Hydra-Matic History: The First Automatic Transmission


By early 1939, Oldsmobile had built 5,000 preproduction cars with a new version of the AST, dubbed Hydra-Matic Drive. Internally, Hydra-Matic was very similar to the AST, but used a third planetary gearset rather than spur gears for reverse. Two oil pumps, one driven by the engine, the other off the transmission output shaft, provided operating pressure for the control valve body. A fluid coupling replaced the conventional clutch; Oliver Kelley, who was the lead designer and in whose name the patent was filed, dubbed the coupling a “fluid turbo clutch.” The coupling had an unorthodox feature that was to become a Hydra-Matic signature: To prevent the car from creeping forward at idle, the engine flywheel drove the fluid coupling impeller through the front planetary gearset rather than directly. As a result, the impeller was always turning slower than was the engine, reducing creep.

As with the Automatic Safety Transmission, Low range provided automatic shifting between first and second gear. Drive functioned much like the AST’s High range, but the governor had been reworked to shift automatically through all four forward speeds. The two-three shift was still mechanically complex and tended to be jerky if the bands and linkages were not in perfect adjustment, but there was no clutch pedal and no need for manual shifting except to select reverse or neutral.

Hydra-Matic quadrant in a 1949 Pontiac Chieftain © 2010 David Cory (CC BY 2.0 Generic)
On early Hydra-Matics, the shift quadrant offered only Neutral, Drive, Low, and Reverse. There was no Park position, but there was a parking pawl, which engaged when Reverse was selected with the engine off. (Photo © 2010 David Cory; used under a Creative Commons Attribution 2.0 Generic license)

1940 Oldsmobile Hydra-Matic chart
Gear ratios for a prewar Hydra-Matic. On postwar cars, the ratio of the rear planetary gearset was increased to 2.63, which also increased the ratio in first gear to 3.82:1.

Unlike the Automatic Safety Transmission, Hydra-Matic was not built by Buick. In early 1939, GM, correctly sensing that the new transmission would be a big hit, established a new Detroit Transmission Division to manufacture Hydra-Matic. Production development began in May and Hydra-Matic went on sale that October as a regular production option for all 1940 Oldsmobiles. At launch, the retail price for the option was only $57, which was actually $18 less than the old AST.

Hydra-Matic hood badge on a 1942 Oldsmobile B-44 club coupe © 2009 Aaron Severson
In addition to its obvious convenience, Oldsmobile claimed that Hydra-Matic returned up to 20% better fuel economy than did a standard three-speed transmission. As with the earlier Automatic Safety Transmission, the claimed edge in fuel economy was due less to the transmission and more to the 3.63 axle ratio included with it, which was almost 20% taller (lower numerically) than the 4.30 axle standard with manual shift. Hydra-Matic cars were fairly thirsty in stop-and-go driving, but gentle highway cruising could return up to 19 mpg (12.4 L/100 km), not bad for a big American car of this era. (author photo)

As Oldsmobile’s none-too-modest advertising proudly declared, Hydra-Matic was a paradigm-changing innovation. Whatever its problems — which on early cars included engine surge, a tendency to harsh shifts, and problems with the transmission’s special “Hydra-Matic Drive Fluid” — Hydra-Matic offered painless two-pedal driving. For American drivers, most of whom had never been enamored of manual shifting to begin with, Hydra-Matic was a compelling proposition, attracting even customers who’d been burned by previous semiautomatic transmissions.

As was GM policy at that time, Oldsmobile had exclusive use of Hydra-Matic for one year in recognition of the division’s considerable efforts (and expenditures) in developing the transmission. When that exclusivity period expired, Buick still wanted nothing to do with Hydra-Matic, which Buick chief engineer Charles Chayne nicknamed “Hydra-Jerk,” but Cadillac added Hydra-Matic to its options list in 1941. Despite higher prices — now $100 on Oldsmobiles and $125 on Cadillacs — more than 40% of Oldsmobile buyers and about 30% of Cadillac customers opted for the self-shifting transmission.

By the time the War Production Board halted civilian automobile production in February 1942, the Detroit Transmission Division had delivered almost 215,000 Hydra-Matic transmissions. Hydra-Matic’s influence and popularity would only increase during and after the war.

1942 Oldsmobile B44 Special Sixty-Six club coupe side © 2009 Aaron Severson
This 1942 Oldsmobile proudly proclaims the presence of Hydra-Matic transmission on big chrome badges on either side of the hood; by this time, about 45% of new Oldsmobiles were so equipped. Note the dull finish of the grille: Like many 1942 American cars, the Olds had less chrome, fewer die castings, and other changes to reduce its use of zinc and other strategic materials. (author photo)


Even as production of passenger cars was winding down, the American auto industry was rapidly converting to military production. The conversion was supervised by William (“Big Bill”) Knudsen, who had been president of General Motors from 1937 to 1940, so naturally GM played a major role in the war effort. (Ironically, 25 years earlier, Cadillac founders Henry and Wilfred Leland had resigned from General Motors after then-president Billy Durant refused to become involved with military production.)

In 1941, before the United States entered World War II, the principal U.S. light tank was the M-3 Stuart, manufactured by the American Car & Foundry Co. The M-3, which British armor units nicknamed “Honey,” weighed 14 tons (12.7 metric tons) and was armed with a 37mm cannon and four 0.30-caliber (7.62mm) machine guns. It was powered by a single seven-cylinder Continental W-670 radial engine with 262 gross horsepower (195 kW). By 1942, the Stuart was in service in North Africa and the Pacific theater with the British Army, the U.S. Army, and the United States Marine Corps.

M5A1 Stuart tank front 3q © 2005 Bukvoed (CC BY 2.5 Generic)
An M-5A1 Stuart light tank, photographed outside the Yad La’Shiryon Museum in Latrun, Israel. (Photo © 2005 Bukvoed; used under a Creative Commons Attribution 2.5 Generic license)

The gasoline-powered Continental engine was in short supply in 1941-1942, forcing some Stuarts to adopt less-powerful diesel engines. In late 1941, Cadillac proposed a new M-3 variant that would substitute two of the division’s V-8 engines for the Continental unit. The two engines were more-or-less stock 346 cu. in. (5,676 cc) Cadillac V-8s, each rated at 148 gross horsepower (110 kW) and each driving one tread via a beefed-up Hydra-Matic transmission called Torq-Matic. The engineer responsible for overseeing this project was one Edward N. Cole, later to become chief engineer of Cadillac and Chevrolet and eventually president of General Motors.

The redesigned tank, designed M-5 Stuart VI, entered production in March 1942. With the dual engines, heavier armor, and other modifications, it was around 5,100 lb (2,313 kg) heavier than the M-3A1, but the twin Cadillac engines provided a comparable top speed. Around 6,800 Cadillac-engined M-5s and M-5A1s were built along with about 1,800 M-8 self-propelled howitzers, which shared the same chassis.

By 1943, it was clear that the M-5 was too lightly armed for the European theater. In April, Cadillac began work on an enlarged version sharing the M-5’s powertrain but trading the 37mm cannon for a 75mm gun, the new lightweight M6 (a.k.a. T13E1) cannon also used on the North American B-25H Mitchell bomber. The new tank, dubbed M-24 Chaffee, was accepted for production in April 1944 and reached frontline service in November. More than 4,300 were built by the end of the war and some remained in service until the late eighties. There were also a number of M-24 derivatives sharing its chassis, engines, and automatic transmissions, including the M-19 gun carriage and M-37 and M-41 howitzer carriages.

M-24 Chaffee tank front 3q © 2009 Joe Mabel (CC BY-SA 3.0 Unported)
An M-24 Chaffee tank at the Fort Lewis Military Museum. (Photo © 2009 Joe Mabel; used under a Creative Commons Attribution-Share Alike 3.0 Unported license)

Military service brought no dramatic design changes to the civilian Hydra-Matic transmission, but the development of Torq-Matic forced GM to resolve many of Hydra-Matic’s early teething problems and later allowed postwar Cadillac and Oldsmobile advertising to boast that Hydra-Matic had been proven in combat. When civilian production resumed, Hydra-Matic was also among the features of GM’s “Valiant” models, which had special driver controls, developed by Oldsmobile, for use by disabled veterans.


Hydra-Matic really took off during the postwar boom. Buick and Chevrolet still disdained it, opting to develop their own torque converter automatics, but Pontiac reluctantly adopted Hydra-Matic in 1948. The transmission was enormously popular despite its high prices, which in 1948 ran to $174.25 on a new Cadillac and $185 on an Oldsmobile or a Pontiac (the latter equivalent to more than $1,600 in 2010 dollar). That year, 73% of Pontiac buyers, 97% of Cadillac, and nearly that many Oldsmobile buyers opted for Hydra-Matic.

