Go Fast Bits Pulley Kits (2001)

Suits WRX/STi MY94-07, Forester MY98-07, Legacy MY03-07)..
In some vehicle applications additional performance benefits are available at high RPM by underdriving the accessories (slowing them down), which reduces the parasitic load on the engine. All underdriven kits are thoroughly tested to ensure that the benefit comes without compromising the accessory performance

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.