GFB Crank Pulley (Not Under-Drive) 2002-2014 WRX / 2004-2021 STI
Install a GFB Non-underdrive light-weight under-drive pulley kit and get better throttle response and more power! How do they do this? 1. Less weight - GFB's pulleys are manufactured on the latest Multi-Axis CNC machines from the highest grade 6061-T6 aluminium billet, making them immensely lighter than their factory steel counterparts. This lower weight yields a large reduction in the rotating inertia of the belt train, allowing the engine to accelerate faster.
The effect is similar to fitting a lightened flywheel and is particularly noticeable in the lower RPM range before the turbo spools up. This is because the power required to accelerate a heavy pulley accounts for a much greater proportion of the available power at low RPM. 2. Under-driving - Additional power increases also come from making the crank pulley smaller, thereby slowing down the accessories and reducing the driven load (particularly at high RPM), without affecting their operation.
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.