You want the stall speed to match the rpm where your motor is making power.

if you have a motor that makes say 500HP but the power band of the motor is from say 4000rpm to 7200rpm then if you have a converter that has a stall speed of 2500rpm the car will come of the line at 2500rpm and fall on its face.

So if you put a smaller diamiter converter in and it's stall is around 4500rpm the car will come of the line at about 4500 rpm which is 500rpm over where the motor is making power and it will launch the car hard.

sounds complicated but really its not.

BG