Underdrive pullies have a smaller crank pulley this will in turn spin your supercharger slower. Therefore your boost will be down, to compensate you can get a smaller blower pulley, but you can only go so small and decreasing it's size leads to belt slippage. If your ultimate goal is boost, then at least keep stock pullies to put back on. Better yet concentrate on mods that help your engine breathe better I.E. long tubes, CMCV delete, heads & cams and when you do slap on the huffer you'll get that much more for it.