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


We may take it as a sign of GM’s considerable wealth and resources that it entered the fifties with three distinct automatic transmissions while most of its competitors were still struggling to develop even one. Until the debut of Chrysler’s two-speed PowerFlite in 1953, the only other automaker to develop its own automatic was Packard, which had introduced its torque converter Ultramatic in 1949. (GM, not convinced that the venerable independent had the resources for such a feat, later sued Packard, claiming Ultramatic’s torque converter design infringed on Buick’s patent.) Both Studebaker and Ford turned to Borg-Warner to develop their first automatics, while most of the other independents either bought Hydra-Matic or went without.

In a later era, GM’s corporate management would undoubtedly have pushed for standardization, but in the early fifties, General Motors controlled nearly half of the largest automotive market in the world and allowing the divisions to remain independent and competitive was an affordable indulgence. Just as important was the fact that GM had invested considerable capital in development and tooling for Hydra-Matic, Dynaflow, and Powerglide, which the corporation was not about to casually discard.

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 Cadillacs, Oldsmobiles, and Pontiacs. This 1950 Oldsmobile Eighty-Eight has the earlier Hydra-Matic, with only a single drive range; the Dual-Range version replaced it in 1952.

Moreover, the divisions’ customers were developing strong feelings about the pros and cons of each transmission, which sometimes resembled the rivalry between competing sports franchises. Hydra-Matic fans extolled its efficiency and crispness; Dynaflow supporters proclaimed the virtues of that transmission’s seamless action, and derided Hydra-Matic’s lurching shifts. Even the chief engineers of Oldsmobile and Buick exchanged good-natured jibes about the comparative merits of “Dyna-Slush” and “Hydra-Jerk,” although the latter camp had to swallow their pride for a while in 1953, when the destruction of the Livonia Hydra-Matic plant briefly forced Olds and Cadillac to use Dynaflow. The irony is that both transmissions emerged from the the same group in the corporate Engineering Staff; browsing through GM’s transmission-related patents from this period reveals many common names and evidence of similar design methodology.

Other manufacturers quickly settled on a compromise — two- or three-speed planetary gearsets combined with a torque converter. Chevrolet went that route in 1953, reengineering Powerglide to start in low and shift automatically to high; Packard adapted Ultramatic to do the same late in the 1954 model year. Instead, GM’s engineering staff spent the next decade refining the Dynaflow and Hydra-Matic concepts.


At the same time the corporate Engineering Staff was developing the controlled-coupling Hydra-Matic, they were also hard at work refining the torque converter concept. Although Chevrolet had backed away from the pure torque converter drive approach, Buick remained firmly committed to Dynaflow and was looking for ways to reduce its inherent limitations.

The first result of these efforts was the Twin-Turbine Dynaflow, introduced for the 1953 model year. Like the first Dynaflow, its basic concepts were developed by Oliver Kelley’s corporate engineering team while the production version was overseen by Buick staff engineer Rudolf Gorsky. As the name implied, Twin-Turbine Dynaflow featured a new torque converter with two turbines, a single impeller, and a single stator. Another important new element was an additional planetary gearset, mounted within the converter housing; insofar as the converter and the transmission proper were separate entities, the gearset was part of the converter, interposed between the turbines and the transmission input shaft.

The ring gear of that planetary gearset was driven by the first turbine. The planet carrier, which was attached to the transmission main shaft, was connected via an overrunning clutch to the second turbine. The gearset’s sun gear, meanwhile, was mounted on a one-way clutch that, like the one-way clutch of the stator, only allowed the sun gear to rotate in the same direction as the impeller.

During torque multiplication, the flow of oil from the impeller would drive the first turbine, whose blading was optimized for that operating regime. Any load on the transmission main shaft would lock the planetary gearset’s sun gear against its one-way clutch, so the first turbine would drive the planet carrier (and thus the main shaft and second turbine) at reduced speed, providing a mechanical gear reduction in addition to the torque multiplication provided by the converter itself.

As the first turbine picked up speed, the second turbine would also begin to accelerate. The first turbine continued to provide all the output torque (acting through the planet gears) until the speed of the second turbine exceeded that of the planet carrier. The planetary gearset would then act as a torque-combining differential, simultaneously driven by both turbines, until the flow of oil within the converter was primarily between the impeller and the second turbine, whose blades were optimized for efficient cruising. That left the first turbine to freewheel.

Once the speed of the second turbine was nearly equal to that of the impeller, the stator would unlock, also unlocking the planetary gearset sun gear, and torque multiplication would cease. Unlike the revamped Powerglide, there were still no discrete gear changes, so the Twin-Turbine Dynaflow remained a continuously variable transmission.

Dynaflow shift quadrant on a 1956 Buick Roadmaster convertible © 2010 Aaron Severson
Although the Twin Turbine and Variable Pitch Dynaflow 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 Variable Pitch Dynaflow, the stator blades would change position if the throttle was floored.

