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

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

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

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


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


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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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


Add a Comment
  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.


      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.

  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.

  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.

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