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

SENIOR COMPACTS

As we mentioned previously, the late fifties were not a particularly good time for radical or elaborate new designs. The new car market, which had boomed in 1955, slumped badly for 1956. By 1957, a national recession had buyers searching for smaller, cheaper, more economical cars.

Chevrolet general manager Ed Cole used this opportunity to push through his radical rear-engine, air-cooled Corvair, but did nothing to help GM’s mid-priced divisions, which had been hit hard by the recession. Senior corporate management responded with the X-100 project, a collaborative program to give Pontiac, Oldsmobile, and Buick their own small (or at least smaller) cars for the 1961 model year, a year after the debut of the Corvair.

Although the X-100 cars were intended to have a high degree of commonality so as to share the substantial costs of the new models, the final products ended up considerably less alike than the corporation originally hoped. The so-called “senior compacts” — the Buick Special/Skylark, Oldsmobile F-85/Cutlass, and Pontiac Tempest/Le Mans — did share the same unitized Y-body shell (an enlarged version of the Corvair body) and various minor components, but there were significant differences in their mechanical layouts and powertrains, including three completely different automatic transmissions.

Ironically, the most conceptually exotic of the trio, the Pontiac Tempest’s rear-mounted automatic transaxle, was probably the cheapest of the three to develop and tool. Dubbed “TempesTorque,” it was a variation of the Corvair’s optional Powerglide transaxle. As in the Corvair Powerglide, TempesTorque’s torque converter was at the back of the transaxle. Since the Tempest had a front-mounted engine, TempesTorque used the Corvair transmission’s front oil pump driveshaft as an input shaft, driving the torque converter impeller through the torus cover. That input shaft was also splined to the hub of the direct drive clutch, just like in a conventional RWD Powerglide. In high, input torque was therefore split approximately 40/60 between the front sun gear, which was driven by input shaft through the direct drive clutch, and the rear sun gear, which was driven by the torque converter. As with Hydra-Matic, this “split torque” layout served to reduce slippage at cruising speed in high gear.

Color diagram of 1961–1962 Pontiac TempesTorque transaxle © 2016 Aaron Severson
More déjà vu: The early (1961–1962) Pontiac TempesTorque — again NOT to scale — bore a strong, non-coincidental resemblance to Corvair Powerglide. The famous flexible driveshaft connected the engine (in front, not pictured) to the central input shaft, a clever re-purposing of the Corvair transaxle’s front oil pump drive. Aside from the location of the engine, the major difference between the Tempest and Corvair transmissions was that in TempesTorque, the direct drive clutch hub was splined to the input shaft rather than the main shaft. As a result, in high gear, the front sun gear turns at engine speed while the rear sun gear (driven by the main shaft) turns at the speed of the torque converter turbine; the output shaft therefore turns a little faster than the turbine. For 1963, Pontiac divorced the input shaft from the direct clutch, instead connecting the clutch hub to the main shaft — as in the Corvair Powerglide — so that in high, both sun gears would turn at turbine speed. (Author diagram)

The 1961–1962 TempesTorque had a lower converter stall ratio than did the Corvair Powerglide (2.00:1 rather than 2.60:1), but the indirect ratios were the same (+/-1.82:1 for low and reverse). As with the Corvair transmission, there was no parking pawl.

Pontiac made a variety of changes to TempesTorque for 1963, the “rope-drive” cars’ final year. Torque capacity was increased to accommodate the Tempest’s newly optional 326 cu. in. (5,340 cc) V8 engine while new planetary gears — possibly borrowed from the latest heavy-duty Powerglide, although we couldn’t swear to that — gave indirect ratios of +/-1.76:1. A new direct drive clutch deleted the previous high-gear torque-splitting feature and the torque converter was redesigned to provide higher stall ratios (2.40:1 for four-cylinder cars, 2.20:1 for the V-8). TempesTorque and the rope-drive Tempest/Le Mans disappeared for good after the 1963 model year.

DUAL-PATH TURBINE DRIVE

The two-speed torque converter automatic transmission offered on Buick’s Y-body compacts, dubbed Dual-Path Turbine Drive, was quite different from TempesTorque and for that matter the twin-turbine transmission used in contemporary full-size Buicks. Today, the Dual-Path Turbine Drive is one of the most obscure and poorly understood of GM’s early automatics, in part because it was used only on the 1961–1963 Buick Special and Skylark. In a sense, it was Buick’s first true automatic transmission, since it was the first to actually include provision for automatic shifts between two distinct stepped ratios in Drive, something the designers had taken pains to avoid with the Dynaflow family.

The Dual-Path transmission seems to represent a merger of two distinct conceptual threads within GM’s corporate transmission group. One was the use of a split-torque clutch to provide direct drive, a concept dating back to the original Hydra-Matic (and essayed in somewhat simpler form around 1957 by Oliver Kelley’s colleagues Robert M. Tuck and James J. Mooney, Jr. — see U.S. Patent No. 2,929,270). The other, developed by Kelley and Gilbert K. Hause, was a three-element torque converter with a stator that could do double duty as a drive turbine. Judging by the earliest relevant patent disclosures (U.S. Patents 2,957,370 and 3,030,823), the latter was conceived as a streamlined and simplified evolution of the Twin-Turbine Dynaflow, applying some concepts from the triple-turbine transmissions to allow the deletion of Dynaflow’s separate planetary gearbox.

It appears that there was some consideration of using the Dual-Path transmission in the other X-100 cars. The patent outlining most of the major mechanical details was actually filed by John DeLorean, then the head of Pontiac’s advanced engineering section, although Hause led the development of (and patented) the hydraulic control system. Interestingly, a subsequent patent filed by Kelley and Hause described several possible rear transaxle versions, although the production Dual-Path transmission was only for front-engine/rear-drive applications. The only significant element TempesTorque ended up sharing was the torque-splitting feature, which Pontiac implemented differently.

Like the contemporary Powerglide, Dual-Path Turbine Drive had a three-element torque converter and a single planetary gearset with dual sun gears, but the mechanical similarities ended there. As in the dual-turbine Dynaflow series, the actual planetary gears were nestled in the center of the torque converter torus, with the turbine hub driving the annulus and the planet carrier driving the transmission main shaft. Unlike previous Dynaflow transmissions (and most other automatic transmissions), the impeller was mounted on the flywheel side of the torus housing, facing backward (i.e., toward the rear axle) while the turbine faced forward, toward the engine, and was connected to its hub by a series of narrow struts.

Color diagram of 1961-1963 Buick Dual-Path Turbine Drive transmission © 2016 Aaron Severson
Dual-Path Turbine Drive’s torque converter (again, simplified/abstracted and not to scale) was ‘backward,’ with the impeller (red) facing toward the rear axle. Several narrow struts connect the turbine (blue) to its hub, which contains the annulus. Note that both one-way clutches (fuchsia with heavy black arrows) shared a common cam, which was anchored to the case by the forward clutch. (Author diagram)

Unlike Powerglide and Dynaflow, Dual-Path used no brake bands. Instead, it was controlled by four multi-disc clutches (the direct drive/converter clutch, reverse clutch, forward clutch, and coast clutch) and two one-way clutches (one for the stator, the other for the planetary gearset’s rear sun gear). The converter clutch was mounted in the hub of the torque converter impeller, allowing the torus cover to be locked to the planetary gearset’s front sun gear. The other five clutches occupied most of Dual-Path’s cast aluminum transmission case.