By 1949, it was becoming clear that not only was automatic transmission a marketing advantage, lacking an automatic was a serious competitive handicap. While Hydra-Matic was never the smoothest of transmissions under the best of conditions and imposed penalties in both performance and fuel economy, many customers didn’t care. Critics like Mechanix Illustrated reviewer Tom McCahill disdained automatics, but American buyers were willing to accept the drawbacks if it meant not having to shift.

Other automakers were soon forced to follow GM’s lead. Ford and Studebaker turned to Borg-Warner to develop three-speed torque converter transmissions while Packard introduced its proprietary Ultramatic in May 1949. Chevrolet launched its first Powerglide transmission late that year for Chevy’s 1950 models.

The Detroit Transmission Division built its millionth Hydra-Matic in January 1949. Not satisfied with that volume, GM began selling Hydra-Matic to automakers who couldn’t afford to develop their own automatics. Curiously enough, one of the first customers was Lincoln-Mercury, which added Hydra-Matic as a Lincoln option mid-year; Ford’s new Fordomatic/Merc-o-Matic wasn’t yet ready and in any case didn’t have the torque capacity for the big Lincoln V-8. Other Hydra-Matic customers included Nash, Hudson, and Kaiser-Frazer. Most outside users didn’t bother to conceal the Hydra-Matic’s GM origins, happy to take advantage of its reputation and name recognition.

Hydra-Matic badge on a 1954 Nash Ambassador Custom sedan © 2009 Aaron Severson
This 1954 Nash Ambassador Custom’s badges proudly announce that it carries the optional Hydra-Matic transmission, which Nash purchased from GM.

With so many new customers, it took GM less than two years to sell another million Hydra-Matics, making the Hydra-Matic the most successful automatic transmission in the world.


Add a Comment
  1. I very much enjoyed the first installment of the Hydramatic story. However, I think Henry Ford deserves a little more credit for the basic blue print of a mass produced planetary gearbox. The Model T which debuted in 1908 had the bands, clutch pack and planetary gear sets which are easily recognized in modern automatics.

    1. Henry Ford didn’t invent the planetary gearset — epicyclic gear trains were used for a whole variety of industrial purposes in the 19th century — but he was definitely their most enthusiastic proponent for automotive use (and he also did an enormous amount of work with planetary gears for tractors). I don’t know how much of a direct link there were between the Model T and later planetary gearsets (including the various preselectors, most of which were invented in the 1918-1920 period), but certainly any automotive engineer would have been aware of its design.

      Ironically, Ford’s own efforts to develop an automatic transmission pretty much came to naught; he was keen to include automatic with his "X car" (which would also have had an X-8 engine), but he couldn’t make it work. Ford Motor Company ended up turning to Borg-Warner to develop Ford-O-Matic. Even more ironically, Howard Simpson, who worked very closely with Ford at Fordson (the tractor division) and at Ford on planetary gears, subsequently designed the Simpson gearset used variously by Cruise-O-Matic, TorqueFlite, and Turbo Hydramatic.

  2. The Continental R-670 is not a rotary (Wankel) engine, but a radial engine.

    1. Whoops — a slip of the keyboard. Thanks!

    2. Sorry, I didn’t mean to click on the thumbs down icon, happened by accident. Just wanted to clarify.

  3. Great article guys!! Keep it coming.

  4. I had vaguely heard of the fire that destroyed the Hydramatic plant in 1953, but this excellent article got me wondering about it. After a bit of searching, I found an interesting photo spread in the November 2, 1953 issue of Life magazine on page 102. It brings home the enormous industrial enterprise and effort that went into the Hydramatic.

    I’m already anticipating next week’s installment…

    1. The Willow Run plant that GM bought to replace the Livonia factory was truly vast. It was built during the war to build heavy bombers. At its peak in 1944, it was churning out 25 B-24 Liberators — four-engine heavies much more complicated than any contemporary car — every day. Kaiser-Frazer leased it after the war, although it had far more capacity than they needed.

      1. It’s still there, and still vast. Note the scale; Willow Run is roughly 4 sq. mi., including runways.

        Willow Run is both a busy freight airport and a GM transmission plant (or [i]was[/i], I’m not sure of Willow Run’s role in New G.M.).

        On a semi-related note, Connie Kalitta’s successful air freight operation is based at Willow Run. Many will recognize his name from drag racing circles.

        In addition to transmissions, Willow Run was the final assembly point for various GM vehicles. For anyone in the Ypsi area, I highly recommend a visit—the grounds are pretty well wide open, since it’s a freight airport. Kind of surreal.

  5. I recall from reading an autobiography by W.O. Bentley that he prided himself on his engines having great “flexibility,” the ability of the motorcar from a standing stop to start off and accelerate smoothly to top speed in high gear.

    In this series, will you cover the Powerglide as well? My grandparent’s ’66 Tempest had one of these (along with the OHC 6 you recently covered), and I still remember the disconnected feeling that came with pressing the accelerator and waiting for things to start happening.

    1. It’s easier to get away without shifting with very large engines — lots of low-end torque. (Given Bentley’s penchant for 4.5-liter fours, he clearly understood that!)

      I’ll touch on Powerglide a bit next week. The A-body Tempest did not use Powerglide, though. The Tempest, Special, and F-85/Cutlass used a different two-speed automatic, which Buick called Super Turbine 300. It was developed by the corporate Engineering Staff to replace the different automatics used by the Y-bodies. It worked a lot like Powerglide, but it wasn’t the same transmission.

  6. I would like to know the series numbers of the tranmissions that were built at the Hydromatic Plant in Three Rivers, MI. Ther years should be between 1979 to around 1994. This is when General Motors owned it.

    1. I’m afraid I don’t have that kind of technical information. The best I can suggest is contacting the GM historical archives and seeing if they have those records. Sorry!

  7. Hello – This morning I found your nice site while attempting to establish the invention and first use of synchonized shift transmissions, thanks for the great read.

    I want to draw your attention to the GM Automatic Safety Transmission for automobiles of 1937-39, which I had understood was the first commercial application of a practical automatic transmission by any maker. This transmission was invented and the prototype(s) made by Oscar Banker in his garage shop in northern Ohio, and subsequently pitched to GM. Although at once interested, by that date GM had invested heavily in H-M development and proceeded to the H-M transmission you outlined (read:’not originated here’ syndrome). Banker’s transmission was a much simpler design, which was offered essentially unchanged as the AST, and he also originated the safety shift pattern we now know (which H-M never had) and lobbied tirelessly as a ASE member for this shift pattern moving reverse range off the bottom in the name of safety. Banker’s transmission design evolved (slightly) in to GM V-Drive used in busses, etc. for decades. His and the transmission’s story is in a well written out of print biography of Banker by Robert ‘Bob’ Hull, and outlines this story in detail with footnotes, including theft of design lawsuits and what followed. I am also a fan of the H-M transmission, but this great story of one inventor getting crushed under (like Ford and Ferguson) rates a mention as his design and use of it passed the test of time – Best & Thanks

    1. Although in any event, calling anything the [i]first[/i] is a perilous and ultimately futile endeavor. The Hydra-Matic was the first really successful fully automatic transmission, in that it was produced in number, worked reasonable well, and was commercially successful, but there were many, many earlier attempts, going back to well before World War 1. There are very few automotive inventions that some inventor didn’t try long before they found their way into production cars.

    2. Hmm. Any sources you can provide on this would be very helpful. Banker’s automatic transmission design, on which he applied for a patent in 1936, does have some resemblance to the Automatic Safety Transmission.

      Everything I’ve seen about the AST suggests that there is a strong relationship between it and Hydra-Matic. There are obvious differences (the arrangement of reverse gear, in particular), but the latter certainly looks like an evolution of the former.

      This bears more investigation, so again, any sources you can provide would be appreciated.

  8. I was surprised to read, “Deliveries to non-GM users didn’t resume until later in the 1954 model year,” because I’ve read elsewhere that GM took the noble step of delivering the first Willow Run Hydra-Matics to Hudson, Kaiser and Nash when the first units rolled off the line in October 1953.

    However, I’ll also concede that your article cites its sources, which is something that can’t be said for the other articles stating that such a gallant gesture was carried out by GM.

    1. You’re right, I’ve read that, as well, although it slipped my mind.

      The 1954 part I believe is correct — although it’s important to emphasize that it was the 1954 model year, not the ’54 calendar year. By the time the Willow Run plant had been converted, I think all H-M users (GM or not) were at least a few weeks into ’54 production. Even if GM sent the first units off the line to its outside customers (which they may well have done, as much for contractual reasons as magnanimity), early ’54 models would already have been built with alternative automatics. However, the “to non-GM users” part is possibly inaccurate, or at least misleading, so I’ve deleted it from the text.