This new arrangement provided greater torque multiplication — up to 2.45:1 at stall — without increasing the converter’s stall speed, which would have hurt fuel economy. Since some of the additional multiplication was now mechanical rather than hydraulic, the Twin Turbine converter also provided some relief from the early Dynaflow’s low-speed throttle lag, although really brisk takeoffs still demanded the use of emergency low. An additional one-way clutch also allowed some engine braking on the overrun, which helped to keep speed under control when descending steep grades.

The Twin Turbine Dynaflow was a definite improvement over the original, but 2.45:1 was still marginal for the prodigious curb weight of contemporary Buicks. To address that limitation, in 1955, the twin-turbine Dynaflow gained a new variable-pitch stator. Like the variable-pitch propellers used on some aircraft, the stator blades could switch from a high angle, for greater torque multiplication, to a low angle, for greater efficiency at cruising speeds. Flooring the throttle would flip the stator blades back to the high position with an effect analogous to a kickdown downshift in a stepped-gear transmission. The following year, Dynaflow added a second, fixed-blade stator, increasing the converter’s maximum torque multiplication to 3.5:1. Low could now be held until just past 60 mph (say, 100 km/h), allowing magazine reviewers to record some rather racy 0-60 mph (0-97 km/h) times.

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. It was nominally optional on base Specials, but very few cars — probably less than 5% — were built without it. With its standard 364 cu. in. (5,957 cc) V8 and Variable Pitch Dynaflow, a ’57 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 2 to 3 seconds to acceleration times.

Although the Twin Turbine Dynaflow was finally approaching the performance and flexibility of contemporary stepped-gear automatics, that was not enough for Transmission Development Group chief Oliver Kelley. Kelley’s team was hard at work on the ultimate torque converter transmission: the triple turbine.


In 1952, engineers P.J. Rhoads and Kenneth Gage of the Detroit Transmission Division began work on the second-generation Hydra-Matic, a project that ultimately cost some $35 million. The new transmission, which went on sale for the 1956 model year, was known by a variety of trade names — Oldsmobile called it Jetaway Hydra-Matic while Pontiac christened it Strato-Flight and AMC dubbed it Flashaway — although all were functionally identical. The patent application, filed in November 1953 by engineer Walter Herndon (who had been part of Earl Thompson’s original Hydra-Matic development team), called it a “controlled coupling” transmission.

The controlled-coupling transmission was a thorough redesign of the original Hydra-Matic. Like its predecessor, it had three planetary gearsets, giving four forward speeds plus reverse. A fluid coupling took the place of a conventional clutch; the engine drove the coupling’s impeller through the front planetary gearset, to reduce creep at idle. There were two oil pumps and an external oil cooler to prevent overheating. With a cast iron case (except the front torus cover, which was aluminum), the complete transmission was fairly heavy, at 240 lb (109 kg), and quite bulky.

Most of the changes to the new Hydra-Matic were designed to smooth its notoriously jerky shift quality. The first major change was replacing the troublesome brake bands with sprag (one-way) clutches for both the front and rear gearsets. (A rear band was retained, but it was used only in Low range.) The second and most dramatic change was the addition of a second, smaller fluid coupling between the front and rear planetary gearsets.

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 1.44; 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 1.44; rear brake off, rear clutch engaged, gear ratio 1.00; reverse cone clutch released; overall ratio 1.44:1. 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 1.44; rear brake off, rear clutch released; reverse cone clutch on, gear ratio -2.99; overall ratio -4.31:1 (reverse).
The chart above shows the gear and band engagements for the original Hydra-Matic; the chart below shows the corresponding positions for the controlled-coupling transmission. Some 1955 single-coupling Hydra-Matics had a front planetary gear ratio of 1.55:1, which made first gear 4.10:1 and third 1.55:1.

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 controlled-coupling Hydra-Matic, necessitated by the removal of the brake bands; in the single-coupling Hydra-Matic transmission, neutral was obtained by releasing all clutches and bands.

The new coupling, which was about 25% smaller than the main coupling, replaced the original Hydra-Matic’s front clutch pack. The coupling’s impeller was driven by the ring gear of the front planetary gearset, while the turbine was connected to the front planetary’s sun gear. Since a fluid coupling can’t be mechanically disengaged, it incorporated valves that would allow the oil in the torus housing to be drained or refilled in only 0.4 seconds. In first and third, the coupling was empty and thus effectively disengaged. In second and fourth, it was full, driving the sun gear of the front planetary at the same speed as the ring gear and putting the front gearset in direct drive.

These changes made Hydra-Matic substantially smoother than its predecessor. The 1-2 shift, which was achieved by filling the front coupling, was almost imperceptible. The 2-3 shift, which still involved the engagement of the rear clutch pack, could still elicit a modest thud, but it was far less pronounced than before. (When the new transmission was first shown to the press, engineers admitted they would have preferred to replace the rear clutch pack with a fluid coupling as well, but cost and space considerations had ruled out that possibility.) The redesign also eliminated the need for periodic band adjustments, which most owners had neglected anyway.