The main shaft was surrounded by four concentric sleeve shafts of varying lengths. The outermost sleeve allowed the torus cover to drive the transmission’s single oil pump, which was mounted in the front of the transmission case, like Dynaflow’s front pump. Within that shaft was a sleeve shaft connecting the converter turbine to the hub of the reverse clutch. The two innermost shafts connected the stator and rear sun gear to the inner races of their respective one-way clutches. Those clutches, which were of the cam-and-roller type, shared a common cam, which was connected to the forward clutch and coast clutch. The forward clutch, like the neutral clutch of the Controlled Coupling Hydra-Matic or Triple Turbine transmissions, allowed both one-way clutches to be selectively neutralized (i.e., allowed to turn freely without locking) by disconnecting the cam from the transmission case. The coast clutch, meanwhile, allowed the cam to be locked to the inner race of the rear sun gear so that all three would turn together on the sun gear sleeve shaft.

Color diagram of 1961-1963 Buick Dual-Path Turbine Drive transmission torque converter and planetary gearset© 2016 Aaron Severson
Simplified detail (still not to scale) of Dual-Path Turbine Drive’s converter and the planetary gearset in its hub. (Author diagram)

All this sounds very complex, but Dual-Path’s operation was reasonably straightforward. Selecting either D (Drive) or L (Low) on the selector (which had a PNDLR pattern) would engage the forward clutch. This enabled the one-way clutches for the stator and the rear sun gear, preventing them from turning backward. The hub of the torque converter turbine then drove the annulus of the planetary gearset, with the stator providing additional torque multiplication in the customary fashion; stall ratio was variously quoted at 2.40:1 or 2.50:1. Reverse torque on the planet carrier locked the rear sun gear’s one-way clutch, putting the planetary gearset in first gear and providing a mechanical gear reduction of 1.58:1.

Selecting L would engage the coast clutch as well as the forward clutch. This allowed the stator to function normally, but prevented the rear sun gear from turning in either direction, ensuring that the planetary gearset would remain in reduction on the overrun. Dual-Path’s hydraulic controls included no provision for automatically disengaging the coast clutch, so in Low, the transmission could not shift out of first gear.

With the selector in D, however, the hydraulic control system would shift automatically between first and second. Upshifts were executed by engaging the converter clutch, which established a mechanical connection between the torus cover and the front sun gear. That drove both sun gears forward, which caused the rear sun gear’s one-way clutch to automatically unlock so that the planetary gearset was no longer in reduction. Engine torque was then split 36.6/63.4 between the front sun gear (which turned at impeller/engine speed) and the annulus (which turned at turbine speed), reducing converter slippage. Downshifts were executed by simply releasing the converter clutch, which caused the rear sun gear to automatically re-lock and put the transmission back in first. Shift points were determined by a combination of throttle setting and car speed. Dual-Path was the first Buick automatic to be equipped with a centrifugal governor, previous Dynaflow and Turbine Drive control units having had no need to measure road speed.

Color diagram of 1961-1963 Buick Dual-Path Turbine Drive transmission one-way, forward, and coast clutches © 2016 Aaron Severson
Detail (not to scale) of the Buick Dual-Path Turbine Drive’s interconnected one-way clutches. With the forward clutch engaged, the clutch cam was stationary. Each race could rotate with the engine (as indicated by the heavy black arrows), but would lock against the cam if turned backward. The coast clutch locked the two races to one another. (Author diagram)

Dual-Path obtained reverse in much the same way as the earlier triple-turbine automatics, although the arrangement was slightly simpler because Dual-Path had only one turbine and one planetary gearset. Moving the selector to R released the forward clutch and engaged both the reverse and coast clutches. The reverse clutch then held the turbine stationary, which caused the turbine and stator to effectively swap roles. With the forward clutch disengaged, releasing the one-way clutch race from the case, reverse torque on the stator caused the stator, its sleeve shaft, and the clutch race to spin backward. Since the coast clutch was also engaged, the reverse rotation of the clutch cam drove the rear sun gear backward. The annulus, which was held in place along with the turbine, then acted as a reaction member, allowing the sun gear to drive the planet carrier in reverse reduction.

The point of this unusual arrangement was to minimize the number of components, keeping the transmission as compact and as light as possible. With its air-cooled aluminum case, Dual-Path Turbine Drive was one of the lightest automatic transmissions ever developed by a U.S. automaker, weighing only 95 lb (43 kg) with fluid. That was a bit less than the Buick Special’s standard three-speed Warner Gear T-85 manual transmission and less than half as much as the full-size Turbine Drive or four-speed Hydra-Matic. The transmission tunnel intruded into cabin space only slightly more than that of the rope-drive Tempest, with its rear transaxle.

1962 Buick Special convertible front 3q © 2010 Aaron Severson
Dual-Path Turbine Drive was used only in 1961–1963 Buick Specials and Skylarks. It was nominally a $189 option, but our guess is that the automatic went into something like 85–90% of all Y-body Buicks. That would bring the three-year production total to around 330,000–350,000 — a respectable total by any standard except GM’s. Specials with automatic had a 3.08:1 axle ratio, but Skylarks with the hotter optional V8s typically had a shorter 3.36:1 axle.

From a performance standpoint, a two-speed automatic linked to an assortment of modestly powered V8 and V6 engines doesn’t sound promising, but contemporary testers found that cars equipped with Dual-Path were unexpectedly spry. Although Dual-Path’s first gear was quite tall — it was only slightly shorter than second gear in the T-85 three-speed manual or, for that matter, third gear in the Controlled Coupling Hydra-Matic — the torque converter provided a starting ratio comparable to first gear in the four-speed Hydra-Matic. With a 3.08:1 axle, standard on automatic Specials, first could be held to 63–64 mph (101–103 km/h), so Low was useful in mountain driving that would be uncomfortably buzzy with many contemporary two-speeds. In all, performance was really not bad, if still somewhat inferior to the available manual transmissions. The closeness of the ratios and the use of one-way clutches rather than brake bands also made Dual-Path’s shifts impressively smooth.

One minor sacrifice was the capacity for push-starting, something allowed by most earlier GM earlier automatics. Hause’s patent (U.S. No. 3,108,493) included an auxiliary oil pump to be used solely for that purpose, mounted at the rear of the transmission just ahead of the governor. However, the production transmission had only one pump, presumably in the interests of reducing cost and weight.

91 Comments

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  1. Hey,how come you can yack all day long about this ones gearset setup,or that ones turbine combination,but no illustrations???
    Just because you can picture the entire mechanical world with words doesn’t mean the rest of humanity can.
    Pictures Please!!!!

    1. Um, no “Thank you for an awesome article and site?”

      There is an illustration of a Turboglide and it’s hardly fair to expect Aaron to write an great article about the development of the automatic AND delve into all the technical details. He does to a degree, but that’s not the overwhelming emphasis of the site, as far as I understand it.

      How about Googling “Turboglide,” “Dynaflow” or “Powerglide?”

