By on May 3, 2022

A few weeks ago, we concluded Abandoned History’s two-part coverage of the Chrysler UltraDrive transmission. Within the comments was a request for more transmission coverage of an equally abandoned nature. Let it be so! Come along as we discuss the vast automatically shifted expanse that was the Turbo-Hydramatic transmission family, by General Motors.

The Turbo-Hydramatic was a consolidation solution at GM in the early Sixties. The new automatic was intended to replace the aged Hydra-Matic transmission, as well as the Dynaflow, a Buick box. The original Hydra-Matic was developed by Oldsmobile and Cadillac and was introduced for the 1940 model year. An important step forward, that gearbox was the first mass-produced transmission that was both fully automatic and meant specifically for passenger cars. It was also the first automatic that used hydraulic fluid. A great success, the Hydra-Matic was used in Cadillacs, and Oldsmobiles, and was an option in Pontiacs in the late Forties.

Developing a high-tech automatic transmission was a costly enterprise, and GM sold the Hydra-Matic to smaller and leaner companies like Hudson, Nash-Kelvinator, Willys, Kaiser, Lincoln, and even overseas at Austin. They applied the Hydra-Matic to the luxurious Vanden Plas Princess through 1968.

Dynaflow appeared later than Hydra-Matic and debuted for the 1948 model year. A transmission exclusive to Buick, it was initially available only on the high-end Roadmaster. A two-speed box, Dynaflow used a torque converter at all times. Low gear was held through 60 miles per hour when the driver shifted to Drive to put the transmission into second. It was considered an automatic even though a shift was required because there was no clutch.

Dynaflow was very smooth, but also very inefficient and slow. At the time, Buick engineering was permitted to emphasize smoothness above all else: Other automatics of the period were largely jerky and unpleasant. The Dynaflow was not exceedingly popular, and absolutely needed replacement by the Sixties.

The so-called Turbo-Hydramatic (hereafter THM) built on the name of the original from the Forties and continued as a three-speed auto for passenger car applications. In reality, though it shared a familial name it was not mechanically related to the old Hydra-Matic.

The first of the THM family was the 400, which quickly became the legendary global star of the THM line. Its first usage was in Cadillacs for 1964. Worth noting, the same transmission when used in a Buick was rebranded as the Super-Turbine 400. The transmission was exclusive to Cadillac and Buick for its first year and spread to Oldsmobile, Pontiac, and some larger Chevrolet models in 1965.

THM400 was made of a combination of aluminum and iron and weighed 135 pounds without fluids. Its entire case was made of cast aluminum, and the bell housing was integrated into the transmission. Key to the THM400’s internal strength was a cast-iron center support, which suspended the transmission shafts.

The new Turbo-Hydramatic three-speed automatic used the Simpson gear design. The Simpson gearset is a compact and mechanically simple design, and it requires a minimal number of bearings compared to other designs. That meant it was lighter, cheaper to produce, and had less internal friction due to its low number of bearings. The design quickly became the standard in automatic transmissions and was used (earlier) in Chrysler’s TorqueFlite line.

Ready to get into the transmission weeds? Early examples of the THM400 produced from 1964 to 1967 sometimes had a special feature. It was not implemented on all examples, but a good number had a “Switch-Pitch” torque converter. The Switch-Pitch converter’s party piece was its variable-pitch stator. Hydraulics changed the angle of the fins on the stator of the converter, which moved the stall speed up or down by up to 1,000 RPM while in motion.

A revised stall speed was effectively the same as altering the gears of the transmission or the rear end’s ratio while in motion. A high stall was advantageous for fast acceleration from a standstill, and the converter automatically lowered the stall speed for efficient highway cruising. GM used the special converter previously on examples of the Dynaflow from 1955 to 1963, and on the two-speed Super Turbine 300 from 1964 to 1967. The innovative Switch-Pitch was discontinued after 1967. The probable cause of death was tied to high manufacturing costs on a component that was not a necessity at the end of the day.

