To the average gearhead, a torque converter's method of function seems fairly straightforward: the converter allows the engine to spin to a certain rpm before it engages the transmission. The basic method is to choose a converter designed to your exact torque output and rated to stall a few hundred rpm below the engine's torque peak. However, turbocharged engines pose a few specific challenges to torque converter selection because the engine's torque curve isn't necessarily tied to engine rpm.
Test your engine's horsepower and torque output. If you haven't installed the engine yet, then take it to a shop with an dynamometer and test it there. If the engine is already in the car and you're looking to change converters, load the car onto a chassis (wheel) dyno and test it that way. If possible, take your car to a facility with the capacity to record boost pressure ratings in conjunction with rpm.
Example: An Evo VIII makes 357 horsepower at 7,120 rpm and peaks at 314 ft-lbs. of torque at 5,000 rpm. The turbo produces 8 lbs. of boost at 2,500, 10 lbs. at 3,000, 18 lbs. at 3,500 and peaks at 20 lbs. of boost at 4,000 rpm through 6,500 rpm.
Find the halfway point on your turbo boost map; that is, the rpm where turbo boost is at half if its maximum. Mark this rpm. Locate your peak torque rpm, and subtract from it the half-boost rpm (HBR) to determine the difference. Divide this difference by two, and add it to the HBR. In this way, you can determine the halfway point between HBR and peak torque, which is you target stall speed.
In our example, 5,000 rpm (peak torque) minus 3,000 rpm (HBR) equals 2,000. That 2,000 derivative divided by two equals 1,000. Finally, 1,000 plus 3,000 rpm equals 4,000 rpm, which is our target stall speed.
Examine the horsepower/torque chart to find out how much torque the engine produces at your target stall speed. The Evo VIII in our example produces about 300 ft-lbs. at the target 4,000 rpm stall speed. Now, all our turbo pilot needs to do is to order a torque converter designed to produce a 4,000 rpm stall at 300 ft-lbs. of torque.
