So what's the talk about stall speed? Stall speed boils down to the rpm at which the convertor effectively locks up the motor to the driveline and multiplies torque at a one-to-one ratio. Since the guts of a convertor consist of a fluid coupling, the two halves of a conventional convertor are never directly locked together like a clutch disk to a flywheel- unless of course it's a modern lockup version with an integral friction clutch. A good convertor should produce less than 10% slippage after the engine rpm has exceeded the stall speed, otherwise the constant, heat building slippage will lead to wasted engine output and an early death for the transmission.
Convertors are related to stall within a certain rpm range. That's not because the convertor companies don't know their product; it's just that your individual engine characteristics and vehicle specs influence stall speed and make a precise number nearly impossible to predict. Even the term "stall speed" itself isn't perfectly defined. Here are the three distinctions you're likely to encounter.
True Stall: The rpm the engine cannot exceed when the driveline is locked. The most accurate way to determine true stall speed is by locking first gear and Reverse with a transbrake and observing engine rpm at wide open throttle (WOT).
Flash Stall: The rpm the engine "flashes" to when launched from rest at WOT. A convertor will often briefly flash to a higher rpm than it's true stall speed.
Brake Stall: The rpm the engine cannot exceed with the brakes locked and the driveshaft not spinning. Brake stall isn't usually an accurate measurement tool since the engine overpowers the wheels before the true stall speed is reached.
