Lightweight Underdrive Crank Pulley for Scion This undersrive crank pulley is exactlly 3% smaller diameter than the stock pulley. Reducing the size of the crank pulley reduces the draw from engine accessories, by slowing them down. That frees up power to the drive the wheels leading to as much as a 15 HP boost. You will need to change to a smaller diameter belt, but will also reduce wear on the new belt with this pulley. CravenSpeed crank pulleys come perfectly balanced, and feature anodizing for superb wear resistance & strength. Releasing Power The rule of thumb is that each pound of weight taken off the engine's rotating assembly is worth approximately 2.7HP. The concept is that the crank is a parasite; it does not put power to the drivetrain but still requires the energy produced by the engine to rotate. When you reduce this parasitic weight, you release power that was previously used only for spinning the supercharger and alternator. Compatible with Scion tC 2005-2008 and xB 2008-2015