      1. the 1953 pontiac production ended on november 20th,1953, quite some time before that date, pontiac was receiving regular shipment of the dual range hydra-matic from the new hydra-matic plant, during the time after the fire, a little over 17,000 pontiacs were built with the powerglide transmission, the 1954 pontiacs started production in december of 1953, and ALL 1954 pontiacs ordered with automatic transmissions had the dual range hydra-matics, the same goes for cadillacs and oldsmobiles. General Motors told rolls royce that they would stop suppling rolls with the dual range hydra-matics in 1966, not 1978 like you wrote. GM had rolls adapt to the buick version of the turbo hydramatic 400 transmission. you should know that the dual range hydra-matic trans was B&M high performance transmission choice, handling alot more horsepower after B&M modifications. the dual range hydra-matic was replaced by GM with the controlled coupling hydra-matic because the public wanted a smoother shifting transmission, not because of rising horsepower ratings from cadillac, oldsmobile, and pontiac engines.

        charles coker
        1953 pontiac technical advisor

        1. Charles,

          Thanks for the comment. Rolls-Royce did indeed switch to the TH400 for most of its cars around 1966, but the big Phantom VI limousine retained the old four-speed until 1978, finally switching to the Turbo Hydra-Matic when the 6,750 cc engine was adopted the following year.

          1. hi aaron, i don’t know where you got your information, but when general motors told rolls royce in 1966, that GM wanted rolls to adapt to the newer turbo hydramatic 400 transmission, that meant for all rolls royce models, not just most models. if it wasn’t profitable to supply dual range hydra-matics for all rolls royce models, it certainly wasn’t proitable to supply for one rolls royce model. i have search the internet, and can find nowhere any search results that backs up your claim of 1978. i know a rolls royce transmission rebuilder in the west los angeles, calif. area, 1966 was the last year for any rolls royce model using a dual range hydra-matic transmission.

          2. Charles,

            I don’t claim to be a Rolls-Royce expert, but if you don’t believe me, I would suggest looking up the Phantom VI in any number of Rolls-specific websites or something like the old World Cars volumes released annually by the Italian Auto Club in the seventies and early eighties.

            The Phantom VI was a formal limousine produced in very, very limited numbers for VIPs like members of the British royal family. The model wasn’t included in most price lists and I think it was available only on a special-order basis. As with the old Toyota Century and Nissan President or the Daimler DS420, which did similar duty, the Phantom VI was almost exclusively chauffeur-driven and then mostly for full-dress affairs. Between that and the extremely limited volume, the Phantom tended to lag well behind other contemporary models in technology. For example, the Phantom VI also retained drum brakes and the older 6,230cc engine long after most other Rolls-Royce models adopted four-wheel discs and the bigger 6,750cc V-8.

            I suspect that with the Phantom VI, there is an important distinction to be drawn between when components were *supplied* and when they were *used*. My assumption is that Rolls-Royce had a stockpile of the older Dual-Range transmissions — which the company would also have needed for service replacements and the like — and just kept using them in the Phantom VI until the supply ran out. Keep in mind that total lifetime production of the Phantom VI was in the hundreds, and that was spread out over a period of more than 20 years!

        2. My father was part of the team that perfected the automatic transmission. He was in GM’s experimental engineering group from 1937 to 1948. He was also chosen to be the driver of the Olsmobile that received the first working automatic after he designed the passing gear fluid system. This test made Michigan news paper headlines when they drove it across the state "first production car without a clutch peddle". My father left GM and opened the first automatic transmission shop in the US. This shop was located in Phoenix Az, from here he supplied rebuilt HM transmissions all over the western half of the US. One of my fathers passions was horsepower and the method to transfer it the pavement. When you see the picture of a B&M Hydramatic opened up with all the modifications you are looking at my fathers transmissions/inventions, B&M bought all thier racing Hydro’s from my father, the only thing B&M ever produced was the B&M sticker they put on it.

          1. Did you father work with Earl Thompson?

    2. Regarding when GM sold Hydramatics to other car builders: My uncle bought a new ’51 Kaiser with an HM. I am quite sure they sold both to Studebaker and Nash about the same time.

      My father-in-law drove a 1953 Cadillar, which he claimed would regularly return over 20 mpg on the highway at 55-60. I don’t doubt that number.

      I’d heard from what I thouht were good sources that the net slippage was in the 3% range. I’m 80, and have had direct experience with most of the early automatics and semi-autos from Chrysler. As well as servicing some of them.

      Currently I am just finishing a novel, with as much good history as I can find, about how the small six car companies could have/should have been saved.

      1. The question wasn’t when GM originally started selling Hydra-Matic to other carmakers, but when they *resumed* deliveries after the fire destroyed the original Hydra-Matic plant in Livonia in 1953. As the text indicates, GM certainly sold transmissions to outside companies before then, but the destruction of the plant obviously interrupted deliveries both to GM divisions and outside customers.

  9. hello, i find some of your statements about the hydra-matic in error. the fluid coupling does not waste fuel or slips alot at low speeds, my own 1953 pontiac chieftain custom catalina would always get 15 to 17 mpg city, and 19 to 20 mpg hwy. the fluid coupling barely slips enough at idle to allow the pontiac straight eight to keep running with a in gear idle speed of 365 rpm’s. the hydra-matic’s low slippage equal ideal transmission fluid temps that only heavy duty cars like taxi’s and police cars needed a fluid cooler installed. as far as handling more than 165 horsepower, you only have to look at what transmission made B&M transmissions famous. the two best automatic transmissions to ever come out of detroit is the the 1955-56 dual-range hydra-matic, and the 1964-67 variable pitch turbo-hydramatic 400. charles l. coker, 1953 pontiac tech advisor, poci.

  10. Mr. Coker,

    Thanks for your comment. The remark to which you’re referring was a general description of the nature of fluid couplings, not specific to the Hydra-Matic. To function as a clutch, a fluid coupling has to have enough slip to absorb the engine’s idle torque, otherwise the engine will stall if the brake is engaged. The amount of slippage at engine idle speed or just off idle is therefore a lot higher than the slippage at cruising speed — 100% versus less than 5%.

    That said, the Hydra-Matic’s fluid coupling had considerably less slip than most, if not all, of its early rivals, and as you note, HM-equipped cars could return quite good fuel economy. Some of that was achieved by using a lower numerical axle ratio than manual-shift cars, but the Hydra-Matic was certainly far more efficient than the early Dynaflow, Powerglide, or Ultramatic. I imagine that if one did a proving grounds comparison between a Hydra-Matic car and a manual-shift car (without overdrive) with the same axle ratios and tire sizes, the manual shift car would probably have a slight edge, but in the real world, it would likely be a wash.

  11. Thanks for the great story and information. My late mother, who began driving before WWII, boasted that she never learned to drive a car with a manual transmission. She learned to drive on her mother’s 1937 Oldsmobile with AST, which my grandfather bought from an Oldsmobile rep traveling through the South. My mother later drove the car throughout the WWII years. She said that other than using the clutch pedal to change the gear quadrants, she left it in “High” and that it shifted very much like Hydra-Matic, which she drove for many years afterward. And speaking of Hydra-Matic, I remember as a boy back in the 1950’s, the women in the family talking about the Hydra-Matic “jump”. When starting out, they would always make sure that the gear selector was in the correct quadrant (especially when choosing “Reverse” on the far right side), give it the gas, and then release the parking break. I also remember Hydra-Matic having a rather unique and pleasing sound that is not heard in cars today.

  12. Two points concerning the fluid coupling in 1940 to 1955 HydraMatic transmissions:
    1. In first gear, the flouid coupling is driven through the front planetary gearset. Thus, the coupling is driven at about 3/4 of engine speed. This reduces “creep” and allows the engine to reach a higher RPM at takeoff.
    2. When the transimssion is in fourth gear, the fluid coupling is effectively locked out and there is no slippage at all. The original HydraMatic was an engineering tour de force, a brilliant design with many patents that frustrated competitors for years.

    1. The article mentions (at the top of page 3) that the power flow is through the front planetary and then the impeller, for just that reason. I’m not sure if fourth gear completely eliminated fluid losses from the coupling — I would have to go back and stare at the shop manual for a while — but if not, it was close to it. Certainly, Hydra-Matic had far less slippage than any of the early torque converter automatics, although comparing it to Chrysler’s Fluid Drive (which had a friction clutch as well as a fluid coupling) would be an interesting exercise. (Before anyone says anything, yes, I’m well aware that Fluid Drive was not a truly automatic transmission!)

    2. Okay, I took a look at a Cadillac service manual for the 1941 Hydra-Matic. Power flow in third and fourth gear still goes through the fluid coupling — it’s not locked out completely. However, the front clutch pack has a torque-splitting function in third and fourth; when it’s engaged, it simultaneously drives the rear planetary gearset (which is locked together) and the fluid coupling impeller. According to the manual, the rear gearset takes 60% of the input torque, with the other 40% going through the coupling. Since the coupling is taking only 40% of the input torque, its maximum slippage is consequently lower. Not a pure mechanical lockup, but certainly a benefit.