    2. (ETA May 30, 2016): Very late, but there are now diagrams! I’m not a technical illustrator by any stretch of the imagination, but you can at least get a sense of how these things were laid out.

      1. There’s a site here that has a diagram of an overhaul of the controlled coupling hydra-matic. I can really see why GM wanted to get way from this design. Although today’s ZF 8 and 9 speeds are probably worse, but then half of the world industry is sharing the development costs for these.

        1. …And yet, they were damn near indestructible. We had a ’58 Pontiac that took a lot of punishment in the snow, yet worked without any issues, other than a small oil leak, until I had to sell it in late 1964.If I remember correctly, it was cast iron and weighed around 225 lbs.

          1. The ’58 edition weighed about 240 lb. GM was able to trim about 10-11 lb for 1960 by slimming down the case a bit.

  2. In the photo of the Hydra-Matic shift quadrant in the ’50 Olds 88, is that an aftermarket turn signal unit? If so, it’s a reminder of how times have changed! I understand that at that time, a heater was an option on many cars.

    1. I believe turn signals were standard on Oldsmobiles by 1949, at least on DeLuxe models. I’d need to find somebody with an Olds dealer book from that period to know for sure, but my information suggests they were standard fit.

      Pretty much everything [i]else[/i] was at least technically optional at that point, including oil filters, wheel covers, hood ornaments, windshield washers, and (at least until after the war) reversing lamps. Heaters didn’t become standard even on Cadillacs until almost the mid-fifties, and they weren’t standard on cheap cars for another decade after that. Very few cars were built without a lot of these items, but they weren’t included in the list price for many years.

  3. At least they did not charge extra for chrome after the war.

    I remember seeing a ’50s car ad that mentioned the [i]reverse[/i] gear was an optional extra. On the other hand many cars (particularly British) came with leather seats only because it was cheaper than vinal.

    1. I don’t know of any cars that late that didn’t come with a reverse [i]gear[/i], although reversing [i]lamps[/i] were still extra on many inexpensive cars at that point. Turn signals, as well.

    2. Just as well they didn’t charge extra for chrome.
      The ’58 Buicks & Olds would have cost a small country to buy.

      Back on topic, thank you once again for an
      entertaining read.

      Cheers,
      Chris

      1. Well, in essence, they did charge extra for the chrome, though fortunately not by the pound. On most cars of that era the amount of brightwork was tied to the trim level, and naturally the higher the trim level, the higher the price. Beyond that, there were often extra-cost dress-up packages (either factory- or dealer-installed) that primarily consisted of additional chrome trim. Such things didn’t really disappear from American options lists until the rise of Japanese-style tiered equipment packaging quite a few years later.

    3. Ahh! Those were the days! Everything from a Roller (that’s Rolls Royce to you Yanks) to a Moggy (Morris Minor) with a leather interior. I remember the smell well as a small child in the early ‘sixties.

      Unfortunately British manufacturers did make the switch to vinyl during that decade for economy reasons and every non-luxury car came with a ghastly black vinyl interior that was composed of shiny paper-thin crap. On hot days (mercifully few and far between in the UK), first degree burns to your back and ass were the minimum you could expect. No wonder parts counters did a roaring trade in textile seat covers — they may have been ugly, especially the furry ones, but sure beat the OEM’s one and only offering of black vinyl by the acre.

      I owned a 1966 Pontiac Bonneville 4-door for a short while in 1979-80 (I sold the engine and transmission to a local drag racer and scrapped the body because it was too rusty to repair). It was white with a turquoise interior (even the steering wheel was see-through turquoise perspex). The upholstery was Morrokide and that was a revelation to me. It just shouted quality and put into stark perspective just how short-changed we Europeans were when it came to cars, forced to pay over the odds for inferior rubbish. The only way to go lower was to buy something from the Soviet Block — not that a Lada or a Yugo could possibly be worse than a Hillman Avenger (Plymouth Cricket in the US). [Aside: Thanks a bunch Chrysler. You took over the Rootes Group, at the time manufacturers of the Sunbeam Tiger, and turned them to manufacturing the most embarrassing pile of dross in automotive history. Shite is shite regardless of whether you brand it as Hillman or Chrysler or Talbot, as happened to the Avenger over its lifespan.]

      Did things get better in the ’70s and ’80s? Not unless you consider flimsy Dralon “better”. As I recall, you purchased a car new paying extra for the “luxury” option and well before it got to five years old the upholstery was torn and stained and looked like a pigsty. I still get nostalgic for that old Pontiac — The body may have been a rust bucket but the interior was palatial.

  4. Thanks for a great website and particularly for the GM transmissions articles. Every article I’ve read has been complete, accurate, and very interesting.

  5. Thank you for the automatic transmission article(s) on GM. Finally, someone has accurately chronicled the myriad development story for us.
    Your site is a valuable and entertaining resource – keep up the great work!

  6. This brought back some memories – I remember when I first got my license driving my Dad’s ’65 Olds F-85 with Jetaway and those 1-2 shifts at about 70mph if you held your foot in it. I have a question – I have an childhood memory of an early 50’s vehicle ( think it was a Chevy ) with a “Torque-Glide” logo on the trunk lid instead of “Power-Glide”, but that can’t be right, can it?

    1. Chrysler had a number of semi-automatics in that period with a variety of bizarre names: Gyro-Torque, Fluid Torque Drive, Fluid-matic, Fluid-Drive, and Plymouth’s Hy-Drive. Maybe it was one of those?

    2. Actually, from 1965 up, the F-85, Buick Skylark, and Pontiac Tempest all utilized the newly available Turbo-Hydramatic 300, which in essence was the same thing as a Powerglide, but with non-interchangeable parts. Early versions had variable pitch and a rear pump. It was with the advent of these new automatics that the shift indicators from that time forward would read P R N D L.

      1. The latter point is correct, but the rest is not. As the text explains, the two-speed transmission used on 1964-on B-O-P A-bodies is not Powerglide, although they’re similar in many respects. Although the two-speed (which Buick called Super Turbine 300) was manufactured by Hydra-Matic Division, it was not called Hydra-Matic. (I know the source you’re looking at, and it’s incorrect.) The three-speed Turbo Hydra-Matic became optional in 1967 with the big engines only and was later supplemented by the medium-duty TH350. The two-speed remained available on low-end models into the early seventies.

        1. You are wrong the turbo 400 was built by the Buick division of GM in1964 and all divisions but Cheyenne used them in full size cars. I have a GM delve that is 3 inches thick telling how to rebuild every automatic transmission they used from 1956 to 1964 with service bullion so from Buick staring in
          1964 I used for 45 years in the transmission business

          1. At least some early TH400s and later TH350s were indeed built by Buick rather than Hydra-Matic Division, that’s true. (My assumption is that it was in part a retooling issue, since Hydra-Matic was still building substantial numbers of other designs, including Roto Hydra-Matic and limited numbers of the four-speed dual-coupling unit.) And some non-Buick users did indeed switch to TH400 for some models in 1964, although not all and not as widely as in 1965. (I assume by “Cheyenne” you mean “Chevrolet,” which first offered TH400 on B-body cars with the Turbo-Jet big blocks in mid-1965.)

