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As far as I understood from this video, there are torque converters and fluid couplers (a hydraulic clutch), but certain types of torque converters kind of work by performing the two functions at different speeds.

I just can't find hydraulic torque converters that serve as reduction boxes in bigger ratios, like gears, pulleys or transmission belts.

I guess the closest I could find was hydrostatic transmission, but this kind of transmission is used with hydraulic pumps and hydraulic motors separated, but connected. Which I couldn't find a precise amount of efficiency, but it was said to be really low.

Fulano
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Torque converters as used in passenger cars commonly operate over ranges of between 2:1 and 4:1 for output RPM. The maximum torque multiplication (and rpm "division") occurs essentially at the stall condition (output shaft prevented from rotating) but at stall, the efficiency is zero (all the power input is dissipated in fluid friction).

It is possible to construct torque converters with higher torque multiplication ratios than this but they are custom jobs used in things like drag racers and not commonly available.

It is also possible to place two torque converters in series to get higher overall ratios, as used in the 1963 Buick Riviera Twin-Turbine Drive, (sometimes known as the "Slush-O-Matic") but the power transmission efficiency of this arrangement is very poor.

A far more efficient arrangement is to combine a torque converter with a two-speed transmission, yielding the famous General Motors/Chevrolet PowerGlide of the mid-1960s.

niels nielsen
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