Torque multiplication exists only when there is a difference in speed between the impeller and the turbine.
The magnitude of torque multiplication depends on load. When the turbine is stalled, it has a maximum value of about 2.2 to 1.
Stall is an operating condition where the turbine is stationary and the engine throttle is wide open, making the rotational speed of the impeller as high as possible.
Stall can be approximated to when a vehicle moves from rest, up an incline, towing a heavy caravan or car trailer.
Instantaneously, as the vehicle begins to move, maximum torque multiplication occurs.
At an engine speed of 2100 RPM, and torque at that speed of 100 Newton meters, the torque input to the transmission will be 2.2 times that value - 220 Newton meters.
This multiplication tapers off as the turbine commences to turn and increases in speed. This subjects the fluid in the turbine passages to centrifugal force, which slows down the circulation between impeller and turbine.
When turbine speed reaches around 90% of impeller speed, torque multiplication falls to zero. Torque transfer from impeller to turbine is then about 1 to1. This is known as the coupling point.
At coupling point, fluid flow from the turbine vanes is relatively low, but it is at high speed, in the direction of rotation. The rapidly turning turbine discharges its fluid against the back of the stator blades. This force unlocks the stator and all three elements rotate as one unit. Unlocking the stator prevents turbulence in the fluid and any braking effect on the engine.