  13. The first car I ever drove was a 1958 Pontiac with Hydramatic – still, after all these years, the best Automatic ever made. Unlike modern computer controlled automatics, the H-M was analog – shifts were controlled by speed, oil pressure, and many other factors too technical for my level of expertise. In short, they made H_M seem as if it had a personality; indeed, each one I drove (Dual Fluid Couplings from 1956 on)had a distinct feel. Can’t prove it, but my feeling is that GM paid more attention to the smoothness of the shifts the more expensive the model. My friend’s parent’s ’59 Olds 98 Convertible had a 2d to 3d shift that you couldn’t feel, only hear the revs drop. My Pontiac, on the other hand, had a very pronounced 2 to 3 shift. And yes, echoing an earlier comment, they had a distinctive sound.

    1. I’m not sure exactly how much difference there was in the dual-coupling Hydra-Matic across divisions. With some of the other transmissions, there was a lot; Turboglide and Flight Pitch Dynaflow, for example, were different transmissions, even though they were each based on the same Engineering Staff concept. On the other hand, those transmissions were manufactured by the divisions themselves, whereas the Hydra-Matic came from Hydra-Matic Division. I would assume that there were at least minor adjustments to tune the transmission for each division’s engines — the Oldsmobile, Pontiac, and Cadillac V8s in those days were quite different, and didn’t have the same torque curves — but I don’t know how extensive those changes may have been, or if there were also significant variations in internal materials, transmission mount design, etc.

      Complicating that question is the fact that there were a lot of minor revisions to the dual-coupling Hydra-Matic during its first few years, to improve reliability, deal with teething problems, and so forth. Furthermore, the dual-coupling transmission was a very, very complex bit of business (part of the reason GM eventually phased it out). Minor production variations between individual transmissions might also make for small but perceptible differences in feel.

      Of course, it’s not improbable that someone at Pontiac told Hydra-Matic Division, “Hey, we’d like a little firmer shift quality, for a sportier feel.” On the other hand, it’s also possible that the transmission in a ’59 Pontiac might feel a little different than one in a ’58, especially if they were at opposite ends of the allowable production tolerances. Hard to say.

  14. Hello!
    In first place i´d like to congratulate your work here. Fantastic! Very informative and entertaining. Now about the topic, and just to measure the historical importance of the first Hydra-Matic, here in Brazil people often call any kind of automatic transmission “hidramático” (it´s how they called them in the late ’40s, early ’50s, and the name is still in use!). But the “hidramáticos” still are kind of rare in our country, people prefer to change the gears by themselves. Only in the 2000s automatic transmissions started to take off in sales, and in 2010 they represented 16% of total sales (compared to just 5.5% in 2005).
    Now a little off topic: You mentioned the Willow Run Manufacturing Plant, was that plant constructed by Ford Motor Company for the construction of aircrafts?
    Thank you for your articles and information!
    Best regards

    1. Gabriel,

      Yes, the Willow Run plant was originally constructed by Ford, building Consolidated B-24 Liberator heavy bombers under license. At the end of the war, it was leased by the new Kaiser-Frazer company, which later bought the plant outright. Kaiser sold Willow Run to GM in 1953.

  15. Sir:
    I am curious as to what oil is used the original Hydra-Matic trans. The military used the Hydra-Matic in the GMC (1950-1955) and used Heavy-duty engine oil as a hydra fluid.
    Can a SAE 10W High-detergent work in this trans?
    Marty Meaney
    Ludlow, VT

    1. Marty,

      I’m afraid I’m a historian, not a mechanic, so I’m really not qualified to give technical advice. I’d suggest talking to a shop that overhauls early Hydra-Matics and see what they recommend.

    2. Oldsmobile in the early days advised that 10W motor oil could be used in an emergency. I bought a 55 Olds 88 in 1978. It had 46,000 miles on it. The “original” transmission fluid did not have the red dye we associate with ATF or Dexron. I taught for Allison Transmission locally from 1978 to 2003. They reccomend Dexron for all automatics but for “off Highway” use you can use “C” approved fluids which are motor oils that have been approved by Allison

    3. the answer is type A, which is still available today, although most original hydra-matic owners today are using dexron with good satifaction.

    4. I am involved part-time with forest fire suppression and in that capacity have driven several different early fifties GMC 6X6s converted for hauling water. Most have 1,000 to 2,000 Imp. gallon water tanks installed. They all had the GM dual-range automatic transmission. They have got to be the roughest shifting automatics I have ever had the displeasure to drive, crashing into gear with no way to control the shift. With shift kits installed that turned them into manually shifted automatics most operators could learn to rev match the shift for a good deal smoother operation. But the factory transmission is no fun to drive.

      1. Setting aside any maintenance issues, I would guess that Dual-Range Hydra-Matics built for heavy-duty applications shifted more harshly than ones intended for passenger cars. Heavy-duty clutch packs and a hydraulic valve body set up to take a lot of punishment aren’t known for smoothness in general; conversely, slow, slurred, gentle shifts are easier on passengers, but not good for transmission longevity under heavy loads.

  16. Can anyone here tell me how many of these 3100 pickups came with the hydromatic? I have the serial# and also the Detroit Transmission Division #off the side of tranny.Please contact me if you can help, and I will give the #s’ Thanks, Rod

    1. I honestly don’t know. The figures GM supplied for Hydra-Matic production unfortunately did not include the truck units, and I have not seen totals elsewhere.

  17. i have a hdra-matic that has the b&m automotive chevy bolt pattern bellhousing on it that is a very interresting transmission love to hear about the history on that transmission it is hard to get information on old transmission

    1. Might be a Hydro stick B and M transmission that was made in the 1960s by B and M. It was one of the first drag race automatics.

  18. Hi, I was tring to find some information, on the 1938 Oldsmobile AST transmission. I cannot understand why the information is so hard to find? I owned a 1938 Olds with the AST and a 6 cylinder engine. I did not like it and let my brother use it. He complained that, it cost more to put oil in the transmission, than it did gas. They used motor oil in the trany. So I sold it.
    My first boss had a 1937 Hudson, with there Electric vacumm shift transmission, which I drove, at times. Both of these early tranys are hard to find info on.
    Art Baethke

    1. Art,

      The AST was short-lived, and really quite rare. As best I can figure, production was no more than about 25,000 units, and as you saw, customer reaction to it was not great. I suspect that there weren’t too many AST-equipped left on the road by the end of the war. Also, I don’t think Oldsmobile encouraged dealers to take them apart; there was a program that let people trade a defective unit for a replacement for about $75. So, I’m not sure if there was even a dealer repair manual for it.

      The Hudson system is a little better documented. (It was made by Bendix, known as the “Electric Hand.”) It was sort of an add-on vacuum/electric preselector system — it added a vacuum cylinder to the gearbox that would execute the actual shifts. There was a selector switch on the steering column that you would use to choose what gear you wanted, and another (usually attached to the clutch pedal) that would trigger the actual shift via solenoid. You could also order Electric Hand with a vacuum-operated clutch that would disengage automatically if you lifted your foot completely off the throttle and then reengage when you pressed the accelerator. (Hudson promoted the combination as Selective Automatic Shift, but it wasn’t exactly automatic, and as far as I know, the preselector and the vacuum clutch were technically separate options.) Hudson-Terraplane clubs would probably know where you could find service manuals or other technical info, if you wanted a more detailed explanation. Likewise, Cord collectors are familiar with it, as the Electric Hand was used on the 810 and 812.

      1. I had a ’36 Terraplane (built by Hudson) with the magic hand. All of these cars were delivered with a "stick" that was under the front seat. It plugged into the transmission, just like any floor shifter of the era. That was the only way to shift the one I had. The standard joke was that the magic hand was gauranteed clear to the end of the dealers driveway, or until the first time you used it.

  19. I remember back in about 1959 we had a 58 Pontiac station wagon that ran real good. But one time our family went to stay a weekend on a farm out in Kansas. It had rained the night before we returned and the dirt road turned to mud. Well that Pontiac really plowed through the mud ok in low, but a week later the transmission went out. The Pontiac dealer said the transmission had burnt up and needed a rebuild. That car had several transmission rebuilds before it was traded in on a 63 Pontiac. The transmission was kind of weak as I remember, even though it shifted just fine. I think it had two ranges on the shifter. Today you would use a FWD truck for that type of use, but back then it was common to use a car like a truck.

  20. The Hydromatic was used in 1950 and 1951 Lincolns with the 337 Cu inch flathead V8.
    The Ford o matic I guess could not cut it behind the big 337. Hydromatics were used in Lincolns up to about 56 or 57. After that, a big Fordomatic design was used. I drove a 1950 Lincoln once and the transmission worked ok.

