A racing pulley is an engine accessory pulley that is either smaller or larger than the stock component, depending on the pulley. Often sold in a set, the racing pulley allows the engine to produce more horsepower to the drive tires than a comparable stock pulley set-up. Generally placed on the crankshaft, water pump and alternator, a racing pulley compensates the speed of the accessory in relation to the high racing speeds of the engine. Along with the change in the pulley diameter, the surface of the pulley is often enhanced to better retain the drive belt at high racing speeds.
When a smaller drive pulley is placed on an engine, it slows down the speed at which the engine accessories are being turned by the engine. This, in turn, allows more engine power to be directed to the drive tires creating more top speed for the vehicle. Often, the water pump racing pulley will be larger than the stock piece in order to avoid overheating when running slower on the race track during caution periods. The danger in pushing the coolant too fast through the engine and radiator is that the radiator may not have sufficient time to absorb the heat out of the liquid if the wrong racing pulley is used.
A vehicle's alternator is perhaps the primary beneficiary of the racing pulley. If allowed to spin at the increased racing speeds, the alternator would very likely be burned out and ruined. The alternator is only effective at producing power until an engine reaches a certain speed; after that speed has been surpassed, the alternator begins to overheat and damage the inner workings. It is the job of the engine builder to determine what the proper driven speed of any component should be and install the applicable racing pulley.
The thickness of the racing pulley is also critical, and the typical pulley is made of a thicker steel than a stock pulley. This prevents cracking not only from the harsh racing conditions the pulley operates in, but also from engine harmonics emanating from the high revving engine. Harmonics are sound waves that can actually destroy the engine's bearings and other critical components, such as the pulleys. By installing a more substantially-built racing pulley, the cracking is not usually an issue. Due to the reduced function of the engine accessories at speeds equal to highway driving speeds, the high-performance pulleys should never be installed on a street-driven vehicle.