Underdriving your alternator is dangerous though, just cause it can kill your battery. Underdriving the crank on a celica is a bad idea because it acts as a harmonic dampener, you rempve that, you get strange noises and you can dammage your engine. If you are at 200whp will 202whp be ANYWHERE of a noticable difference? 2whp when you are at 120whp you will probably notice, but there is absolutly no reason to take the associated risk with an underdrive pulley if you go turbo or SC IMHO. It just doesn't make any sense. And what ryanceli said also makes sense, since you are driving the supercharger with the crank, an underdrive crank pully may underdrive the supercharger, I think that's a safe guess.