    1. The Hydra-Matic was used in Lincolns from 1950 through 1954. In 1955, it was replaced by Turbo-Drive, a three-speed/torque converter automatic. I’m not sure if it was a bigger Fordomatic in concept and design; I’ve always assumed so, but I haven’t looked into it in any detail.

      1. Rolls Royce used a Hydromatic for a while. Some where I seen a list of transmissions that were used is different cars back then. I drove a military two and a half ton truck once that had a Hydromatic. What a slow truck it was. I thought I was going to have to get out and push a few times. It was a real big transmission as I recall, but it looked just like a Hydromatic for a car. Had a real cool looking gear shift on a stand. No one wanted to drive it, but I did not mind if you was not in a hurry to get some where.

        1. Rolls-Royce built the Dual-Range Hydra-Matic under license for many years. In the mid-sixties, it was phased out of Rolls-Royce and Bentley passenger cars in favor of the Turbo Hydramatic (purchased from GM’s Hydra-Matic division), but Rolls continued using it in the big Phantom limousines until 1977 or 1978. I don’t know how long they used it in military vehicles or trucks.

  21. I have a 1940 oldsmobile 6 cylinder can you tell me what engine size it has its a hydromatic.

    1. Manuel — The 1940 Oldsmobile six was 230 cubic inches (3,764 cc). It had 95 gross horsepower.

  22. The 1905 Sturtevant offered a patented “automatic” transmission.This device was a was a “three speed,consisting of a series of clutches,operated by centriugal force.”The make was high dollar($5,000)and was
    offered 1905,’06,and ’07 with the automatic.

    Push-button electric shifting made by Hammer-Cutler came out about 1913 and appeared in the SGV,Pullman,Haynes,etc.I believe it was called
    a Vulcan,and was a 4 speed.

    Around 1917 another expensive car came out with
    an electric transmission.The Owen-Magnetic had no gears to shift.

    1. I took a look at the Sturtevants’ patent for the device (U.S. Patent No. 766,551, issued 2 August 1904), which is an interesting piece of work. The patent describes a two-speed transmission, although it notes that the same principles could be used to add additional speeds by adding additional clutches. I wonder how well it actually worked.

      I’m not familiar with the Hammer-Cutler system or the Owen-Magnetic, but I’ll look into them — thanks!

      1. Hmm, there was also a continuously variable transmission invented by Frank Hayes that was offered briefly in 16 and 18 HP Austins from 1933. It appears to have been broadly similar to the modern roller-type CVT (such as Nissan’s Extroid unit), but it retained a manual clutch, which had to be used for stopping and starting.

  23. I’ve got car 74(1905 REO)and I forgot to mention that 1933 was the year REO offered the
    Self-Shifter.I think it came on either the
    straight eights or the sixes.It must have been
    functional as they were offered multiple years
    and there are several surviving cars with this

    I recently saw a 1938 Studebaker for sale with an electric hand Bendix Shifter.It was my
    understanding that it was a factory option.

    1. The Reo Self-Shifter was introduced in the spring of 1933; it was standard on eights, optional with the six. It apparently worked reasonably well, but it was not an automatic transmission in the modern sense — instead, it was a four-speed semi-automatic, analogous in concept (though not in mechanical layout) to the later GM Automatic Safety Transmission.

      One of the things that can be very confusing about this subject is that for many years, the terms “automatic” and “self-shifting” were applied rather loosely to all manner of transmissions, only a few of which we would consider automatic. Some were semi-automatic transmissions, like the Reo Self-Shifter or the AST, while others were preselector transmissions along the lines of the Cotal or the Bendix Electric Hand. Preselectors were not automatic except in the sense that the actual gear change was executed by some kind of remote mechanism, rather than through the direct movement of a shift lever or cable linkage. The driver still had to decide what gear to use and initiate the actual shifts. (I would draw an analogy to the way the term “automatic” is applied to pistols or rifles; it’s often used to describe weapons that are simply auto-loading, rather than truly automatic.)

      Like the AST, a lot of the semi-auto and preselector transmissions retained the clutch pedal (although in a preselector transmission, it was essentially converted to an engagement switch to execute the actual shift) and generally required at least some manual shifting. Few of them did well commercially, particularly during the Depression — buyers were reluctant to spend the extra money and risk the potential reliability hassles if they still had to change gears and operate a clutch pedal.

  24. GREAT GREAT ARTICLE…It helped me a lot

    I have a 1941 Cadillac Series 61 (Model 6109D). Is my shift pattern as shown above?
    Neutral, Drive, Low, and Reverse
    Do I need to pull in (or push out) the shifter to get it into any gear?

    Once again Thank you

  25. My Great Uncle, Kenneth E Snyder was an engineer at GM for 30 years. He worked there from approx 1936 to 1966. On the back of the watch given to him by GM states: Service Award 1961 Detroit Transmission Division GMC K.E. Snyder 25 years. My Great Aunt Marion, his widow, gave the watch to my son after my uncle died. Uncle Kenneth was on the team which developed the hydramatic, from all the stories that I’ve been told, plus, the last time I was at his home in FL , I saw the award GM gave to him which was a small trophy of a transmission. I wish I knew exactly what it said. Our family has always been very proud of his work at GM and always knew how smart and gifted a man he was. I’m sure the whole team knew they would never receive individual recognition, auto companies never worked that way. Anyway, I just thought all you car people would enjoy hearing my story. My brother, Kenny, lives in FL and really knew my Uncle MUCH better than I and would be a much better source of information.

  26. Hello! Certainly interesting historical information here. Thanks for the attention to detail. I was hoping that someone here might offer some advice on an HM unit that I’ve put in my 41 Cadillac 6227D coupe. The car was originally HM, rare for a coupe and I’ve just about completed a frame off restoration. I found an older gentleman who used to work for GM HM to do a rebuild of a ’48 HM unit (41’s are really not too good). He did it and I have pictures of each step of the rebuild. He’s now passed on. I have installed the unit, filled it properly, and adjusted the bands according to the 48 shop manual. The car moves in Hi, Lo, and R. R seems fine but the manual shift out of R is a little tough. [b]Lo has one speed, no shift and I’m not sure if that’s correct. In Hi, the car takes off, but misses the shift and feels as if it’s shifted to N. [/b]I’m hoping that an adjustment to the throttle control linkage may be the issue and that it’s not something internal. I’ve tried adjusting the bands 1/2 and 1 full turn tighter with no effect. There are no strange noises associated with the problem. Again, in Hi/D upon acceleration the car gets up to about 10mph and then feels as though it shifts to neutral rather than up shifting to 2nd.
    Any help would be appreciated.
    Ken Karrer 1941 Cadillac 6227D coupe

    1. Ken,

      I’m not qualified to perform troubleshooting or repair advice, but to the best of my knowledge, the early Hydra-Matic IS supposed to shift between first and second in Lo range (comparable to the “2” position on later automatics). I would suggest digging up a shop manual for the ’48. You might even find at the library — I was able to find and check one out while researching this article.

    2. Ken

      It’s been a while since your post, so you probably got your questions answered … but if you’re still having issues here’s a few comments.

      I agree with Aaron, don’t want to try to diagnose your problem here … but I can’t help making a couple quick comments …

      I’m an Olds guy … so I did a little quick research on your questions since Olds, of course, used the same Hydra-Matic as Cadillac (or mostly the same as far as I know). We had several Oldsmobiles with Hydra-Matic back when I was in high school, that I used to work on. And I have always been interested in transmissions (I’m an engineer by training, though I don’t work in engineering).

      It sounds like you have two issues/questions:

      1) What is correct operation in "Low" range?
      2) Bad 2nd to 3rd shift (goes into neutral)

      Item #1 – In Low range the trans should start in first and shift to second, but at a somewhat higher speed than in Drive range. If shifting into Low range at cruising speed, the downshift sequence gets a little more complicated, but that’s another discussion.

      Item #2 – The 2nd to 3rd Hydra-Matic shift is quite complicated due to the required timing of the apply and release of the friction elements. The 2-3 shift is probably the major contributor to the Hydra-Matic’s reputation as having harsh shifts.

      In second the front planetary is locked up with the clutch applied, the rear planetary is in reduction with the rear band on. In third this is reversed: front planetary goes into reduction by releasing the front clutch and applying the front back; the rear planetary up shifts by releasing the rear band, applying the rear clutch. If the timing of these "release" and "applies" is off then one gets 2-1-3 (front planetary downshifts faster than rear planetary upshifts) or one can get 2-4-3 (rear planetary upshifts before front planetary downshifts) … or 2-neutral-3 if either the front band apply is too slow (after the front clutch released) or the rear clutch apply is behind the rear band release. As I think about it, mis-adjusted bands seems like the most likely cause of a 2-neutral-3 shift, though it isn’t explicitly listed in the Diagnosis section of the Service Manuals I looked at. If you have a mis-adjusted band (singular) you should be able to figure out which by observing other shifts, but not necessarily.