Even without the special torque converter, the THM400 continued on in overall GM usage. Most examples were of the Cadillac, Oldsmobile, Buick, and Pontiac variety. Cars from those four brands used one bellhousing design for the THM400. Chevrolet put the THM400 in its heavy-duty trucks and vans for a time and used a different bellhousing. Its use was limited on Chevrolet passenger cars and normally reserved for high-performance small-block vehicles, such as the Corvette.

Outside of GM vehicles, the THM400 built a reputation for utmost toughness, reliability, and longevity. It became a favorite of other brands that required an automatic transmission capable of shifting big power, torque, weight, or some combination of all three. We’ll start with British examples.

Between 1965 and 1980, Rolls-Royce used the THM400 in the Silver Shadow. After that, they ported it over to the Shadow’s successor the Silver Spirit, which remained in production through 1992. Because Bentley was under the same ownership at the time, the equivalent Flying B offerings with the THM400 were the T1, T2, Mulsanne, Brooklands, Turbo R, Corniche, and Continental. Jaguar also implemented the THM400 in its 12-cylinder XJ12 from 1977 through 1994, and on the XJ-S from 1977 through 1990.

Ferrari supplied its first-ever automatic transmission upon the introduction of the 400 Automatic coupes in 1976 and used the THM400 to wrangle the V12’s power. The 400 was updated to the 412 in 1985 and continued to use the THM through the model’s cancellation in 1989.

The THM400’s most illustrious usage was probably in Japan when it was selected for a very special limousine by the name of Nissan Prince Royal. The Royal was built between 1966 and 1967, a total of five examples were made especially for the Japanese imperial household. All five cars required a transmission capable of handling the 6.4-liter V8 engine, and THM400 was the obvious choice. The Royal is worth its own Rare Rides entry.

And finally, we circle back to some additional American THM400 usage outside GM. AM General found a use for the THM, where it became the automatic of choice for the Humvee and earlier examples of the civilian Hummer H1. AMC applied it to various models within the SJ platform, like the large and in charge Wagoneer and Grand Wagoneer, as well as the Gladiator pickup. Given AMC’s limited budget, early THM400s in Jeeps required an adapter between engine and transmission bell housing to make it fit. AMC ordered up its own housing later for inline-six and V8-powered Jeeps.

But nothing lasts forever, and by the turn of the Eighties, the THM400 was being phased out for passenger car applications. It was heavy, and the government made new demands for efficiency in passenger cars that weren’t friendly to a Sixties rear-drive three-speed transmission. Later in THM400’s life, it changed identities, spawned some updated variants, and set the stage for the rest of the THM transmission line. More on that in Part II.

[Images: GM, Rolls-Royce, Bentley, Jaguar, Jeep]

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27 Comments on “Abandoned History: General Motors’ Turbo-Hydramatic Transmissions (Part I)...”


  • avatar
    dukeisduke

    The THM400 is a great transmission, and is still one of the top choices for hot rods, for high torque applications. And switch-pitch converters are still desirable.

  • avatar
    thornmark

    “The new Turbo-Hydramatic was the first three-speed automatic with Simpson gears. The Simpson gearset is a compact and mechanically simple design, and it requires a minimal number of bearings compared to other designs. That meant it was lighter, cheaper to produce, and had less internal friction due to its low number of bearings. The design quickly became the standard in automatic transmissions and was used (earlier) in Chrysler’s TorqueFlite line.”

    hard to follow that

    how could it be the first 3 speed w/ Simpson when Chrysler already did it w/ the TorqueFlite – seems a tortured way of giving GM credit for something you write that Chrysler had already done

  • avatar
    Jeff S

    Adam on Rare Classic Cars covers these transmissions. https://www.youtube.com/watch?v=WMhV8Eoe8rA&ab_channel=RareClassicCars%26AutomotiveHistory

  • avatar
    ajla

    The thing about 3 speeds is that it isn’t enough speeds. With shorter gearing highway driving sucks and with taller gearing around town acceleration suffers.

