These Steeda Supercharger Pulleys are made from Steel not aluminum and have holes drilled around the center of the pulley to lower the rotational weight. This is the highest quality pulley available for the Cobra. 2.81" pulley 4-5psi over stock, requires SCT XCalibrator 2 Tuner & shorter belt. Testing has shown gains of 60+ horsepower. Serpentine Belt for 2.81" When running a 2.81" pulley you will need a shorter serpentine belt to keep tight on your blower for maximum boost.