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CATEGORIES (articles) > Transmission > Automatic Gearboxes > Olds Mobile Hydramatic Gearbox

Olds Mobile Hydramatic Gearbox

Hydramatic (also known as Hydra-Matic) was an automatic transmission developed by General Motors's Oldsmobile division. Introduced for the 1940 model year, the Hydramatic was the first fully automatic mass-produced transmission developed for passenger automobile use.


During the 1930s various automakers sought methods of reducing or eliminating the necessity of shifting gears. At the time, synchromesh gears were still a novelty (and confined to the higher gears in nearly all cases), and shifting a manual gearbox required more care and skill than most drivers cared to exert.

GM had previously experimented with the Automatic Safety Transmission, a project initiated at Cadillac division in 1934 and later developed by Buick and Oldsmobile. Oldsmobile offered the AST from 1937 to 1939; Buick, which felt the results uninspiring, offered it only in 1938. The AST was a semi-automatic transmission using planetary gears and a conventional friction clutch.

Unsatisfied with the Automatic Safety Transmission, Oldsmobile launched a new program, headed by engineer Earl Thompson, to combine hydraulic operation of a planetary gearbox (which would allow much shifting to be automated) with a fluid coupling instead of a friction clutch, eliminating the need for de-clutching. The transmission would have four forward speeds plus reverse, providing a broad range of torque multiplication. It incorporated a parking pawl when the selector was placed in reverse with the engine off, although there was no separate Park position.

The result, dubbed "Hydra-Matic Drive," went into production in May 1939 for the 1940 model year. The first Oldsmobiles so equipped were shipped in October 1939. Advertising proclaimed it "the greatest advance since the self-starter."

In 1940 the Hydra-Matic added 57 dollars to the car's price, rising to 100 dollars for 1941. In 1941 it also became an option on Cadillacs for 125 dollars. Almost 200,000 had been sold by the time passenger car production was halted for wartime production in February 1942.

During the war the Hydramatic (mated to a Cadillac V8 engine) was used in a variety of military vehicles, including the M5 Stuart tank and the M24 Chaffee light tank. The extensive wartime service greatly improved the postwar engineering of the transmission, which was subsequently advertised as "battle-tested."

Starting in 1948 Hydramatic became optional for Pontiacs, although Buick and Chevrolet chose to develop their own automatic transmissions. One million Hydramatics had been sold by 1949. In the early 1950s various manufacturers that could not afford to develop an automatic bought Hydra-Matics from GM. Users included:

  • 1950-1956 Hudson
  • 1950-1956 Nash
  • 1951 Frazer Nash
  • 1951-1954 Kaiser
  • 1954-1955 Willys
  • 1950-1954 Lincoln
In 1952 Rolls-Royce acquired a license to produce the Hydra-Matic under license for Rolls-Royce and Bentley automobiles. It continued production through 1967.

The Hydramatic underwent several revisions through 1955, before being replaced by the substantially redesigned Controlled Coupling Hydramatic (also called Jetaway) in 1956. The new transmission incorporated a secondary fluid coupling and a pair of sprag clutches in place of the former friction clutch and brake bands, shifting in part by alternately draining and filling the secondary coupling. It allowed the driver to hold the transmission in low or in third gear until the maximum allowable upshift points, for improved performance in traffic or in mountain driving, and incorporated a separate Park position.

The new transmission was substantially smoother than the original, but also more complex and expensive to produce, so in the early 1960s it was phased out in favor of the lighter Roto Hydramatic torque converter automatic and, ultimately, the Turbo-Hydramatic.

The original Hydramatic continued to be used in light trucks and other commercial vehicles well into the 1960's. It was subsequently replaced in that role by the Turbo Hydramatic, whose simplified design was much less costly to manufacture. Despite the name, the Turbo Hydramatic (THM) was not mechanically related in any way to the original Hydramatic.

The Hydramatic was a complex design that was expensive to mass produce. Nevertheless, despite some early teething problems, it was fairly reliable and was so rugged that it was widely used in drag racing in the 1960's. It was not as smooth as some competitors (notably Buick's Dynaflow, the ultimate in slip 'n slide), but it also sacrificed less performance. It paved the way for the widespread acceptance of automatic shifting.

Hydramatic is now a trade name for GM's automatic transmission division, which produces a variety of later transmissions, the most notable of which is the Turbo-Hydramatic from the 1960s to the 1990s.


The Hydramatic used a two-element, fluid coupling (not a torque converter, which has at least three elements, the pump, turbine and stator) and three planetary gearsets, providing four forward speeds plus reverse. Standard ratios for the original Hydra-Matic were 3.82:1, 2.63:1, 1.45:1 and 1.00:1; the redesigned Jetaway Hydramatic used 3.96:1, 2.55:1, 1.55:1, and 1.00:1.

A unique feature of the Hydramatic design was the manner in which the fluid coupling was interposed in the power flow. In modern automatics, all engine power passes through the torque converter and then on to the gear train. Unless the converter includes a clutch to lock the turbine to the pump, some slippage will always occur. This was not the case with the Hydramatic.

In first gear, power flow was through the forward planetary gear assembly (either 1.45:1 or 1.55:1 reduction, depending on the model), then the fluid coupling, followed by the rear gear assembly (2.63:1 reduction) and through the reverse gear assembly (normally locked) to the output shaft. That is, the input torus of the fluid coupling ran at a lower speed than the engine, due to the reduction of the forward gear assembly. This produced an exceptionally smooth startup because of the relatively large amount of slippage initially produced in the fluid coupling. This slippage quickly diminished as engine rpm increased.

When the transmission upshifted to second gear, the forward gear assembly locked and the input torus now ran at engine speed. This had the desirable effect of "tightening" the coupling and reducing slippage, but unfortunately also produced a somewhat abrupt shift. It wasn't at all uncommon for the vehicle to lurch forward during the 1-2 shift, especially when the throttle was wide open.

Upon shifting to third, the forward gear assembly went back into reduction and the rear gear assembly locked. Due to the manner in which the rear gear assembly was arranged, the coupling went from handling 100 percent of the engine torque to about 40 percent, with the balance being handled solely by the gear train. This greatly reduced slippage, which fact was audible by the substantial reduction that occurred in engine rpm.

Many Hydramatics did not execute the 2-3 shift very well, as the shift involved the simultaneous operation of two bands and two clutches. Accurate coordination of these components was difficult to achieve, even in new transmissions. As the seals and other elastomers aged, the hydraulic control characteristics of the unit changed and the 2-3 shift would either cause a momentary flare (runup in engine speed) or tie-up (a short period where the transmission is actually in two gears at the same time).

The shift from third to fourth gear locked the forward gear assembly, producing 1.00:1 transmission. The fluid coupling now only handled about 25 percent of the engine torque, reducing slippage to a negligible amount. The result was a remarkably efficient level of power transfer at highway speeds, something that torque converter equipped automatics could not achieve without the benefit of a converter clutch.

Use in Pop Culture

In the 1978 musical Grease, Danny Zuko (played by John Travolta) describes his beloved hot rod as automatic, systematic and hydra-matic. Why it's Greased Lightning!

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