    Despite it being maligned by journos I actually quite enjoyed the 5A in my Charger R/T. It maybe needed a touch more final drive but overall it was a good setup. That trans was around for a long time so maybe you can do something on that one some day?

    • 0 avatar

      That’s the Mercedes 5G Tronic isn’t it?

      • 0 avatar
        ajla

        Yes. Wikipedia says it came out in ’96 but I’m pretty sure it was available in the ’94 S320. The last ChryslerCo application was the 2020 Charger Pursuit and I think that was its last use unless you turn up something in Bulgaria still using it.

        Confirmed, 1994 S320 offered the 5A (pg23):
        oudemercedesbrochures.nl/W140_USA_1994.html

    • 0 avatar
      Mike Beranek

      Three gears is more than enough when you’re pumping out 400+ foot-pounds of torque at 1500 RPM. It was the perfect transmission for big-cube V-8s.

  • avatar
    relton

    The Simpson gearset was first on the 1950 Studebaker Automatic Transmission. Studebaker developed their own automatic transmission, and licensed Borg Warner to manufacture it. It had the basic layout for automatic transmissions for the next half century, or more. Simplified torque convertor, 3 speed planetary gearset, and a lever control that was PRNDL. Chrysler’s Torqueflite was the first to use overrunning clutches for the 1st gear ratio, so that 2nd gear was engaged without disengaging 1st. Th THM400 used the same arrangement for all 3 gears and used clutches for all normal shifts instead of a band like Chrysler. The THM400 had a much mor sophisticated shift arrangement, using all 3 gears on slowdown, instead o jumping from 3rd to 1st, as many other transmissions did. THM400 was also the first to use a vacuum modulator instead of a throttle linkage. THM400 also developed tooth profiles for the park pawl and gear so that if the driver attempted to put the lever in Park at speed, the teeth would no engage until the car speed was nearly 0.

    I started my career at Hydra-matic in 1965, and I’m very familiar with these transmissions.

    The THM425 was also a development of he THM400, used for the front wheel drive Toronado and ElDorado.

  • avatar
    islander800

    My one experience with owning a THM400 was with a 1967 Riviera with 72K original miles I obtained in 2001 in excellent original, having spent its life in Florida.

    Matched with the new-for-1967 Buick modern free-breathing big block 430 cu. in. V8 (pushed to 455 by 1970), it was extremely smooth and shifted flawlessly – until the transmission cooler lines that ran through the original radiator corroded enough to allow coolant to circulate through the trans. A simple trans rebuild and rad re-core later, and a new separate rad for the transmission, it was back in business. The 430 was a torque monster, putting out 475 lb.ft., and the THM400 was a perfect match to get two tons up to speed with gusto. It’s still, I understand, a transmission of choice for drag applications (along with Chrysler TorqueFlites) because of its bullet proof reputation – as is that Buick big block: it has a high nickel content, meaning lighter and stronger than 100% iron, plus massive main bearings, making it virtually indestructible as well. The 1967 Riviera had the Switch Pitch THM400 and it almost seemed like it had another whole set of gears when it kicked in but I can understand it was not cost effective for GM as it wasn’t really needed.

    If GM did nothing else right, they did invent one of the best automatic transmissions ever.

  • avatar
    MRF 95 T-Bird

    I owned a 66 Pontiac Tempest with the 230-OHC-6 and the Super Turbine 300. Pontiac referred to it as Tempest torque and Oldsmobile called it Jetaway. It was air cooled and didn’t have the cooling lines that ran to the radiator.
    Even though it was a two-speed aluminum unit it didn’t share much with in common with the Chevrolet Powerglide. The earlier Powerglide until 62 had an iron case. I know that first hand from wrestlling the one out of my dads 54 Chevrolet using my skateboard.