            I’m familiar with the type of service manual you’re describing; I may even have referred to the same one you have. While manuals like that are handy from a technical standpoint, they aren’t ideal historical sources, which of course isn’t their function. Their technical information may be more or less correct at the time it was originally written (although it’s not altogether uncommon to find errors in that as well), but manuals like that often don’t do a great job of reflecting running production changes and the intricacies of what was offered on what model/in what combination and when are beyond their scope.

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

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

    2. Try this… as good an explanation of your problem as I’ve ever understood: https://www.youtube.com/watch?v=rLDgQg6bq7o

      1. He talks about your differential girdle spring at starting at ~1:10. It’s supposed to be hooked onto the upend of the gramys.

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

  9. I have a 62 Buick,Skylark,with the dualpath Tranny.the trans is in direct drive,only goes foward,no neautral,park orreverce,is thier a fix for this.

    1. Can some one HELP.
      I have a 1962 Olds Cutlass F 85, Auto Hydro Matic floor shift.
      I had the transmission rebuilt 3 times already.
      and the problem is that when the car warms to operating temp
      it starts to jerk and gos into neutral. it clears once i accelerate.
      RPMs Are normal. trans just dosnt stay in low gear when moving at 10mpg or at a stop. Thanks- Robbe California

      1. @Robert: I’m afraid I’m not at all qualified to offer repair or troubleshooting advice — sorry!

  10. this article was great. It answered my question as to why the 52 Super I just inherited doesn’t shift….that would be because it isn’t made to shift automatically….I read a blog online saying
    1952 Buick – the slowest car I ever loved….so true!

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

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

    1. Well, there’s an old saying to the effect that you can make anything fit if you have a big enough hammer. I honestly don’t know how much trouble would be involved in interchanging them, but since they were never designed to be used behind the same engines or in the same cars, I imagine it would take some work.

      At one time, Buick Nailhead engines were popular with drag racers, so if you were asking this question in, say, 1964, there might have been aftermarket kits to mate an older Buick V-8 with a beefed-up Powerglide. (Some drag racers used Powerglide because it consumed relatively little power and they didn’t need a lower first gear.) Today, I suspect you’d have more luck finding some way to put in a Turbo Hydramatic. I’ve never looked, though.

      This is a question that would probably be best put to a performance transmission manufacturer or a shop that specializes in parts for older transmissions.

    2. No the dynaflow and the powerglide are not interchangeable. the dynaflow is about three times heavier and will not fit up to any engine that was made for the powerglide. The powerglide came in two models first being the cast iron model that was used through 1954 then the aluminum powerglide after that. both very good transmission, and easily rebuildable.

      1. The earliest Powerglide is very similar to the early Dynaflow, although I doubt they’re easily interchangeable. As the revised text explains, Powerglide had several phases: the early dual-impeller variety, used through 1952; the later iron-case version with a three-element converter, used, with various evolutionary changes, from 1953 to 1962–1963; and the late aluminum-case version. The aluminum Powerglide (for RWD cars — all Corvair Powerglide units had an aluminum case) was introduced for some models in 1962 and for others in 1963.

  13. chevy had 2 auto transmissions in 61and62 1 was a turbo glide the other was –glide that changed by fluid. there was no gears in the trans. on the gear selector was P R D G G was for grade as going up a hill. what was the name of that trans?

    1. The two transmissions were Powerglide and Turboglide. Powerglide was the familiar two-speed-plus-torque-converter Chevrolet automatic, while the transmission you’re thinking of was Turboglide, which is described in the text.

      The G position was for Grade Retarder. It was intended not for climbing hills, but for descending them; it was supposed to mimic the effect of engine braking, of which the Turboglide otherwise didn’t allow very much. The Grade Retarder was not useful for acceleration or hill climbing, although some people had problems because they assumed it worked like the Low position on Powerglide, which was definitely not the case!

  14. Re read this as a refresher on the development of the automatic. Thank you again. Your site is an invaluable resource and I cannot thank you enough for doing what you do.

  15. Thank you for your clear and concise explanation of Dynaflow, and how it differs from the other two GM automatics. As we were a “Buick family,” the innate superiority of Dynaflow was never a question; it was an article of faith. I remember the feelings of incredulity and betrayal I felt when I was told for the first time that Dynaflow was “Just Powerglide with a different name,” and that Hydramatic was obviously better, because Olds and Cadillac used it. You have restored my faith in Dynaflow.

  16. We have recently inherited a 53 Roadmaster. I think it is an early model serial #26854377 because the 322 nailhead has a weighted pully instead of a rubber loaded harmonic balancer. The Dynaflow is now in the transmission shop and we are finding puzzles. According to the shop manuals the 53 should be the new twin turbine with only 1 pump and one stator. This trans has the words “twin turbine” cast into the bellhousing. But inside it has 2 pumps and 2 stators. Do we have a transitional factory job or a trans shop hybrid? Was the change made to save money (fewer parts) or to improve performance? Will our new Roady rise and fly?

  17. Fascinating info.

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

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

  19. Just wanted to say this is a great article. I started out looking to find the difference between the hydra-matic dual range and the strato-flight and wound up learning a lot more.

  20. The article refers to the Hydramatic’s jerkiness. Actually, many Hydramatics were so smooth that you could not even feel the shift; you could just hear the drop in engine speed. I remember in 1959 riding in a 1949 Lincoln with Hydramatic; it accelerated quickly and so smoothly that I could not feel the shifts. The same was true with some other cars with Hydramatic in which I rode, including a 1950 Pontiac, and those were all before GM introduced the Hydramatic with the second (controlled) fluid clutch in 1956. On the other hand, I rode in a 1953 Cadillac with had very firm shifts.

    The downshift resulting from flooring the accelerator were another matter; they were always accompanied by a mechanical clunk.

    1. The issue with the original Hydra-Matic was that because its shifts were mechanically complex (particularly between second and third, which was the most complicated sequence), its smoothness depended a great deal on how well the bands were adjusted, the condition of the transmission fluid, and other maintenance- and condition-related factors. If everything was perfectly adjusted, it would be quite acceptably smooth (particularly by the fifties, by which time GM had made a lot of minor refinements). If not, it would throw off the shift timing just enough to make the shift jerky, albeit not necessarily enough to really impair the transmission’s function. I suspect a lot of owners who complained to their dealers or mechanics were told, “Ehh, they all do that.”

      Even some of the engineers who originally designed the Hydra-Matic thought it was too complicated for its own good, which is why they subsequently got into the torque converter automatics, which didn’t shift at all. The original Dynaflow was very much the antithesis of the Hydra-Matic in a lot of these respects.

    2. My experience with Hydromatic cars was that they were fairly smooth in shifting. PowerGlide cars had a very pronounced jerk when shifting. When my city purchased GM buses in the sixties, the Hydromatic was very rough when shifting with an easily heard lowering in engine sound as speed increased.

      1. The difficulty with making blanket statements in this area is that each of these transmissions was around for a long time in several quite distinct versions, not all of which felt or acted the same.

        As the text explains, early Powerglide cars did not provide any automatic shifting in Drive, relying on torque converter multiplication exclusively. Powerglide was revised in 1953 to start in first and shift automatically to second. So, early Powerglides (or Dynaflow) were smoother than even a well-adjusted early Hydra-Matic, albeit not especially quick or efficient. After that, there were early (iron-case) and later (aluminum-case) Powerglide transmissions, tuned in different ways for different engines.

