By spinning the alternator slower than stock you can free up some of the HP robbed in the upper RPM bands. You can still use the stock accessory belt with this alternator pulley. Using a water pump underdrive pulley and an alternator UD pulley requires a larger belt.
Pulleys are anodized black and made from 6061 aluminum. Using aluminum allows weight savings over the factory steel pulley.
These are only compatible with the 97-03 alternators.