    • 0 avatar
      Wizweird

      Omg! Had a friend with that same setup. Guy brought the car in, says it makes a funny noise going around corners. We jump in, and the first corner, he nails the gas to the floor. The column shifter jumps literally out of drive, into reverse, made a racket, then pop! Back into D! He’s like I can’t figure it out! He never saw the shifter move! A couple of motor mounts later, it was good as new! Ty for bringing back that memory

  • avatar
    css28

    My dad bought a ’66 Buick LeSabre to replace our ’56 Oldsmobile and ended up buying the LeSabre 400 because he couldn’t stomach a 2 speed automatic that would have come with the base trim. As I recall, the fuel mileage was pretty bad, as the 400 package included a more powerful engine with 4 bbl carburetor.

    I remember him boasting about the “switch the pitch” torque converter at the time.

  • avatar
    rpol35

    A few thoughts on the THM 400. I have owned cars with both the switch pitch and non-switch pitch torque converter and could never tell the difference.

    Chevrolet used this almost indestructible box more on big-block cars than small-block-equipped examples. True, SBC Corvettes from ’68 to ’76 employed it, as did ’70-’74 Z28 Camaros but it made its mark in B-Body big-block vehicles as well as A-Bodies like the SS396 Chevelle and El Camino.

    Fun fact, in ’68 Chevrolet, made it available on the lowly 307 CI V8 if installed in an Impala or Caprice. About twenty-five years ago, I looked at a very clean ’68 Impala SS coupe that was for sale and it was so equipped. I was really surprised at that configuration! The THM 400’s only downside is its parasitic nature, it consumes a lot of HP to spin – a minor price to pay for its strength and smoothness.

    Thx for the post, I enjoyed it!

  • avatar
    bumpy ii

    Fun fact: GMC offered the 400 with the big-block V6 from 1966 to 1969 (and maybe later?), and used a factory adapter plate to mate the (probably) Chevy case to the V6 block.

  • avatar
    28-Cars-Later

    Very nice work Corey.

  • avatar
    Aussie V8

    Great article, however you have completely neglected to mention GM usage of the transmission outside of North America. In Australia GM’s local subsidiary Holden used these transmissions extensively in their locally manufactured cars from 1971 until 1977 when they were replaced by the Turbo 350, and later the locally manufactured Tri-Matic (Marketing name of the THM180 in Australia).

    First they were fitted behind the imported 350CI Chevy V8, however later they were fitted behind the Holden manufactured 308CI 5.0L V8. The Holden V8 blocks were cast with either the “Turbo” bellhousing pattern at the back of the block, or the smaller “Tri-matic” pattern which was fitted to the engines supplied with manual or later Tri-matic automatic transmissions.

    The THM400 equipped cars were noted for their smooth shifting and bullet proof reliability, although they certainly sapped more horsepower and used more fuel than the later Tri-matic cars.

    • 0 avatar
      eng_alvarado90

      He didn’t, though. He mentioned its extensive usage on British luxury marques as well as Ferrari.
      The THM400 was almost ubiquitous hence I’d understand Corey can’t mention everything it was put in.

      • 0 avatar
        Aussie V8

        I said that he didn’t mention GM usage outside of North America. Rolls Royce, Jaguar and Ferrari are not GM subsidiaries.

        I’m sure more of these transmissions were fitted to Holden manufactured vehicles than were fitted to Ferrari or probably Rolls Royce vehicles.

    • 0 avatar
      Bike

      Trimaticcame out with the HT around 1970?? Not 1977?

      • 0 avatar
        Aussie V8

        Yes, correct. The Tri-matic was released in 1970 for the very last of the HTs. The 308 and 253 V8s could be optioned with this transmission. The 350 chev V8 still got the powerglide. In July 1971 Holden started fitting the THM400 behind the 350, replacing the powerglide. In October 1974 the 308 V8 got the THM400 instead of the Tri-matic. The smaller 253 V8 retained the lighter Tri-matic. Then the 308 went to the THM350 in May 1979. In November 1981 Holden went back to the Tri-matic behind the 308. So yes, the Tri-matic was available from 1970 to 1974, but then a 7 year gap before it was reintroduced behind the 308…

    • 0 avatar
      Bike

      Trimaticcame out with the HT around 1970?? Not 1977?

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