        Similarly, the early (1940 to 1955) and late (1956-1964 dual-coupling) Hydra-Matics were significantly different mechanically — albeit still related — and felt quite different.

        So, while it may sound pedantic, it’s important to qualify statements like, “X was smoother/rougher than Y.”

      2. Those GM buses had a 1 speed automatic Allison transmission. Great roaring noises as the variable torque converter changed pitch and allowed the bus to gradually accelerate to 25 mph, then an almighty clonk as the torque converter was locked-up with a mmm-uhh-mmm vibration that gradually settled down as the engine bounced up and down on its mounts. Crude or what! Engine note and speed decreased at point of lockup.

        I blame those buses, their braying, outlandishly noisy two-stroke GM diesels and the pathetic transmission for ruining the quiet of our city at night when introduced. Went to London for grad work in 1969, and it was obvious that a AEC 4 stroke diesel packing all of 120 hp and four speed preselector gearbox not only got a double-decker bus going from stop much quicker than a GM bus, it was at least 10 times quieter doing it.

        Speaking from my point-of-view as a mechanical engineer. In those days as a student I had to ride buses and had a keen interest as to why the GM was so unrefined and the engine so noisy. No domestic competition would be my guess.

        1. Noisy or not, I loved those old roaring GM buses, when in “hydraulic drive” mode. That mode would seem to be not very fuel-efficient; a 4-speed pre-selector as you mention, should indeed have been more fuel-efficient (as well as quicker, as you mention). I have read that a later version of this Allison transmission arrangement actually had a second gear, making for a true two-speed, plus lockup in high. I cannot confirm that, though.

  21. I’ve heard a story about the Hydra-Matic, as follows:

    Supposedly Rolls-Royce acquired a Hydra-Matic for evaluation. They liked it but thought one particular part had too rough a finish. When they fabricated a smoother-finished version of the part and incorporated it into the reassembled Hydra-Matic, the transmission didn’t work. True, or urban legend?

    1. I’ve heard that story in regard to the Turbo Hydramatic (not the original), which Rolls-Royce also built. The way I’ve heard it is more that they tightened up the tolerances, which didn’t necessarily work out well. I don’t know if it’s true or not, but it’s not implausible. There’s an analogy to be made with pistols, where getting everything “tuned” to tight tolerances improves accuracy, but makes the action less tolerant of dirt or debris. (This is why police and military sidearms are not built like target pistols.)

      1. I am reasonably certain that while Rolls Royce licensed & built in England the original HydraMatic, it imported the Turbo HydraMatic 400 from GM in the states.

        1. You’re correct; my previous comment was based on a point I was only half-remembering. They did import them, but asked for higher-than-standard tolerances.

  22. Thank you for this very complete summary. I have been curious about these transmissions for quite some time, and this is quite helpful. Your research is impressive, as is the writing.

  23. The main problem with reliability of the Slim Jim was the weakness of the front oil pump cover; they cracked. An improved pump with webbing on the cover was designed to replace failed units. RHM 375 Model 10’s made at Willow Run ceased in 1962. The THM 350 signalled the beginning of a long slide toward mediocrity by GM.

    1. I have to wonder if the Roto Hydra-Matic’s various weaknesses, including the propensity for leaks and the issue you describe, were exacerbated by the very high operating pressures. As mentioned, the RHM’s operating pressures were substantially higher than the earlier dual-coupling HM’s, which is a lot of added stress to put on what was still fundamentally an adaptation of the earlier transmission.

      I’m not sure how your last statement follows. The THM350, which didn’t arrive until five years or so after the RHM expired, was effectively a replacement for the Powerglide and Super Turbine two-speed automatics, and in that sense were an improvement in most respects. (There have been some harsh criticisms of the later TH200, but that’s a different story.) Since most rivals had long since offered three-speed automatics for most engines, the TH350 was also arguably overdue. It wasn’t quite as heavy-duty as the TH400, but it wasn’t designed to be, trading off some torque capacity for lighter internals and lower power consumption.

    2. I would disagree; I had very good luck with the THM350 in my 1973 Nova 350; it reached 185,000 miles, with no issues other than some fluid leakage. Shifting was still quick and firm. I have not heard of a lot of issues with this tranny.

      1. The lighter TH200 has gotten a pretty bad rep, but I’ve never heard anything particularly bad about the TH350.

  24. I had a 1949 buick super with dynaflow, four door. It averaged about 8 mpg. It took everything I earned as a super market clerk to keep the transmission running, most repairs were $300 to $400.

  25. Studebaker developed their own automatic and introduced it in 1950. Ford wanted to license it, but Studebaker turned them down. Studebaker started using the Borg Warner later, when manufacturing costs of theirs got too expensive. If I recall, a European manufacturer bought the tooling, and used it in their own cars?

    1. I believe the Studebaker automatic became the basis of the Borg-Warner DG, which was used on a number of British and European cars of the ’50s.

  26. Thanks so much for the great overview.

  27. Great job like the article ? would you have any info on the olds roto hydromatic . I have a 62 any m having some small issues
    Thank you Mike

    1. I’m not able to help with any kind of troubleshooting or repairs, sorry!

  28. Thanks again for a great resource. I find myself returning to it for a periodic refresher when a relevant vehicle appears. (Today’s is a 1961 Buick.)

    1. Thanks, Ed! I’m actually in the process of updating this article as I recently did with the Hydra-Matic story, to fix some minor factual glitches, clarify the technical details (which is a major project, let me tell you), and add some new info.

  29. All this effort and expense just so drivers don’t have to clutch and shift? Turns out major beneficiaries of automatic transmissions are texters. Who cause many of the accidents on the road now!

    1. Given the timeframes of the respective inventions, I would said that definitely constitutes an unanticipated side benefit…

  30. I believe that the first automotive use of planetary gears was in the Model T. As I recall, you would press down on one pedal to get the car going (1st gear), then move the gear lever and let the pedal up for high gear. It wouldn’t have taken much to use a servo to make these motions and a combination speed and throttle position sensor to determine when to make them. That could have been an early two speed automatic. The original Hydra-Matic is just a more sophisticated, four-speed version with a fluid coupling, isn’t it?

    1. That is how a Model T transmission worked, although it was not the first automotive application for epicyclic transmissions; a number of other cars, including Cadillac, used planetary gears before the Model T was introduced. (I’m always leery of pointing to anything as The First just because it’s often wrong unless you add a lot of qualifiers — a surprising number of innovations were tried or at least considered decades earlier than you might expect, even if manufacturing or machining technology wasn’t up to making it work.)

      It is certainly true that Henry Ford remained a stubborn proponent of planetary gears, which he continued developing for tractor use even after he was persuaded to allow a conventional gearbox in the Model A. (One of the engineers who worked closely with him in that, Howard Simpson, went on to design and patent the “Simpson gearset,” licensed by many other manufacturers including GM and Mercedes-Benz.) However, the Model T certainly wasn’t automatic and it would have needed some other control mechanism to execute shifts without driver intervention.

