[blue00gt]

A stall converter is a viscous coupling that you could compare to a clutch as far as what it's function is. As the engine rpms come up, it allows less and less slippage until it reaches full lockup at the stall speed - at that point it's like having the clutch all the way out. I don't know how it actually works inside. All automatics have and need them because otherwise you could never get the car going from a stop. The idea behind having a higher rpm stall converter is the same as slipping the clutch off the line to get a better launch - keeps the engine from bogging and allows it to get into the powerband quicker. It is actually a good mod for even a stock auto stang, especially modular motored stangs because they have less low end grunt than the pushrod 5.0s so they are harder to get out of the hole quickly.
You just wouldn't want too high of a stall on a street car because if you are trying to cruise with the rpms below full lockup then it is always slipping and you are wasting gas and power and generating excess heat.