      Despite having adjusted the bands once, I’d recheck adjustment.

      If bands are ok, next stop would be looking for sticking valve in valve body.

      If you’re still having problems .. post something here and we can communicate outside this forum.


  27. This article is fascinating to me. At my current age–66–I drive a Grand Marquis LS 4 door sedan. What I would give for a dual range hydramatic drive transmission. Before starting at OSU in Stillwater, OK, I worked at Edward’s AFB and purchased a fast back 1948 Cadillac with hydramatic drive. I wish I had never parted with that car! Thanks to the authors of this engrossing article/history!

  28. This article contains much useful information about the Hydramatic. Earl Madman Muntz put the Hydramatic in the Muntz Jets from 1950-1953. A project Jet I’m working on has a 337 cui Lncoln flathead mated to a 1952 Hydramatic. Take 8 qts. of Type A fluid througha funnel in the dipstick hole. Hope to get it running. Right now it seems to have mineral oil in it. I guess it was used back in the day.

  29. Hi,
    first of all thanks for the nice reading, it’s the first time i see this site, i’ve run into it casually!

    "The best I can suggest is contacting the GM historical archives and seeing if they have those records."

    I’m afraid they don’t have that information, I already checked.


  30. I really enjoyed the story and have a question about the production facilities. GM built the Livonia facility in 1948 according to the Livonia history website. I’m curious as to where were they produced from 1939 until 1948? Was there a dedicated factory?

    1. You know, I don’t know. That’s a very good question.

      1. Ah-ha! Finally something I can contribute to! lol. I was actually searching for info on the Detroit Transmission Division of GM when I stumbled on this page. Except for the real technical stuff, I actually found it very interesting! Especially the GM history. (my dad retired after 42 yrs at GM and my mom got an early retirement at 27 yrs so we lived and breathed GM. lol)
        Anyway, the plant you are asking about, where the transmissions were produced starting in 1939? That plant is the reason I’m scouring the internet on a sunny Saturday in Detroit. The first building GM built the Hydra-Matic transmissions was located on the corner of Riopelle and Farnsworth in Detroit. It was near Warren and I-75, on the east side of Detroit. The building started out sometime around 1917 as Fisher Body Plant #10, where presumably, Fisher made car bodies for GM and many other car companies at that time. (until a deal was struck in 1926 for them to become the in-house body maker for GM, according to Wikipedia) It’s a little unclear to me if they continued to make bodies for GM there until 1939 when the Detroit Transmission Division was formed and housed there or if there were other operations in between.
        The H-M was built here until the new facility in Livonia was ready in 1949. We all know what happened within a few years of that. Below is a link to a brochure on the GM Heritage Center’s website, showing a timeline history of the Detroit Transmission Division, with the first line saying that the division was organized "in one-third of an old six story building on Riopelle Street in Detroit".

        [www.gmheritagecenter. com/docs…elcomesYou.pdf]

        Although I don’t see a date this was printed, it was after 1957 since they list that year on the timeline but I’m sure it was before 1960. But what I find funny is, they write off the original building as "old" when, in 1939, it was just barely over 20 years old and even if it was 1959 when they printed this brochure, it was just barely over 40 years old! Now, this was an Albert Kahn building, top of the line, re-enforced concrete that was made to stand the test of time. But, because it wasn’t the "new hotness" like the Ypsi plant they’re advertising in the brochure, it was just an old building. I’m surprised they even named the street it was on. Anyway…
        After Detroit Transmission moved to Livonia, eventually GM moved some Cadillac production in there for quite a few years and then used it as a warehouse until the early 80’s. And GM people know what happened then; plants started closing left and right. My parents were both working at a plant that closed in 1987, I believe, and I think that one lasted longer than some others. (Luckily my parents both ended up at the Warren Tech Center and were able to leave GM on their terms, not GM’s.)
        It’s a little fuzzy at this point, as far as this plant but I know a food wholesaler was in there for a while in the 90’s – Total Foods, until the people I work for bought it in 1998. They used it as a rental property (it was 500,000 sq ft) but used a big chunk of the space as a warehouse, shipping facility and eventually an assembly operation was added, for the manufacturing company they also own, Palmer Distrib. This is the company I actually work for but I am basically the admin asst for both of the owners, but in their main office in St Clair Shores. I never worked in the warehouse.
        This building did stand the test of time, for a total of just under 100 years. I’m sure it would have lasted a lot longer too but a nasty, nasty fire took it on February 5th, 2014. It started on the 3rd floor and swept thru the building like it was a house of cards. It burned for 5 days, smoldered for 2 days and then burned for 2 more days. Someone close to the Detroit Fire Dept put together the video at the link below, if you’re interested in seeing the first couple days in 7.5 minutes: [youtu. be/wKe63ZdRsec]

        That happened at the beginning of February and the last part of what was left of the building was knocked down at the end of May. I guess they’re still cleaning up the rubble and rest of the property. I think I heard mid-July is when they expect to be done with that.
        So, now I’m helping someone do some research on the history of the building for a paper they’re writing. This is what brought me here. A lot of the early history I shared came from that other person’s research. I am obviously more familiar with the more recent uses of it. I have one more link to share, in case any one is still skeptical that this building I’m referring to is in fact part of the Detroit Transmission Division of GM….

        [www.emporis. com/ building/detroit-transmission-division- general-motors-factory-detroit-mi-usa]

        1. Christine,

          Thanks for the information! I will have to check this out.

          1. Aaron,
            I’m glad I could contribute to the conversation. My friend is supposed to be posting his paper about this building on his website within the next couple of days. I will post the link to it once it’s up. It should include pictures from after the fire but before the demo – inside and out.

  31. Thank you so much for all of your effort here, and with the article!
    In my years( 83-03) as a New Car Manager & U/C Mgr in a multi-point GM store, I guess I have driven everything G.M. ever produced domestically, say from 1970 and newer, and many a lot older. (just a guess) I never thought to bring this up to the Factory rep, who may have been able to comment on my question. MY mother has driven Cadillacs going back to 1966 through today all except one of them being Sedan Deville’s, the exception was an ’82 or ’83 Coupe powder gray beauty, I was not permitted to drive them, of course, but now at 53 she may allow me to gas it up for her.. Loves her Cadillacs!
    Why does a Cadillac tranny shift feel differently to me, it is just a unique feeling of smoothness & power? Now, that’s me. Have you ever heard this or experienced for yourself?

    1. Well, even where different division share the same basic transmission, each division’s engineers have to tune the transmission to suit the torque characteristics of the division’s engines and their cars’ identity. With planetary gearset automatics, the transmission’s performance depends a lot the timing and firmness of clutch engagements and so forth; slower, slurred shifts tend to be smoother but less efficient, while firm, fast shifts are the reverse.

      Finding the right balance for street cars is as much an art as a science because it tends to involve a lot of compromises. GM divisions tuned the Turbo Hydramatic (which most Cadillacs between 1965 and 1981 used) in various ways; some were very smooth, others produced back-slapping shifts (intentionally).

      Cadillac had two advantages during much of that time: first, their engines generally had lots of torque, and second, Cadillac customers weren’t looking to extract maximum quarter mile performance. As a result, their engineers could strike a more relaxed balance on shift quality without sacrificing too much performance. They could also more thoroughly isolate the transmission mounts from the car in ways that Supercar drivers would’ve complained about endlessly.

  32. I’m very pleased you acknowledge the part the English Daimler Company played in the birth of Hydramatic. What Kettering & co aimed to do was give the Daimler transmission a brain, and the hydraulic servo system, controlled according to speed and load, was that brain.
    But the basis remained the same – four speeds, fluid coupling and steering column lever control, so allowing three to sit abreast in front.
    The Rolls-Royce made hydramatic was licenced for 5,000 units per year, I believe, so they sold hydramatics to Jensen, Armstrong-Siddley and I think a couple of other posh low-volume British marques.

  33. I’ve enjoyed reading several articles on here today and they’re mostly very good, but I have to take some issue with parts of this – mostly detail and tone… possibly from a need to brutally economise to get it within 4 pages despite having an obvious wealth of information to include, IDK…!

    First up… yep, "sliding-gear" manual transmissions ARE desperately out of date, obsolete, irrelevant, etc… and indeed, they were so well before WW2. One of the later cars to continue using actual "crash" gears, with straight-cut gears moving around in relation to one another, was actually the Ford Model A with its 3-speed manual version (not all old Fords were 2-speed epicyclic!). Even before synchromesh took hold, manufacturers had gotten the idea about constant-mesh, helical gears and dog clutches – the kind of setup still used in most motorcycles to this day, in racing cars (with straight-cut gears), and for a surprisingly long time in certain cars like the Fiat 500 (which never received synchro as far as I know – downshifts generally needed a blip of the throttle, but upshifts were merely a bit rough rather than grinding).

