Vehicles equipped with an automatic transmission use a torque converter instead of a clutch, which allows the vehicle to idle without propelling the vehicle. Once the vehicle's engine reaches a certain speed, the torque converter routs the power produced by the engine to the transmission. This is referred to as the torque converter's "stall speed."
A torque converter actually does three tasks: first, a torque converter transfers the power generated by the engine to the transmission's input shaft. Secondly, it multiplies the torque produced by the engine to give the engine a mechanical advantage by about a 2:1 ratio. Thirdly, a torque converter turns the transmission's front pump to produce the hydraulic pressure the transmission requires to properly function.
A torque converter should be thought of as two halves. The half connected to the engine is called the front shell or "impeller," while the half connected to the transmission is called the rear shell. Between the impeller and the rear shell is the stator, the turbine and the sprag.
The turbine connects to the main shaft of the transmission, and the impeller attaches to the crankshaft, which causes it to spin at engine speeds. As the impeller spins, it draws in transmission fluid through its center, then forces that fluid out of its fins. The highly pressured fluid then acts on the blades of the turbine, which causes the turbine to spin. When the impeller spins faster than the turbine, the stator operates to redirect the transmission fluid back to the impeller to increase the pressure by which the fluid exerts against the fins of the turbine at low speeds, which multiplies torque. If the transmission is in gear and the engine is idling, applying the brake causes enough drag to allow the impeller to spin independently of the turbine.
Stall speed is the maximum speed at which the turbine can be held steady at wide open throttle. Simply put, a converter's stall speed is the speed at which an engine must turn, as measured by revolutions per minute or "rpms," to begin moving the vehicle. As the engine speed increases, the stator within the converter spins along with the rest of the converter, and is responsible for getting the vehicle moving when the accelerator is depressed.
How to choose a torque converter's stall speed is by far the most common question regarding torque converters. The important factor to remember is that the converter's stall speed is the speed at which the engine must turn before the vehicle will begin to move. Car enthusiasts often replace the stock converter with one which allows the engine to reach a higher speed before the vehicle begins to move. Delaying the point at which the vehicle will begin to move allows the engine to reach the speed where it begins to make its power. Stock converters typically have a stall speed of between 1,400 to 1,800 rpm, while slightly modified engines may use a converter with a stall speed of approximately 2,400 rpms. Race engines frequently use torque converters with a stall speed of 3,500 rpms.
