some cause a vibration in the crank angle sensor at high RPM. I had an old Perrin that did, they have since redesigned. I've got a Worx Tuning pulley which won an award at SEMA for having a design to elimiate the vibes.. it's easy to do. loosten the belts to the alt and PS and AC pulleys put a long breaker on the crank pulley, put it in 4th and get a buddy to hold the brakes then loosen it up and swap the pulley out. Watch not to lose the key that keeps it from spinning around.