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.
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.
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.
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.
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.
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.
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.
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.