Basically the stall speed is determined by the torque input to the converter. Example: The 9 1/2 inch converter in my race car stalls at about 3200 rpm behind a 170 cid engine. The same converter in the same car, but with a 225 CID engine stalls about 4,000 rpm. Bigger engine, more torque, higher stall. That same converter could be used behind a 400+ cid small block and the stall would be in the stratosphere. That is why you don't buy a converter by advertised stall speed. Means nothing without knowing the input torque.