Hydra-Matic. Neutral: all brakes off, all clutches disengaged. First: front brake on, front clutch released, gear ratio 1.45; rear brake on, rear clutch released, gear ratio 2.63; reverse cone clutch released; overall ratio 3.82:1. Second: front brake off, front clutch engaged, gear ratio 1.00; rear brake on, rear clutch released, gear ratio 2.63; reverse cone clutch released; overall ratio 2.63:1. Third: front brake on, front clutch released, gear ratio 1.45; rear brake off, rear clutch engaged, gear ratio 1.00; reverse cone clutch released; overall ratio 1.45:1. Fourth: front brake off, front clutch engaged, gear ratio 1.00; rear brake off, rear clutch engaged, gear ratio 1.00; reverse cone clutch released; overall ratio 1.00:1. Reverse: front brake on, front clutch released, gear ratio 1.45; rear brake off, rear clutch released; reverse cone clutch on, gear ratio -2.97 (compound); overall ratio -4.30:1 (reverse).
The chart above shows the gear and band engagements for the 1952–1954 Dual-Range Hydra-Matic; 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.

The redesigned transmission maintained the same basic 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 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)

Chart of internal gearing and brake/clutch combinations for the 1956–1964 Controlled Coupling Hydra-Matic. Neutral: front sprag clutch locked, overrun clutch released, front coupling empty; neutral clutch released; rear sprag free, overrun band off, rear clutch released; reverse cone clutch off. First: front sprag clutch locked, overrun clutch engaged (Low or D3/S only — released in D4), front coupling empty, gear ratio 1.55; neutral clutch engaged; rear sprag locked, overrun band on (Low or D3/S only — released in D4), rear clutch released, gear ratio 2.55; reverse cone clutch released; overall ratio 3.97:1. Second: front sprag clutch free, overrun clutch released, front coupling full, gear ratio 1.00; neutral clutch engaged; rear sprag locked, overrun band on (Low or D3/S only — released in D4), rear clutch released, gear ratio 2.55; reverse cone clutch released; overall ratio 2.55:1. Third: front sprag clutch locked, overrun clutch engaged (Low or D3/S only — released in D4), front coupling empty, gear ratio 1.55; neutral clutch engaged; rear sprag free, overrun band off, rear clutch engaged, gear ratio 1.00; reverse cone clutch released; overall ratio 1.55:1. Fourth: front sprag free, overrun clutch released, front coupling full, gear ratio 1.00; neutral clutch engaged; rear sprag free, overrun band off, rear clutch engaged, gear ratio 1.00; reverse cone clutch released; overall ratio 1.00:1. Reverse: front sprag clutch locked, front coupling empty, overrun clutch engaged, gear ratio 1.55; neutral clutch released; rear sprag free, overrun band off, rear clutch released; reverse cone clutch on, gear ratio 2.78 (2.41 from 1958 model year on); overall ratio -4.31:1 (-3.74 from 1958 model year on).
Shift sequence for the second-generation Controlled Coupling Hydra-Matic. The neutral clutch was another new feature of the dual-coupling Hydra-Matic, necessitated by the addition of the sprag clutches. Hydraulic controls for these elements 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.

The redesigned transmission also deleted the earlier Hydra-Matic’s front brake, 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 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; reverse 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.

Color diagram of 1956-1964 Controlled Coupling Hydra-Matic transmission fluid couplings and front sprag clutch © 2016 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 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.

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.

91 Comments

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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. 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.

  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.

      Cheers,
      Chris

      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. anyone have a diagram of the dual path? It stopped shifting from low into second and I found a spring in the bottom of the pan. Where does it go?

    1. Sorry, I’m not qualified to give repair advice. You might try seeing if your local library has a service manual for it — I was able to find a copy of the Pontiac dual-coupling Hydra-Matic shop manual that way.

    2. 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.

  8. [quote=steve dill]anyone have a diagram of the dual path? It stopped shifting from low into second and I found a spring in the bottom of the pan. Where does it go?[/quote]If you could provide a picture of the spring, I could look it up in my various manuals and give you an answer.

  9. 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!

  10. 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!

  11. anybody know where I can buy the flexible black fresh air vent tubes? Darn Mice

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

    1. Well, 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 Hydramatic. 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.

  13. 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!

  14. 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.

  15. 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.

  16. 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?

  17. Fascinating info.

  18. Hi can any body help me
    I have a 1958 Buick Road Master fitted with a Dynaflow Flight Pitch
    gear box can any one tell me where i can get spares for the gear box
    and will ship them to England

    1. I’m not able to help with technical issues or buying parts — sorry!

  19. 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.

  20. 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.

  21. 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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. Thanks so much for the great overview.

  27. 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!

  28. 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.

  29. 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…

  30. 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.

  31. 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.

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

  33. 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.

  34. 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.

  35. 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!

  36. 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.)

  37. 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.

  38. 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.

  39. 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.

  40. 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!)

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