GFB 04-10 WRX/STI 3 Piece Under-Drive Pulley Kit w/ Belts (Crank Alternator & Power Steering) Go Fast Bits 2010

It might be hard to imagine how reducing the mass of a pulley makes a car go faster, but accelerating a heavy factory pulley from say 3000-7000 RPM in a second or two saps a measurable amount of torque. GFB Lightweight Pulleys reduce inertia by up to 85 percent, so most of the torque that used to accelerate the pulley can now be applied to the wheels.