    The main similarity between an old sliding-gear transmission and a modern 5-speed manual is that you operate it yourself with a pivoting stick, and that it uses gears acting directly on each other. The actual mechanical actions that take place when you change gear bear much less resemblance to each other, and the modern day iteration is perfectly fine to and relatively idiot-proof. The newest 3-shaft versions even lend themselves to dual-clutch semi-automation with the minimum of actual alterations… drill a couple holes, extend the shafts out, split the clutch into two, and shove servos on the release and selector arms…
    (OK, that’s not 100% technically correct, but you get the idea)

    In said more modern designs, the gears stay static and never go out of engagement, and so don’t need slamming back INTO engagement, which causes the grinding, chipping and balking. What moves are the selector/engagement/dog clutch rings (call them what you will), sliding along and spinning in engagement with the splined parts of the input shaft (the input-side gears themselves being freely-rotating on a smooth part of it). When you select a gear, the gearstick moves the selector arm which moves a selector fork on its own shaft, driving one of the rings towards one or other of the gears facing it. Large, rounded-off "dogs" protruding from the face of it then mate with similar outcrops (or indents) on the face of the gear and form a positive engagement for transmitting drive. Thanks to their size and shape, and the lego-brick mode of action (like jamming a stick in a bicycle wheel, rather than trying to get two bike wheels to line up and mesh nicely with each other… whilst both are spinning), this happens rather more easily, smoothly and quietly than in a crash box – though it still often requires the two sides to be turning at least roughly at the same speed. It’s easier for upshifts as the engine speed naturally falls as you take your foot off the gas (and the input shaft speed falls relative to that of the gear assembly even with the clutch depressed) and it’s just a matter of getting into the rhythm, whilst downshifts demand a deliberate raising of engine speed… particularly an issue if you’re trying to brake (doubly so on a steep hill) at the same time!

    (For some reason this isn’t too much of a problem with motorbike transmissions… even though they’re sequential, without even any neutral gaps between gears other than 1st & 2nd for you to blip the throttle in with an engaged clutch, downshifts are usually a perfectly simple press of the gear pedal… I suspect there’s some additional trickery in how they’re made, maybe with a little more play engineered in – clutchless upshifts usually come with just the slightest of jolts suggesting that; additionally, despite their higher engine speeds, there’s usually a more-than-compensating reduction gear between the crankshaft and gearbox input (with then a generally smaller step-down from output to the also-larger wheel), so everything turns rather slower, which together with the overall much lower weight (overall and per-gear) leads to rather less inertia and probably a minor auto-synchronising effect due to the whirling oil bath everything’s soaked in. All I know is I’ve tried to figure it out with mine, with the help of an exploded diagram from the workshop manual that shows how there definitely aren’t any synchros in there (vs the similar diagram for my car), and ended up just giving in and consigning it to the "life’s little mysteries" cupboard.)

    Early synchro boxes tended not to have it on 1st gear for several reasons … first being one of strength. Early synchros weren’t as strong as all that, and drivers were used to giving the lever quite a bit of welly, especially if it balked (which is to be expected when doing a difficult change with a weak synchro – just hold pressure on the lever for half a second more and it should go in…). This, along with the greater torque and rpms that would get shoved through them (even when fully engaged) thanks to the extreme gearing in 1st and the multiplying effect of a suddenly-engaged clutch meant the potential for them to break was rather higher – but plain dog-clutch engagement is very strong, which is why it’s used for race cars and for all but the most modern auto-shifting trucks (and this is why truckers have a gear-jammer reputation… they don’t have synchro and so have to become skilled to avoid dying on long downhills!). It’s also why you can identify when an old car is in 1st gear from the whining noise… the same stresses applied for the gears themselves as well as the synchros, and the one benefit helical-cut gears don’t impart, despite being smooth, quiet and efficient, is strength. Straight-cut gears are rather stronger – and noisier.

    On top of all that, in such transmissions, Reverse and 1st usually shared a selector shaft/ring, being directly opposite each other on the H-pattern, especially with 3-speeds. And you really don’t want to run the risk of the accidents or mechanical mayhem that could happen from accidentally and far-too-easily shifting directly from one to the other whilst moving at some speed… versus that, a bit of a grinding, rattling noise telling you "don’t do that, idiot!" is positively welcome. (This is why reverse is still rarely a synchro gear even in modern cars – in fact, some manufacturers even deliberately engineer it with an otherwise primitive sliding-gear arrangement, specifically so you can only engage it with any ease when stopped or moving very slowly, even if already moving backwards. This is the source of the "crunch" noise you get if you try to slam into reverse too quickly after braking fairly hard from speed… even though the clutch is dipped, the input shaft hasn’t stopped spinning yet, and the reverse idler gear is rather unhappy about being shoved into engagement with it vs the stationary output… the sound has a rapidly dropping frequency as output actually acts as a brake on the input shaft, with the idler gear as its proxy, and it all slips into place only once the input has slowed quite a bit).

    Another concern was one of cost. Synchro was both mechanically complicated, and had to be licensed thanks to the patents and copyrights. The less synchronisers you could get away with using (e.g. a single shared one on the 2nd/3rd selector ring in a 3-speed-plus-reverse box), presumably the less royalties you had to pay, as well as the fewer expensive, complicated and fiddly to produce and install parts you had to fit.

    Don’t forget that a lot of old cars were relatively slow, and also relatively low- (and wide-) geared compared to present day ones, which is why people came up with overdrives. First was usually only for starting off, low speed manoeuvring and hill climbing, and might be all over and done with by 15, MAYBE 20mph (why do you think the old automatic manufacturers thought little of habitually starting in 2nd?). General motoring was almost all achieved in 2nd and 3rd, and quite a few cars even in the 4-speed, all-synchro era were strangely proud of being able to boast that they could run as low as 10 (or even 5!) mph in top gear and still pull back to high speed without needing a downshift (even if it took a whole minute to get up to 50mph again). 2nd gear was plenty flexible enough for most low speed situations. Then – as now, actually, thanks to much better low-end torque and stronger clutches – there would be little call for downshifting into 1st on the move, rather than after having slowed to walking pace (which would allow a smooth re-engagement at idle) or a complete halt, because unless you were approaching a hill that you knew would require (a throttle-blipping double-declutched downshift into) 1st to surmount, there wasn’t that much real-world performance benefit to be had, and it would only be useful for descending the very steepest of hills, the ones that would cause you to stop briefly at the top of them and take a deep breath. Thus, there was little actual need to fit synchro, and again, it could even be regarded as a safety measure to stop gung-ho motorists from slamming the shift into 1st at far too high a speed and blowing the engine – rather more likely with 2nd-1st (or 3rd-1st!) than 3rd-2nd. If you couldn’t rev high enough in neutral to enable the speed-matched downshift into 1st, then you probably had no business being in that gear.

    Now compare all that to the behaviour of the contemporary automatics … there’s not that much of a world of difference, is there. There’s a lot of the same problems, but what the autos do is let the machinery take on all the dirty work, even if it does it about as badly as you do 😉
    These days, manuals are somewhat more developed and refined … though I’m personally not sure what to make of the rather flimsy, cable-change affair in my most recent car. It seems like a step back from the solid, positive-feeling rod change one I had to give up. Makes it harder to do a quick, smooth shift thanks to how notchy and uncertain it is. But, that’s a minor gripe really, on the whole it works fine, and demands nothing more than "realise when it’s time to shift… dip clutch… move lever… lift clutch".

    Moving on from that, we also seem to have skimmed over the whole "epicyclic manual and semi-automatic" thing other than the brief mention of preselectors. If you consider the quite common* addition of a 2-speed rear axle to the Ford Model T, and the pedal rearrangement mods that went along with, these 4-speed variants could be sort of considered such. The "clutch" action was integral to the gear selection movement, whether by footpedal or hand lever; basically there were four different clutches (five including reverse) you could choose to engage by operating the gearshift mechanism, and going into low or reverse from a standstill involved careful take-up of engine drive so as not to stall it… but after that, it was pretty much the same as various other semiautos that came after, like the saxomat. Move the lever/pedal to the next position, and you’re in gear. With the simple addition of, say, a centrifugal clutch that would allow you to come to a halt with one of the gears still engaged, maybe a rotary-sequential selector device to co-ordinate the 2-part shifts (particularly between High-Under and Low-Over), and some device that would move the selector in response to the difference between driveshaft speed and throttle setting, it would have been a "proper" automatic with no hydraulic parts at all, and the world could have been rather different. As it was, it stayed effectively "manual" in all its guises despite strongly resembling an "AST" where the driver had to deliberately regulate the uptake of drive from rest, and command the in-range (1/2 and 3/4) shifts as well as the between-range (2/3) one.

