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


During the 1930s automakers sought to reduce or eliminate the need to shift gears. At the time, synchronized gear shifting was still a novelty (and confined to higher gears in most cases), and shifting a manual gearbox required more care than most drivers cared to exert. The exception here is Cadillac's break-through Syncro-Mesh fully-synchronized manual transmission, designed by Cadillac engineer Earl A. Thompson and introduced in in the fall of 1928.

Cadillac, under Thompson, began working on a 'shiftless' transmission in 1932, and a new department within Cadillac Engineering was created, headed by Thompson and including engineers Ernest Seaholm, Ed Cole, Owen Nacker and Oliver Kelley. During 1934, the Cadillac transmission group had developed a step-ratio gearbox that would shift automatically under full torque. This same group of engineers was then moved into GM Central Research, building pilot transmission units during 1935-36 which were then handed to Oldsmobile for testing.

The Automatic Safety Transmission was a tangent outgrowth of this work. The AST was a semi-automatic transmission using planetary gears and a conventional friction clutch, requiring the clutch to shift between forward & reverse gears, but not between the 2 forward gears. Oldsmobile offered the AST from 1937 to 1939; Buick offered it only in 1938. The results were not quite what GM Research had in mind.

The HydraMatic was designed to combine hydraulic operation of a planetary gearbox (allowing 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. Oldsmobile was chosen to introduce the H-M for two reasons: economies of scale, and to protect the reputation of Cadillac in case of market failure (an unnecessary precaution in that Cadillac developed most of the design). 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 was used in a variety of military vehicles, including the M5 Stuart tank (where 2 of them were mated to twin Cadillac V8 engines) and the M24 Chaffee light tank. The extensive wartime service greatly improved the postwar engineering of the transmission, later 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 did not have the resources to develop an automatic transmission bought Hydra-Matics from GM. Users included:
*1950-1956 Hudson
*1950-1956 Nash
*1951 Frazer
*1951-1955 Kaiser
*1954-1955 Willys
*1949-1954 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 four-speed 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 second gear or third gear until the maximum allowable upshift points, for improved performance in traffic or in mountain driving, and incorporated a separate Park position.

The Jetaway was substantially smoother than the original Hydramatic, but also more complex and expensive to produce. In 1961, a somewhat less complex, but also far less reliable three-speed Roto-Hydramatic also dubbed the "Slim Jim" Hydramatic (in which the "dump and fill" shifting principle was retained) was adopted for all Oldsmobiles as well as Pontiac's full-sized Catalina, Ventura and Grand Prix models, while all Cadillacs and Pontiac's Bonneville and Star Chief models retained the older four-speed "Jetaway" unit (not to be confused with the two-speed Jetaway automatic used in various Buick, Olds and Pontiac intermediate cars from 1964 to 1969). Both of those Hydramatic transmissions were ultimately replaced by a new three-speed torque converter automatic transmission called Turbo-Hydramatic in 1964 and 1965, whose design was more similar in principle to the Chrysler TorqueFlite and Ford's Cruise-O-Matic than the fluid coupling Hydramatic the "Turbo" replaced.

The original Hydramatic continued to be used in light trucks and other commercial vehicles well into the 1960s. It was subsequently replaced in that role by the Turbo-Hydramatic (THM), whose simplified design was much less costly to manufacture. Despite the name, the Turbo-Hydramatic has no mechanical relationship to the original Hydramatic.

The Hydramatic was a complex design that was expensive to produce. Despite some early problems, it was reliable and was so rugged that it was widely used in drag racing for decades. It was not as smooth as some competitor's transmissions (notably Buick's Dynaflow), but was more efficient, especially at highway speeds. The Hydramatic paved the way for widespread acceptance of automatic shifting.

Hydramatic is a trade name for GM's automatic transmission division, which produces a variety of 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 in automotive applications, and 4.08:1, 2.63:1, 1.55:1 and 1.00:1 in light truck and other commercial applications. The 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, which can have a significant negative effect on efficiency and fuel economy. 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 when the shift occurred.

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.

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 transmission's seals and other elastomers aged, the hydraulic control characteristics 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 in two gears simultaneously), the latter often contributing to failure of the front band.

