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

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

IMPORTANT ADDITIONAL NOTE

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

TORQUE CONVERTER DRIVE

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

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

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

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

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

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

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

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

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

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

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

M18 HELLCAT

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

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

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

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

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