Originally Posted by semtex

Someone else could probably explain this better than me, but let me give it a shot. The whole purpose of underdrive pulleys is to supply power to secondary items such as the A/C. Power supplied to those items is power diverted away from the wheels (you can actually feel a slight loss when you turn the A/C on if you pay careful attention). Thing is, the pulleys usually supply more power than those secondary things actually need, hence there's room to reduce the amount of power supplied by the pulleys without negatively impacting the ability of those secondary systems to function properly. And the power that is saved goes directly back to power at the wheels. Now enter in superchargers. Superchargers draw their power in the exact same way as other secondary systems such as A/C. So whereas it's perfectly okay to divert some power away from the A/C unit, you don't want to do that to a supercharger, because then it'd be as though you're purposely handicapping your supercharger. Make sense? Let me know if it doesn't, and I can ask Josh to chime in with a better explanation.