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Torque converters (TCs) are fluid couplings installed between an engine and transmission that allow the engine to idle without engaging the transmission. Torque converters have a number of advantages over a standard clutch setup, including torque multiplication.
History
The torque converter was first used in 1934 on Chrysler's hybrid automatic/manual transmission called the FluidDrive.
Purpose
A torque converter allows an engine to idle without engaging the transmission below a set RPM. This keeps the engine in its ideal operating range and prevents stalling.
Construction
A torque converter has two internal turbines. One is mounted to the case and powered by the engine, the other is connected to the transmission input shaft.
Function
The engine-side turbine is constantly pushing transmission fluid through the vanes of the transmission-side turbine. Once this fluid flow overwhelms the tranny-side's capacity to pass it without moving, the TC's "stall speed" is reached, and the car moves.
Torque Multiplication
During the moments just before full lock-up, the engine-side turbine is turning slightly faster than the transmission turbine. This creates a gearing effect, where torque input to the transmission is greater than that exerted by the engine.
Source:
Rod and Custom magazine: Understanding Torque Converters
Car Craft magazine: What's Up With Stall?
Chevy High Performance magazine: TCI transmission and Torque Converter Technology Insights
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