    (* Damn it, I can’t remember the general name for them right now – as a single company made a decent majority of all those fitted – but I’ve probably seen more in-game models and example videos on youtube of T’s and TT’s with 4-speed twin-epicyclic transmissions fitted than I have ones without. Generally it was in the form of a "splitter" type gear rather than an overdrive, thanks to the T’s rather wide ratio gap between low and high; the axle would offer something in the region of a 1.66/1.00 gearing (relative to whatever the actual rear diff ratio was) to the T’s default 2.73/1.00, with a choice of either multiplication or division relative to the norm. Meaning you could have a roadster with a multiplier one giving 2.73 – 1.66 – 1.00 – 0.60, for better top speed and improved lower-mid-range acceleration and hill climbing, or a truck with a divider giving 4.53 – 2.73 – 1.66 – 1.00 for better load hauling on steep grades and less tendancy to rapidly slow to the original low-gear speeds on shallower ones… Oh, and of course, two reverse gears. Rather pointless on the roadster unless you liked scaring the crap out of your passengers, but pretty handy for the truck. Naturally, if you couldn’t be bothered with the more complex twin-shift arrangement but couldn’t afford the time/money/care to modify it into an H-gate setup, you could just choose to leave one of the transmissions in its high or low position, and only shift the other one, reverting to either a 2.73 or 1.66-to-one 2-speed modus depending which you picked (…probably safest to shift the original transmission, I would guess, as the axle would be intended to experience at least some of its changes whilst the original was in neutral going between Hi and Lo at the combined shift point?).

    Interestingly, some drag racers still use a similar sort of system, when uprated Powerglides and such no longer suffice – a bunch of daisy chained 2-speed epicyclics, with a single clutch that locks them into "forward reduction" when engaged and "direct" when released, and a regular plate clutch (or heavy duty "dumb" TC) at the head of it all. Bunch of little levers in a row forming the control panel. Shove them all (say, 2, 3, or 4 of them depending on your power and expected top speed) forwards, roll to the start line, do your burnout… rev up… lights go green, dump the clutch, then over the next three seconds or so rapidly slam each lever down in turn, jumping up through the ratios quickly, seamlessly – and completely manually – as each reduction clutch is released. Usually a uniform sequence is used, but there’s no reason a roadable dragster couldn’t have a back-and-forth type splitter gear in there which could be rocked one way and the other whilst the other, wider gears were being mostly changed in just one direction.

    And, well…

    All this sort of begs the question…

    Given how the gear selectors in my bike box are quite happily synchronised in their operation by a single up/down footpedal despite the box internally more resembling some notional 5-speed version of the Fiat 500 (three different selectors, whose forks move back and forth just as if they were being pushed and pulled by a regular lever in an H-gate)… and operators of the Ts and TTs with 2-speed axles presumably didn’t suffer badly from jolty shifts even when having to personally synchronise a pedal and a hand lever… and even just now I manage to envisage controlling and tightly synchronising the latter using the former’s cam-like peg-in-groove shift barrel and some simple adaptions to make the output act on brake bands instead of dog-clutch selector forks…

    …how come the makers of the AST and the 4-speed Hydramatic couldn’t dream it up? Heck, the former was almost there, they just needed something to hook up that 2nd-3rd change.

    No complex hydros, no strange lag, anything like that. The barrel turns, the pegs located in the matching grooves are driven one way or the other, and the attached mechanical parts are forced to move in perfect synch. In fact, the shifter itself will balk and refuse to move unless they do, as unsynched movements will jam it before it can turn any further (an actual safety feature on my bike uses a torque-sensing ratchet pawl to do that on purpose to prevent you downshifting more than two ratios in one go without letting the clutch back out, as a 3-gear jump would generally represent a difference in rpms that would cause engine damage, dangerous loss of rear wheel traction, or both; soon as you let the engine hook up for a split second, it frees right back up). Very easy to control with an automated system because all it needs to know is whether there’s a need to go "up" or "down" from it’s current condition (say each shift takes half a second, and it needs to jump to 1st from 4th; "need to go down"… it shifts 4>3 … "need to go down" (still)… shifts 3>2 … and there’s still that pesky "need to go down" signal, well ok let’s go 2>1… and hey, that was "only" 1.5 seconds, probably quicker than a human would have figured it out!), and what the limits are (hi/lo ranges, or e.g. "S" or "2" position – and of course, the point at which 1st becomes Neutral becomes Reverse). The only outputs from whatever control logic is used – mechanical, electrical, hydraulic – would be "up" and "down" from the current position (and, if it’s an actively clutched setup rather than a fluid/centrifugal/TC one, some kind of release bearing servo). Could even add a tiptronic-style sequential manual shift option (either with positions 1-4, or a ratcheting push-up/push-down motorbike/flappy paddle setup), and have Park as something triggered by pulling the lever sideways from N.

    But then, I am looking at this with 20/20 hindsight and the eyes of someone who’s already versed in everything that has come along since. I have a feeling bike manufacturers themselves didn’t hit onto the sequential/barrel shifter idea until after WW2 anyway. And although it could have been achieved just as easily with cams on a rotating shaft (produced using the same tooling as the engine camshafts!) actuating other parts in synch, and it’s almost certain the Victorians did similar things in order to make steam engines work properly, both of those would probably have been viewed as primitive, old tech solutions that should be left in the coal-burning age rather than as practical options for the modern early-20th-century gasoline-burning automobile maker. Hydraulics are where it’s at, don’t you know! So much better than cams and cables and all your old rot. It’s alright, we’ll work this out sooner or later…

  34. My father’s first car with automatic was a ’53 Pontiac–with Hydramatic, of course. He was a buttoned-up guy and usually not prone to getting openly excited about things. For weeks after getting the Poncho, he was telling people, "You just step on the gas, and it goes! You step on the brake, and it stops!"

  35. Automatic transmissins in Indian traffic conditions have not proved effective due to high congestion oftraffic, unruly traffic and slow moving traffic too. Frequent shifting of gear ration result in fast wearing out of brake bands and aslo clutch plates of interlocking clutches. Slow speeds cause slippage in fluid couplings leading to poor fuel economy. Thus Indians mostly prefer manual shift gear boxes. Earlier we had ceilo a korean car with auto transmission that failed in indian conditions. Many customers got manual shift gear boxes fitted.Ofcourse some woman driversprefer auto gears and but are limited.

    1. Automatic transmissions have generally been slower to catch on in markets where fuel prices, tax considerations, and available income favor cars with smaller engines, in part because automatics have traditionally been a serious handicap for smaller engines in performance and fuel economy.

      I’m not terribly familiar with the Daewoo Cielo, but if I understand it correctly, it was essentially a restyled version of a car sold in the U.S. as the Pontiac LeMans, fitted with a smaller 1,498 cc engine and offering a rather elderly three-speed automatic. With only 94 lb-ft of torque, I imagine the automatic didn’t do much for either acceleration or fuel consumption.

  36. I had a 64 Olds 88 in the early 80s. The HM was worn and 1st gear overran. I was annoyed to learn the HM had no exxternal means to adjust the bands. One trans mechanic said they basically self -destruct.

    1. The ’64 Eighty-Eight did not use the original Hydra-Matic, but the later three-speed torque converter version, introduced in 1961. (That transmission is described in Part 2.) One of the things the later Hydra-Matics attempted to address about the original four-speed H-M was that it was very sensitive to proper adjustment of the bands — if they weren’t set up just right, it would throw off the shift timing and make the shift jerky, which was the cause of many complaints.

  37. Very interesting article and comments.

    Does anyone know where I might find a new Speedometer Pinion for a 1948 Cadillac Fleetwood 60 Special Hydra-Matic Drive?

    Thanks for any information anyone can provide.


  38. Would anyone know of a modern transmission I could use to replace the original transmission in my 1950 Series 62 4 door sedan.
    Any help greatly appreciated.

    1. I’m not able to help with modifications or repairs — sorry!

  39. W. G. Wilson, did I miss mention of him?

    Even worse a complete omission of that genius of the 20th century Frederick Lanchester!

    1. Wilson is mentioned, albeit briefly, Lanchester is not. This article is about the origins of GM’s Hydra-Matic; it is not and does not purport to be a complete history of the automatic transmission or the fluid coupling, which would be an account several times the present length. The Wilson and Cotal preselector transmissions represent a separate (if not unrelated) and complicated subject in and of themselves.

  40. I have a 1954 Rolls Royce Silver Dawn that uses the "Hydra-Matic" built by RR under a license from GM. The transmission was replaced under warranty in 1964, at about 75,000 miles, and the second transmission is still doing fine at 150,000 miles. An interesting feature is a servo-power takeoff on the side of the transmission that provides mechanical power assist to the braking mechanism. A very complicated set of levers and rods tying that all together but I’ll have to say it works very well.

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