      As Part 1 of the Hydra-Matic article touches on, there were various efforts to do that, many of which used planetary gears because the brakes and clutches could be controlled hydraulically, electromagnetically, or by some other remote mechanism. So, there is a parallel, but it only goes so far and there were a lot of steps in between.

  31. Minor glitches: The TH 400 was used by Buick AND CADILLAC in 1964. The variable-pitch stator was not used on the TH 400 in ’64, but was available on some Olds, Buick, and I guess Cadillac vehicles from ’65–’67. Ironically, the variable-stator design was used on the “big” engines in the more-expensive cars; the small-blocks and six-poppers needed the torque boost more than the big-blocks.

    For the record, the ’64 TH 400 uses a substantially-different valve body and in-case fluid channels than the ’65-newer TH 400. The valve body of the front-wheel-drive version (the TH 425) uses the ’64-style system. Therefore, a “shift kit” for a 65-newer TH 400 won’t fit a ’64 TH 400 or the TH 425, but a shift kit for a TH 425 will work in a ’64 TH 400.

    The TH 350 was actually a joint development of Chevrolet and Buick engineers, both divisions looking for replacement of the two-speed transmissions they were currently using (Powerglide and Super-Turbine 300) with the resulting “350” produced by the Hydra-Matic Division.

    1. Thanks for the notes — I’m aware of both of the errors you note and they’ll be fixed in the extensive revamp of this article on which I’m currently working. (See the most recent post for details.) I won’t be getting into a detailed discussion of Turbo Hydra-Matic in the revised version, which is already monstrously long and has been eating my brain for months.

      TH400 wasn’t offered on all 1964 Cadillacs, incidentally; it was initially available only on De Ville, Eldorado, and Fleetwood Series Sixty. I wasn’t aware that the TH425 used the original valve body pattern, though. (I know generally how the TH425 is laid out, but I can’t say I’ve ever looked at its hydraulic control layout.)

      1. Okay, the revision is now complete and those corrections are now reflected in the text.

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

  33. I think I can appreciate how big an undertaking revising this article has been. Hats off to you Aaron, for possibly the best explanation of early GM automatics expressed in laymans terms.
    GM didn’t swallow its pride and licence the Simpson system and tried to develop practical cost effective alternatives in its various divisions until the ’60s. Seems a classic case of corporate wilful blindness until we remember hindsight is the only exact science.
    In 1966 “Motoring Which?” the UK’s equivalent to “Consumer Reports” published a test of three 1.5 liter automatic British sedans, a Ford, a Hillman, and a Vauxhall. Vauxhall is the UK subsidiary of GM. The Vauxhall had a GM two speed transmission, the others both used a Borg Warner 35 three speed. They noted that they all had slightly worse performance and fuel economy than their stick versions, but the Vauxhall also had a big gap in its performance between 35-50 mph just when it was most needed. It was likened to driving a stick four speed using only second and top gears. The article also mentioned “Consumer Reports” had harsh words for GM cars using two speed transmissions, I’m guessing Ford, Chrysler, and AMC had all switched to three speed transmissions by then?.

    1. By 1966, I think Ford’s two-speed Fordomatic may still have been available for the cheapest U.S. Falcon models — I would have to double-check, as it may have been dropped after 1965 — but otherwise the other U.S. automakers all had smaller three-speed units for their low-end cars by then. (The light-duty TorqueFlite was one of the big pluses of Chrysler’s compact Plymouth Valiant and Dodge Dart, in my view.)

      The general attitude of GM engineers in this era was that a two-speed torque converter automatic was a perfectly reasonable substitute for a three-speed manual transmission while being simpler, lighter, and cheaper than a three- or four-speed automatic. The latter was of course perfectly true and the former was at least a supportable position. I also suspect some of the transmission engineers were soured a bit by experience with the small three-speed Hydra-Matic, which was little better than a decent two-speed automatic. (The transition from the smaller three-speed unit in the 1961–1963 Y-body Oldsmobile F-85 to the two-speed Super Turbine 300/Jetaway in the 1964+ A-body equivalent was certainly no great loss and probably an improvement in some respects.) On the other hand, by the mid-sixties, very, very few Americans still bought three-speed manual transmissions and it was certainly clear that a good three-speed torque converter automatic was considerably better than the best two-speed. It was also a bigger deal for non-U.S. cars and the later U.S. ventures into the “subcompact” [sic] realm, since having 3 or more liters’ displacement to fall back on masks an assortment of deficiencies.

      I don’t think GM was willfully blind so much as having a fair bit of (understandable) inertia. As this article should hopefully make very clear, GM had invested an absolutely staggering amount of money in automatic transmission development and engineering, accumulating a towering stack of basic patents. The tooling alone was a king’s ransom — in the early fifties, Detroit Transmission built more Hydra-Matics each year than the entire contemporary British auto industry built cars, and that wasn’t even GM’s only automatic! So, a reluctance to completely reinvent the wheel or to unnecessarily license outside technology isn’t difficult to understand. (To be clear, what GM licensed from Simpson and Simpson’s estate was a specific arrangement of planetary gears, not a complete transmission. Part of the reason that arrangement ended up being so widely licensed was that Simpson, like Pol Ravigneaux a decade or so before, had patented many different variations that there was no getting around them.)

      1. I’d forgotten three speed manual transmissions were still commonplace in the USA in the timeframe we are discussing. A two speed automatic makes a lot more sense then.
        I wasn’t suggesting GM was willfully blind, but had missed a trick in not adopting the Wilson system (or at least parts of it).
        As you say, GM spent vast amounts developing their transmissions. I wonder how much it cost Chrysler Corp to licence and develop their transmissions, which I think were superior to any other automatic transmission available at the time.

        1. To be clear, what’s commonly called a “Simpson gearset” really just refers to any compound planetary unit sharing a single sun gear, just as a Ravigneaux gearset is a compound planetary unit sharing a planet carrier and at least one planet gear. There were actually multiple variations of each, most of which Howard Simpson and Pol Ravigneaux dutifully also patented. While each of those layouts has certain advantages, particularly as regards packaging and cost, the invention, as was, didn’t encompass how the gears were selected and chosen. In fact, while there were a bunch of automatic transmissions that used these gear layouts, including Chrysler’s TorqueFlite and GM’s Turbo Hydra-Matic, each was quite a bit different. So, the credit for the functional effectiveness of TorqueFlite or Turbo Hydra-Matic really goes to the Chrysler and GM engineers who developed them. I’ve never seen anything to suggest how much any of the companies paid to license Simpson’s gearset patents, although there were so many users that if there was any kind of per-transmission royalty, Simpson and his estate would have made out quite handsomely.

          Developing an automatic transmission was a very costly business in general, I have no doubt, but in Chrysler’s case, they developed fewer of them — the original PowerFlite two-speed torque converter automatic, the early iron case TorqueFlite, and then lighter aluminum TorqueFlite units with a variety of evolutionary changes — and used them across all the automotive models. GM, by contrast, had three distinct transmission families (Hydra-Matic, Dynaflow, and Powerglide) that each went through several generations and iterations, each notably different, but with a lot of what a software designer might call legacy features. (The outliers there were Turboglide and Flight Pitch Dynaflow, which were not “clean-sheet” designs in a conceptual sense, but shared little with Powerglide and earlier Dynaflow transmissions mechanically and later contributed various ideas and some components to subsequent versions.)

