When only the lightest will do! This pulley set was developed by Norm Floyd and Nick Engels to reduce the rotating mass on the end of the crankshaft of their IT race car to just about nothing and greatly reduce parasitic power losses through the water pump and alternator. NOT FOR STREET USE! This crank pulley weighs less than 10% what the OEM pulley weighs!
Using the crank pulley alone, it will spin the alternator at about 1/4 speed. Using the alternator pulley alone, at about 2/3 speed. Using both together, about 1/5 speed. That's why they are advertised as NOT for street use. The alternator doesn't really charge much until about 4000RPM.
The water pump runs at about 1/4 speed - which is plenty at racing speeds. It doesn't cavitate, but you can't idle it for very long.
This is only a single-groove pulley, so no A/C drive (or PS pump on FWD cars - although there is a set on an ITB Prizm GSi).
The benefits, other than the obvious rotating mass thing, are that there is less parasitic drag from both the alternator and water pump, as well as less working loss. Norm and Nick predicted a 4 HP increase at 6K - based on computer modeling.