1957 Oldsmobile 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 controlled-coupling transmission was introduced; the earlier transmission was dropped by the end of the model year, although it was still used on trucks into the early sixties. The new Hydra-Matic was standard on all Cadillacs and on big Oldsmobiles, like this 1957 Oldsmobile Ninety-Eight Holiday Coupe.

Like the earlier Dual-Range version, the controlled-coupling Hydra-Matic sought to provide a gear for every occasion. It retained the Dual-Range H-M’s second (D3 or Super) Drive range, which would keep the transmission from shifting to fourth gear at speeds under about 75 mph (120 km/h); Low range kept the transmission in second up to about 40 mph (64 km/h). The new Hydra-Matic also offered both part-throttle and full-throttle kickdowns for passing.

Owners soon discovered that the new Hydra-Matic was not quite as efficient as its predecessor was, nor was it as durable. Aggressive driving could destroy the sprag clutches, operation in extreme temperatures could be erratic, and the aluminum torus cover was prone to hairline cracks. Nonetheless, most buyers considered the new Hydra-Matic a welcome improvement.

Thanks to its daunting production costs, the new Hydra-Matic found fewer users than its predecessor did. It was used by Cadillac, Oldsmobile, and Pontiac and was sold to AMC for use in some 1956-1957 Nash and Hudson models, but GMC and Chevrolet trucks stayed with the earlier Dual-Range transmission until the early sixties, as did Rolls-Royce, which built the Dual-Range H-M under license.

Shift quadrant of a 1958 Oldsmobile Ninety-Eight Holiday hardtop © 2010 Aaron Severson
A useful minor addition to the new Hydra-Matic was a Park position — the original H-M had a parking pawl, but it engaged only in Reverse with the engine off. Reverse was still below Low on the shift quadrant, which drew scathing criticism from some experts, including transmission engineer Oscar Banker, who insisted it was unsafe; GM nonetheless retained this shift pattern until the mid-sixties. The “Safety Sentinel” speedometer offered on Oldsmobiles sounded a buzzer if you exceeded a preset speed.

Over the next few years, GM made many minor modifications to the dual-coupling transmission, most aimed at improving its durability and cold-weather performance. The most noticeable change came in the 1960 model year, when the case was redesigned to reduce its considerable bulk. None of these revisions altered the controlled-coupling Hydra-Matic’s basic operation, nor did they make it any cheaper to build. It remained one of the most expensive transmissions of this era and only its ample production volume kept the price within reason.


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. Well, the problem is that unlike the cars themselves, the transmissions don’t have devoted fan clubs who like to show off their cunning internal components at shows. I can’t legally scan pages from the shop manuals, so I’m limited to what diagrams I can create myself.

      I’m not a technical illustrator by any means, and some of these designs are very complex — I don’t know that I would be able to competently render (even in a simplified way) how the pieces interconnect. I had hoped to create one for the controlled-coupling Hydra-Matic, but after staring at the shop manual for days, I mainly understood why GM decided it was too complex and too expensive to manufacture. If I have time and feel inspired, I may create some additional diagrams later and add them back in, although I’m not going to be putting any professional designers out of work…

      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 was designed as a collaborative venture between those three divisions. (Each used their own trade name, which can be confusing; Buick called it the Super Turbine 300.) Only Buick and Oldsmobile used the variable-pitch stator and only through 1967; Pontiac never used the switch-pitch converter. Contrary to popular belief, ST-300 is not Powerglide and resembles the Chevrolet transmission only in certain broad strokes. The three-speed Turbo Hydra-Matic became optional in 1967 with the big engines only and was later supplemented by the medium-duty TH350. I don’t recall off-hand how long the two-speed survived and would have to look it up, but I believe it remained available on low-end models into the early seventies.

  7. Someone left an anonymous message noting that the description of the Twin Turbine Dynaflow was garbled, which is true — I had gotten confused by the written description. In fact, the primary turbine drives the ring gear; within the primary turbine’s operating speed range, the sun gear is locked and the pinions and planet carrier are driven at reduced speed. I’ve amended the text accordingly. Thanks to anonymous for the note!

  8. 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.

  9. [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.

  10. 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!

  11. 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!

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

  13. 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.

  14. 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!

  15. 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.

  16. 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.

  17. 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?

  18. Fascinating info.

  19. 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!

  20. 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.

  21. 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.

        Early Powerglide cars did not shift at all in normal driving — like the Buick Dynaflow, the original Powerglide took off on the converter alone in drive. (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.

  22. 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.

  23. 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.

  24. 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.

  25. 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.

  26. 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.

  27. Thanks so much for the great overview.

  28. 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!

  29. 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.

  30. 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…

  31. 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.

  32. 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.)

Leave a Reply

Your email address will not be published. Required fields are marked *

Click here to read our comment policy. You must be at least 18 to comment. PLEASE DON'T POST COPYRIGHTED CONTENT YOU AREN'T AUTHORIZED TO USE!
Except as otherwise noted, all text and images are copyright © Aaron Severson dba Ate Up With Motor. (Terms of Use – Reprint/Reuse Policy) Trademarks referenced herein are the property of their respective owners and are used here for informational/nominative purposes.