          The three-speed manual transmission occupied a very peculiar space in the American automotive firmament in the sixties and seventies, being simultaneously ubiquitous and rather uncommon. It was notionally standard on a great many cars into the late seventies, but you’d hardly ever see one. The real rationale for its existence, so far as I can tell, was to allow a greater retail markup on the automatic transmissions (or four-speed manual transmissions) most people actually bought. By this point, no one pretended that Cadillac or Imperial buyers would have a manual gearbox, even the carriage-trade versions, but the three-speed was still nominally standard equipment on some quite improbable big sedans.

  34. Great job Aaron, you’ve outdone yourself. I enjoy coming to this site to expand my knowledge. It’s a fantastic resource indeed. I also enjoy your clarifications on “Curb Side Classics” and can faithfully know that any input you offer will be well reasoned and researched. You offer a great service to like minded Auto Industry nuts.

  35. Wow! My brain has tech-overload.I’m going to have to re-read the article in sections to have any hope of absorbing all the new information. Fantastic job on the revision, Aaron, it was well worth the wait. Thanks for the monumental effort!

  36. Great article! One point of contention is some of the THM-400 transmissions fitted to Chevrolets did have the “switch-the-pitch” feature I remember working on a 67 Impala station wagon, with the 327″ engine and THM 400 which had the pitch angle switch on the throttle linkage. This was in the early 1970’s and this appeared to be an O.E. Installation on a stock automobile.

    1. Hmm. To be honest, I had thought until this afternoon that TH400 wasn’t offered with the 327 at all — a number of vintage car magazines complained about that, in fact — but I found one brochure that indicated the 327/THM combination was indeed optional on the ’67 Impala and Caprice. (It may have been a midyear or late introduction.) I’ve never seen any indication that the TH400 fitted to the big Turbo-Jet engines (396/427) had the variable-pitch stator, but it’s possible the ones used with the 327 did. If so, it was likely short-lived, as the switch-pitch stator was dropped for 1968. However, a 327 with switch-pitch THM actually sounds like a pretty nice combination. It would be much more flexible than Powerglide, that’s for sure!

      (I tried very hard not to get sucked into a more involved discussion of Turbo Hydra-Matic in this article for what I imagine will be obvious reasons, but I wanted to mention the variable-pitch stator because it was really one of the only Dynaflow/Twin Turbine/Turbine Drive features to survive into the later era.)

  37. I was under the impression that Chevrolet division never used the variable-pitch stator design, but regarding the 327/THM combo for big Chevrolets – it seems likely. Olds offered the THM 400 as an option on it’s small-block (330/350) powered 88 models for sure in ’67 & ’68, not positive about ’65-66. Both my ’67 Delmont 88 330 and my ’68 Delmont 88 350 came with THM400’s rather than the usual Jetaway 2-speed (ST300). The ’67 is a variable-pitch model, the ’68 is fixed. In normal operation, I don’t really see a pronounced performance advantage to the variable-pitch stator.

    1. The other divisions’ experience isn’t necessarily suggestive regarding TH400 availability. Buick, for example, offered it on the smaller-engine LeSabre (with the 300 cu. in. engine) as early as 1964, whereas the loosely comparable Oldsmobile Jetstar 88 was available only with the two-speed in ’64 and you could still get Jetaway on a base-engine Delta 88 until 1969. Chevrolet didn’t offer Turbo Hydra-Matic at all until mid-1965 and until 1967, it was only available on full-size cars with the 396 or 427. I think part of the rationale was that TH400 was bulkier and consumed more power than Powerglide (hence the later TH350), although the 327 obviously could have benefited from an extra gear.

      When Oldsmobile dropped the variable-pitch stator for 1968, they also gave both Jetaway and TH400 higher-ratio torque converters, so there really isn’t much difference in all-out performance. The point of the variable-pitch stator vanes was to keep the converter “tight” in gentle driving while still providing extra multiplication for fast starts or quick bursts of acceleration, even if you were over the maximum kickdown speed. With the kind used on Turbo Hydra-Matic and Jetaway/Super Turbine 300, it also limited creep on a closed throttle. (The old Buick and Turboglide stators variable couldn’t do that because the stator servo valve was triggered by throttle movement rather than electrically.) So, it was about flexibility more than anything else.

  38. Terrific article with this latest revision!

    The first car I can remember was a ’56 Oldsmobile and by the time I was 8 years old or so my dad had described to me how the “fill and flush” coupling worked in cushioning the shifts. Anytime we were driving I kept track of which was in use. Walking to school I would hum to myself as I walked, imitating the engine speed ramping up in each gear, pretending to be a car with Hydramatic.
    The Oldsmobile was replaced by a Buick LeSabre. We ended up buying the “400” version in order to avoid the two speed automatic. The “switch the pitch” stator was what got Dad’s attention in this car (even if its actual operation wasn’t very noticeable).
    Stuff like this is what motivated me to become a mechanical engineer.

    Thanks for all of your work. It brings back good memories.

    1. Thanks, Chris. I can see that the Controlled Coupling Hydra-Matic would be sort of a crash course in mechanical engineering, since it has a little of just about everything. Bands! Couplings! All kinds of clutches — disc, multi-disc, cone, and sprag! If it had a torque converter and a lockup clutch, it would be a veritable omnibus of early automatic transmission ideas. (If they’d used Walter Herndon’s lockup clutch concept, it wouldn’t have been a complete lockup in the sense of a modern torque converter; it would just have locked out the smaller coupling.)

      What I love — and GM accountants presumably did not love — about the second-generation Hydra-Matic is that it incorporated a bunch of changes that make its basic operation smoother and mechanically simpler, but each change then required a bunch of belts-and-braces stuff to make up for the minor drawbacks created by the simplification, such the need to still use separate overrun brakes so as to not end up freewheeling down every steep hill. It’s a useful reminder that just because something is cleverer doesn’t necessarily mean it’s better.

