Roto Hydramatic

Roto Hydramatic (sometimes spelled Roto Hydra-Matic or Roto-Hydramatic) was an automatic transmission built by General Motors and used on some Oldsmobile and Pontiac and Holden models from 1961–1965. It was based on the earlier, four-speed Hydramatic, but was more compact, providing only three forward speeds plus a small 8" fluid coupling with a stator inside of the fluid coupling. Oldsmobile, one of the users of this transmission, called the fluid couplings stator the "Accel-A-Rotor." The lightweight, aluminum-cased transmission was sometimes nicknamed the "Slim Jim." HydraMatic Division calls the Roto a four range, three gear HydraMatic. It counts the stator multiplication @ 3.50 to one as a first gear, and when road speed and the two coupling halves speed match, it counts the same gear with fluid now passing straight through the stator as 2nd gear at 2.93 to one. 2nd gear or third range has a ratio of 1.56 and because the fluid coupling is drained for this gear ratio making the front clutch apply makes this a rare automatic that is in FULL mechanical lock-up (coupling drained) in second gear. Fourth range the coupling fills releasing the front clutch makes a ratio of 1 to 1. This transmission, like single and dual range, and dual coupling hydramatics also have the feature of split torque in the transmission whereby in fourth or high gear only 40-to 50% depending on transmission, 40% in Roto's case, but because of the design the coupling is only required to carry 40% of the engine torque. The rest is (60%) is in full mechanical connection making these hydramatics the most efficient automatic until the wide spread use of the lock-up torque converter.

Roto's downfall was the 2-3 range or 1-2 gear change because it is not only a huge ratio jump from 2-3 range or 1-2 shift from 2.93 to 1.56, but also there is no fluid slippage in the coupling because the coupling drains (4 tenths of a second) to engage or apply the front clutch and so the trans goes to full mechanical connection. The other downfall of Roto HydraMatic was the extreme oil pressures in the small 8" fluid coupling which caused transmission leaks. All HydraMatic transmissions suffer some shift quality with today's ATF fluid as Type "A" fluid is not available. The main ingredient of Type "A" was banned years ago, however Japanese cars that have CVT automatic's still use that ingredient in their special CVT fluid. That CVT fluid can only bought at the dealer. The ingredient is Whale oil.

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