The remaining 60% is in full mechanical connection, which made these Hydramatics the most efficient automatic until lock-up torque converter came into widespread use.
The other disadvantage of Roto HydraMatic was the extreme oil pressures in the small 8" fluid coupling, which caused transmission leaks.
The Roto Hydramatic was phased out after the 1964 model year in favor of the two-speed Super Turbine 300 and three-speed Turbo-Hydramatic.
It is believed Pontiac was made to take Roto in the short wheelbase models ( Catalina, Ventura, Grand Prix) by the Corporation.
The power flow from the engine crankshaft to the transmission input shaft was connected via a flywheel containing a set of springs to absorb torsional vibrations at the flexplate.
Not only was the transmission shifting to a very much higher range ( 2.93 to 1.56, but it was also simultaneously emptying the fluid coupling and making a mechanical lock-up.
Because of the mechanical lock-up, there was sometimes a shudder felt as the transmission shifted into third gear (fourth range) by refilling the fluid coupling.
Bonneville and StarChief continued to use the 4 speed dual coupling called Super HydraMatic until the end of the 1964 model run.
The lock-up feature of Packard's Ultramatic and Studebaker's DG 250 is commonplace in virtually all modern 4+speed, overdrive automatic transmissions.
It is 9 lb (4.1 kg) lighter and much shorter in length — a necessary design parameter considering its placement in compact cars.
Buick's Dual Path was an air-cooled 2-speed unit with a planetary gearset inside the flywheel-mounted torque converter.
The performance was claimed to be almost as fast as with synchromesh gears and the 2-3 shift could be so gradual in gentle driving that the driver did not always realise it has happened.