  39. Great information.
    Drawing on personal experiences from cars my friends and I owned when we were young men two speed automatics, mostly powerglides, were something we wanted to get rid of if we could afford it. I had a ’65 Pontiac Laurentian (283-2 speed) ’64 Chev Impala SS (283-2 speed) and a “68 Camaro ( 327-2 speed).
    I put a Turbo 350 in the Camaro later and it was a nice addition.
    I know the racecar guys like them but we had full size ’60’s sedans with 283’s and 235’s, not 800-2000 horsepower racecars.
    To this day ( I’m 60) I would rather have a manual than automatic transmision I think because of powerglides.
    In the late ’80’s I learned about Variable Pitch converters some Turbo 400s had, bought the pieces from Kenne-Bell, converted my ’80 GMC ( 350, later 454) heavy half and ’74 Olds (455) Delta 88 convertible over to them. I also added the 2.75 low first gear kit to the Olds also because I’m married to the 2.73 rear gears ( 9 3/8 ring gear diamter) so I’m looking for mutiplication wherever I can get it.
    With the warmer than stock cam (268 Comp Cams) It gives me way better traffic drivability than I had before, particularly when towing a trailer on holidays.
    According to a book I once had, it claimed the fixed pitch 400 converter stator angle is 24 degrees if my memory serves me correctly. I think the Variable Pitch swings between around 18-26 degrees. I have to get another book to be more accurate. I have a variable pitch stator and if you put it through its motions you can see how it would give different stall angles, all you have to do is compare it to boat or aircraft propellers.
    According to my information a fixed pitch 400 converter gives up to 2:00-1 multiplication and variable pitch goes up to 2.5:-1. That helps in a heavy car with tall rear gears.
    Over the years i’ve been to a few “burger stand or shopping mall car shows” and described the variable pitch converter system the guy has on his car and he generally has no idea what i’m talking about.
    Some years of Oldsmobiles (the ones I’m most familiar with) had a switch in the speedometer cable and was in high pitch until a certain speed and some had it in their throttle linkage.
    If one is not careful when they have their transmission rebuilt the variable pitch stuff is not put back in and fixed pitch stuff substituted.
    Transmission repairmen, if not familiar with it tend to think it’s an earlier fluid coupling and primitive garbage from the days before “real” transmissions were made. They”re usually pushing a modified TH700R4 which, in my humble opinion, is not designed for a big motor in a heavy vehicle.
    That being said, the decendant, the 4L60E, is doing just fine in my stock ’96 Impala SS and that 700 would have been a huge improvement in our old ’60’s cars.
    However, some know exactly what that VP is, and if the owner has no idea what he has and someone they know wants one, it’s gone.
    This happens to the factory low first gear kits that are in motorhomes and heavier trucks too.
    The variable pitch really shines when you run more cam or a turbocharger, in high stall they let the engine get above 2500 rpm before they stall and let the engine wind up, making more power.
    In high stall it’s too high to have all the time and in low stall it would leave you wanting more in stop and go traffic, particularly when towing something, but together a nice blend.
    They, along with 2.75 or 3.00:-1 low first gear and overdrive kits were the darlings of the motorhome crowd until heavy versions on the overdrive automatics came along. Those in the know had them, guess where some of them pieces came from. Not everyone in this world has scruples.
    GM made two sizes, the mid size “A bodies” had 10 inch and full size sedans had 12 1/2 inch.
    The big fixed and variable pitch torque converters were the same size and the stators interchanged but the varibable pitch ones were referred to as 12 1/2 inch and the fixed pitch ones as 13 inch.
    I was told the reason was that’s how GM differentiated between the two.
    Several things that I have read over the years described the phasing out of the variable pitch according to GM was it was “a feature that only engineering types seemed to understand plus some customers complained about the whirring noise they made”. And,” with the new large displacement engines coming out it is unecessary”.
    Why spend money on a feature something very few people understand?
    With an overdrive kit, and a variable pitch a TH400 becomes a 12 speed. Not a cheap proposition though.
    Thank you and enjoy.

    1. Thanks for your thoughts, Wayne. The pitch angles of the TH400 variable-pitch stator were 32° and 51°, at least as GM measured them. Fixed-pitch TH400 converters actually varied quite a bit in stall ratio depending on the application, from 2.00 to about 2.50:1 for street applications, whereas the standard variable-pitch units were 1.8/2.2. The switch-pitch stator didn’t necessarily mean greater maximum multiplication. As you note, the main advantage is that you have the higher stall speed when you need it and aren’t stuck with high-stall converter blues the rest of the time.

  40. A monumental amount of work involved in this revision. A labor of love really. Congratulations on unraveling the details in all these GM transmissions, and presenting the results so clearly.

    My further kudos in your even responses to comments where old wives’ tales and “my friend the transmission overhauler tells me you’re wrong” comments seem intent to belittle you. Haven’t seen anyone conclusively prove you incorrect, possibly because you know about 10 times more than they do, and I’m speaking as a retired mechanical engineer who’s had people who just don’t understand that they don’t understand try to sell me a line of magic dreamed up in their heads! It’s how myths and legends are born. AWD systems seem to be completely misunderstood by just about everyone but the engineers who designed them, for example. Especially that particular group of people known as Marketing and their adjunct advertising copywriters.

    If one goes back a bit further to the brief time interval between synchromesh and the first Hydramatic, my speculation for the real reason an automatic transmission was needed was because so little effort was ever applied to designing a half-decent shift linkage and low clutch effort. That’s why people hated driving those clunkers – they were awkward to say the least. Try a ’49 Pontiac three-on-the-tree. Blech.

    So when we youngsters got to drive Austins and euro Fords in the 1950s and heaven! the first Volvo 4 speed manual, the ease of use was outstanding compared to the US stuff. No longer was shifting a chore, it was fun, column or floor shifter. I mean Chev thought the Powerglide more important to introduce than replacing the oil dippers on their six cylinder engine and giving it proper full pressure lubrication, so designing an ergonomic manual shifter was obviously beyond them. Strange attitude to me other than dreams of golden showers of dollars raining upon them for presenting no-shift motoring at a premium.

    Even early to mid ’60s 4 speeders needed a manly-man to shift their obdurate levers. No snickety-snick there. The Corvair 4 speed was an outright laugh compared to the Volvo, but in those days the scorn heaped on “tiny” foreign cars meant Americans in general somehow believed that foreign ideas came from the dark ages and were no good. Same in Canada where I live and lived through endless Ford versus Chev arguments in both high school and college where nothing was ever settled.

    All that personal reflection aside, I must reiterate you’ve put forward a first class effort here and deserve much praise. It’ll probably become a reference work.

    1. Thanks, Bill. It’s certainly true that the shift linkages of domestic cars had a lot to do with the preference for automatic. Three-on-the-tree is mildly amusing to the modern driver as a novelty, but a regular dose of it — particularly with a non-synchronized (or indifferently synchronized) transmission — would be a strong argument for Powerglide. As for the sixties four-speeds, I assume part of the problem was that they were intended primarily for racing homologation or drag racing, rather than something your average consumer might buy (a thesis strongly supported by the fact that a four-speed typically cost as much as or more than automatic).

      On the other hand, there’s a strong argument to be made that automatic transmission is a natural evolutionary development of automotive technology, just like, say, automatic spark advance (another development that was still fairly recent when Hydra-Matic first came on the scene). Even with excellent modern five- and six-speed gearboxes, effective synchros, and low-effort clutches, it’s hard for me to argue that manual shifting is a lot of work of a kind many drivers are perfectly happy not bothering with. The strongest arguments for it, aside from it being a moderately entertaining diversion, are that it makes the most out of smaller engines without a lot of torque and that it spares you the exasperation of delegating a complicated chore to an automated subordinate of often questionable judgment, both of which have become progressively weaker as engine and transmission technology improve. (I say this, mind, as someone who has never owned a car with automatic transmission and who had to learn to drive on a manual gearbox.) So, I can understand, though not really defend, why Detroit engineers treated manual transmissions as a legacy system only being (grudgingly) retained for buyers too cheap to pony up the extra $200-ish.

      (What’s harder to understand, frankly, is that GM let O.K. Kelley and his guys keep churning out different automatic transmission designs of several very different flavors for an astonishingly long time before they finally decided to consolidate around yet another, mostly unrelated design!)

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