A. Both CravenSpeed and Alta offer 15% and 17% supercharger pulley’s and they are designed to decrease rotational inertia of the supercharger and increase boost. The 15% reduced pulley is a popular option that produces a smooth, safe power increase of between 15-17 bhp. This size is most suited to those MINI owners that don't want to invest in a performance intercooler or continually track their MINIs with a stock engine. The 17% reduction builds boost at lower RPM giving even more power when it's needed, this supercharger pulley is better suited to those MINI owners that want more power for occasional short bursts, however, if you are choosing this size then we recommend fitting an uprated intercooler to your R53 to accommodate for the increased temperatures.