From 1939 through 1950, the reverse anchor was used to lock the reverse unit ring gear from turning by engaging external teeth machined into that ring gear. From 1951 on, a cone clutch did the same thing when oil pressure was up, and a spring loaded parking pawl was allowed to lock the same ring gear in the absence of oil pressure. This worked better as the anchor would not grind on the external teeth if that ring gear were turning (that is, unless the engine stalled as reverse was engaged). Reverse was obtained by applying torque from the front unit (band on, in reduction) through the fluid coupling to the rear unit sun gear. The planet carrier of this gearset was splined to the planet carrier of the reverse unit. The rear unit ring gear hub had a small gear machined on its end which served as the reverse unit sun gear. Because the rear unit band was not applied for reverse, the rear unit and reverse unit compounded causing the combined planet carriers to rotate opposite to the input torque and at a further reduced speed. The output shaft was machined onto the rear unit and reverse unit planet carriers.

Shutting off the engine caused the transmission oil pressure to fall off. If the selector lever was in reverse or moved to reverse after the engine stopped, two mechanical parts combined to provide a parking brake. The reverse unit ring gear was held stationary by the reverse anchor. The drive shaft could still turn causing the reverse unit sungear and attached rear unit ring gear to rotate at a very high speed, were it not for the fact that the rear unit ring gear band was now applied by a heavy spring. Usually, bands are applied by a servo and released by spring pressure, but in this case, the band was held off by the servo and applied by spring pressure (actually, when the engine was running, the band was applied by a combination of spring pressure assisted by oil pressure). With the engine off, this brake band acting on the rear unit ring gear had a tremendous mechanical advantage. Since the rear unit ring gear with its attached reverse unit sun gear and the reverse unit ring gear were both locked to the transmission case, the planet carriers and driveshaft could not turn. As such, it provided and effective driveshaft mounted parking brake to be used alone or supplementing the hand brake.

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".


*A massive fire that destroyed GM's Hydramatic plant in Livonia, Michigan on August 12, 1953 left the corporation and the three divisions that used this transmission scrambling for other sources of automatic transmissions to complete that year's model year production. As a result, Oldsmobiles and Cadillacs during the downtime were assembled with Buick's Dynaflow transmission, while Pontiacs used Chevrolet's Powerglide, both two-speed torque-converter units. Non-GM makes that bought Hydramatics from the corporation, including Ford Motor Co.'s Lincoln division and independent automakers Hudson, Kaiser and Nash, ended up looking for other sources of automatic transmissions as well, with Lincoln using the corporate Ford-O-Matic transmission, while the other automakers purchased automatics from Borg-Warner during the downtime.

*About nine weeks after the Livonia fire, GM opened up a new source for Hydramatic production at Willow Run, Michigan. By the time the 1954 models debuted in late 1953, Hydramatic production had returned to normal levels and all '54 model Cadillacs, Oldsmobiles and Pontiacs with automatic transmissions were once again equipped with Hydramatics.

*The first-generation Hydramatic (not the Jetaway version that succeeded it in 1956) did not have a separate park position as found in modern automatic transmissions. The driver had to shut off the engine and then place the transmission in reverse in order to lock the driveline to prevent the car from moving. Also, the original Hydramatic required periodic band adjustment as a routine maintenance item that later versions did not.

*The first version of the Hydramatic used a mechanical pawl to lock the planet carrier of the reverse gearset when the driver wished to back the vehicle. This arrangement did not work well in practice and was replaced with a cone clutch.

*The all cast iron Hydramatic was the heaviest automatic transmission ever produced for automobiles.

*Both the "Jetaway" and "Roto" Hydramatic transmissions used an unusual gear selection sequence of P-N-D-S-L-R rather than the more common P-R-N-D-S-L sequence used on most other automatics during the 1950s and early 1960s and also adopted with the Turbo Hydramatic design.

*Other manufacturers to utilize GM's various HydraMatics over the years have been: Ferrari, BMW, Volvo and Jaguar. Rolls Royce continued to use GM's HydraMatic (the THM-400) from the end of their license agreement in the mid-1960s through 1986, when the near-identical 3L80 took its place until 1992.

External links

* [http://www.gm.com/explore/technology/gmpowertrain/ General Motors Powertrain Division]

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