A torque converter provides slip and torque multiplication in an automatic transmission. It allows the engine to operate at speeds that are not linearly related to the rear wheels.
When selecting a torque converter several key variables need to be considered including stall speed and lock-up capability. Sometimes also called "flash stall", the stall speed of a torque converter is the rpm the engine will go to when the vehicle is at rest and the engine is accelerated to the wide open throttle position (WOT). It is important to keep in mind that the stall speed of a torque converter is not fixed but varies with engine power output and load. A torque converter with a 3000 rpm stall speed on a 300 hp engine may have a 4000 rpm stall speed on a 400 hp engine.
The benefit of a "looser" torque converter is that it gets you into the power band of the engine earlier. If the torque converter allows 20% slip and the output shaft is turning at 3000 rpm, the engine will be turning at 3750 rpm. If the engine makes more than 20% more power at 3750 rpm than it did at 3000 rpm, then the slip of the torque converter has actually increased power delivery to the rear wheels. A "loose" torque converter often hurts driveability and this must be taken into account when selecting a converter for street/strip applications.
A "lock-up" torque converter offers the ability to virtually eliminate the slip in the torque converter and improve its efficiency. Lock-up torque converters are used in most newer production cars to help improve fuel economy. Some drag racing classes do not allow lock-up torque converters. Once you are in the power band of the engine and the slip in the torque converter is greater than the gain in horsepower, it is usually beneficial to lock the torque converter. This will improve elapsed time and especially trap speeds.
Torque converter design and selection is an important but often complicated issue. Many of the performance torque converter companies and distributors offer technical support that can be very beneficial in selecting the proper torque converter for your application. Torque converters have many variables and can often be optimized for different racing (either drag racing, auto-x or road racing) and street applications.
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