High stall torque converters are very desirable in drag race vehicles because they allow the engine to develop maximum torque (say 5000 rpm), before the vehicle leaves the starting line. This type of converter is designed to slip more, or have a higher stall speed then the typical 1300-1900 rpm factory converter. Modifying an engine for increased power will raise the rpm it produces at this new usable horsepower. When the engine power is increased, it will require matching the proper high-stall torque converter. This is important, in order to allow the engine to reach its usable horsepower range before the converter reaches its stall speed, and hydraulically loads the engine. Stall speed is the maximum rpm that an engine can obtain against the hydraulic load of the torque converter with the transmission shaft held stationary.
The best performance is obtained with a converter that has a stall speed slightly higher than the rpm of the engine when it starts to produce its strongest usable power. Too low of a stall speed will result in a bog, and launches will be slow, since the engine will not be in it's usable power range. Too high of a stall speed will result in the engine reaching its peak power range below the rpm. Since the converter can efficiently transfer power, this could result in excessive slippage.
The stall speed is controlled by the converter, engine power, and the ability to hold the transmission input shaft stationary. When engine power is increased, stall speed will go up. Likewise, when engine power is decreased, stall speed will go down. Stall speeds with this type of torque converter are generally rated as what's termed the "Flash Stall Speed" or shock. This is the approximate rpm that the engine will go to when the throttle is quickly moved from the idle position to the wide open throttle with the transmission engaged.
This "Flash Stall Speed" will vary depending upon engine power, rear end ratio, and car weight. It is very important to match all of the engine components; cam, carburetor, torque converter, rear end gear ratio, etc, to work in the same rpm range for the best results. When modifying the engine, do not over cam or over carburete the engine. These are two of the most common mistakes.
Select a rear-end gear ratio that will allow the modified engine and the high stall torque converter to perform efficiently in the rpm range they were built for. A general recommendation, would be to use gears in the following ranges:
3.10 to 3.50 for 2,400 to 2,800 rpm stall converter,
3.50 to 4.10 for 3,000 to 3,800 rpm stall converter,
4.10 and higher range with higher stall converters.
Keep in mind, for a given stall speed, the turbine shaft is held stationary with either the wheel brakes or the transmission brake until the driver is ready to take off. The use of a trans-brake allows the driver to make maximum use of a given stall speed. Stall speed will be limited in cars without a transmission brake. This is because the gear reduction of the transmission and the rear end would provide enough mechanical advantage to overcome the car's wheel brake system. When using the wheel brakes, often the best launch technique for a high stall converter is to leave from a fast idle. This will allow the converter to momentarily flash to a higher stall speed (Flash Stall Speed). This is better then bringing the engine rpm up against the converter and car brakes and also preloading the suspension. When using this launch technique, the engine must idle clean and have a stumble free response to throttle movement.