Hydramatic Transmission

The Hydramatic (also Hydra-Matic) transmission was introduced by General Motors in the 1940 model year as an alternative to the automatic synchromesh-based transmission invtented by Earl A. Thompson in 1928. Although Thompson's design was being used in GM's Cadillac line, the company wanted something in-house (proprietary) and better performing.

Several renditions of the Hydramatic were conceived before its introduction. Once introduced, it was offered as an option on the Oldsmobile (in order to provide a larger trial base and preserve the Cadillac name plate's reputation). It was not a resounding success, but was far from a failure, so it eventually made it through to most of the GM line. It was an option on many GM-made vehicles through the 1960s.

The Original Hydramatic

The Hydra-Matic got its name because it uses liquid (fluid) instead of a friction clutch to facilitate shifting. This eliminated the need for de-clutching, one of the clunky problems with the original Thompson design.

Although it was expensive to produce, the Hydramatic proved robust and popular and so as variants entered the market, the original design stayed largely intact as a popular option for pickup trucks and commercial vehicles into the 1960s and was a popular option in stock car racing as well. Licensed, after-market versions of the original Hydramatic are still available for restoration and custom vehicles.

The automatic transmission division at GM is still called Hydramatic today.

How The Hydramatic Works

Not to be confused with other, newer transmissions with the Hydra-Matic name (which are mechanically very different from the original), the first Hydramatics stayed basically the same through to their end of manufacture in teh 1960s.

This transmission used a two-element fluid coupling rather than a torque converter and three planetary gear sets (four speed plus reverse). Various ratios were produced for different applications.

Two pumps pressurized the hydraulic control system and provide lubrication. One pump was electrically driven while the other was driven manually by the movement of the output shaft of the transmission. This allowed the front pump to be used at low speeds and idle. Its design made it neutralize at higher speeds where the gear-driven pump could operate all functions itself.

As the gears moved through, less and less engine torque was sent through the coupling, essentially improving efficiency and reducing slippage as the vehicle gains speed. In first gear, the fluid couplings are handling about 100% of the engine's torque, but by fourth gear, it's handling about 25%. This reduces slippage and increases efficiency from engine to drive tires.

This reduced transfer made the Hydramatic more efficient at highway speeds than its torque converter equipped cousins (until the converter clutch was introduced).

Once the engine is shut off, pressure in the transmission dissipates almost immediately. When the selector lever was put in reverse without pressure, the Hydramatic would physically engage a sort of parking brake by locking the ring gear and band with a heavy spring. This became one of the most effective drive-shaft-mounted parking brakes ever invented.

Overall, the Hydramatic was one of the most innovative designs in automated transmissions ever conceived.