A supercharger pulley is a device used on a supercharger to drive the unit through use of the engine's accessory belt drive system. Mounted at the end of the supercharger's input shaft, the supercharger pulley is designed so that its size is directionally proportionate to the speed that the supercharger turns, resulting in a predetermined power potential of the unit. By altering the size of the supercharger pulley, the speed the unit is turned affects the amount of boost or pressure at which the unit will push the fuel mixture into the engine. Most pulleys are made of steel and are designed to operate with a supercharger pulley-type, ribbed or Gilmore-style belt drive.
A supercharger is nothing more than an air compressor mounted on top of a vehicle's engine. The unit is powered by a belt running around the engine's accessory drive belt and the supercharger pulley. In some applications, the drive belt may be exclusive to the supercharger and run only from the vehicle's crankshaft hub to the supercharger pulley. Commonly an aftermarket-type supercharger, the factory-installed units commonly utilize the engine's fan belt and design it to not only serve the engine's components such as the water pump, alternator and fan, but to also pass around the supercharger pulley.
As the supercharger spins, it packs the engine's cylinders with an air fuel mixture much denser than a traditional fuel system. This allows the engine to produce much more horsepower than a similarly non-supercharged engine package. Engine builders are able to adjust the amount of power that an engine is capable of producing by altering the size of the supercharger pulley. As with any chain or belt-driven component, changing the size of the pulleys or sprockets will change the speed at which the device turns, thereby altering its performance.
The common supercharger pulley is pressed onto the tapered input shaft and secured with a single bolt or nut. The pulley is held in place with the fastener and prevented from spinning on the shaft by a key that fits in a machined groove in the shaft and the pulley itself. On some supercharger units, the drive pulley is attached to the input hub by six bolts. On this type of supercharger, often referred to as a roots-type blower, the top and bottom drive pulleys are different sizes and are interchangeable, enabling the unit to be over-driven or under-driven. This allows the engine to run with one supercharger drive ratio, and by swapping the pulleys, a different drive ratio is applied.