A torque converter is the direct connection between the engine and transmission. It works much the same way that a stall converter works, but to understand a stall converter, you must first understand how a torque converter works. The torque converter is attached to the flexplate by three bolts and the transmission input shaft slides into the torque converter. It meshes into the converter by a set of splines on the male end of the input shaft and into a hole with splines in the torque converter.
The torque converter has four major components. The first component is the impeller or pump, which is directly attached to the converter. The pump is turning continuously. This produces the hydraulic pressure. The turbine is the next component and is not attached to the converter. It is free floating and attached only to the splined input shaft from the transmission. Its purpose is to supply the torque to move the vehicle. The next is the one way clutch that locks up the converter to the input shaft. The last component is the stator. The stator is designed to multiply the torque or pressure and reduce the speed of the impeller.
The best way to explain the operation is to imagine having two fans facing each other. Turn on just one fan and soon the second fan will begin to spin. If the second fan is held from moving, the first fan that is turned on will of course continue to spin. This is how a car can come to a stop and not stall the engine. Now add a stator in between the two fans. Turn the working fan down to low. The driven fan or the one that is not turned on will have much more pressure although it can still be held. This is similar to an idle or the car sitting still.
Turn the working fan on high, which is similar to stepping on the gas. The driven fan comes to life and can no longer be held. This is basic, but it best explains the way the converter works. The rpm that the torque converter begins to apply torque is dependent on the curvature and type of blades used on the stator and impeller. The normal efficiency of the torque converter is 90 percent of engine speed, hence the lock up clutch. The other 10 percent allows the engine a little flash time for accelerating without lugging the engine. For fuel economy, the lock up will only occur electronically at a cruise or 50 miles per hour and up to a point where no further torque is required. This makes 100 percent 1:1 ratio that lowers the engine rpm and saves fuel.
A stall converter does what a torque converter does, but has a modified stator that requires a higher than normal rpm before it locks up. Lock up is determined by the amount of rpm the engine can achieve with the brakes on. Stall converters can be purchased with a wide range of stall from 1200 rpm to 6000 rpm. Ideally for the best launch or acceleration, the stall should be 80 percent of the rpm necessary for maximum torque. Stall converters are generally used in racing applications or towing applications, but it is not unheard of for a high horsepower street vehicle to be equipped with a stall converter instead of a torque converter.
