...It took the addition of a small device called the stator to increase the efficiency of the fluid coupling and transform it into a torque converter. The stator redirects fluid from the turbine to the impeller, radically increasing the force of the fluid entering the impeller and multiplying the torque input from the engine. Basically, the fluid exits the center of the turbine and is redirected by the stator back into the impeller. This is a very slick trick and can be worth up to 2.5 times engine torque! This means that at a 2:1 torque multiplication ratio, an engine that is making 300 lb-ft of torque at a stall speed of 2,500 rpm will actually apply 600 lb-ft of torque to the input shaft of the transmission as the car leaves the starting line.

This sounds really great, and it is. But the catch is that this torque multiplication doesn't last very long. The multiplication factor is based on the speed difference between the turbine and impeller. At stall speed, the impeller is spinning at engine rpm and the turbine is stopped. This creates the greatest speed difference between the turbine and the impeller and therefore the maximum torque multiplication. Once the brakes (or the transbrake) is released and the vehicle begins to move, the turbine spins up and the speed difference between the two diminishes. Generally, by the time you reach the 60-foot mark on the dragstrip, torque multiplication is effectively eliminated and the turbine and impeller speeds are close to achieving what converter manufacturers call coupling speed.