Power pulley is a term used to describe specially modified drive pulleys designed to reduce parasitic power loss in automobile engines. The pulleys are generally supplied in model-specific sets meant to replace all standard manufacturer pulleys on alternators, water pumps, power steering, and air conditioner units. A typical power pulley is manufactured from lighter materials and features a slimmer profile than the standard manufacturer-fitted unit. The pulleys may also differ slightly in outside diameter with an associated reduction in driven component speed. Although power pulleys do increase the available power ratings for any given engine, their use is often controversial due to problems which may arise as a result of departures from manufacturer's specifications.
Any machine produces a finite amount of power and, in complex systems such as automobile engines, not all of it is available at the machine's intended output point. This phenomenon is a result of the power losses incurred by driving the components that make up the engine's ancillary systems, such as the alternator, power steering unit, and air conditioner pump. This is known as parasitic power loss and is a operational reality in any automobile engine. The power pulley is an after-market part fitted as a measure to win back a portion of parasitic power loss by making the pulley lighter and reducing the speed at which the relevant component is driven.
These departures from manufacturer design specifications are carefully calculated to achieve the desired result without negatively affecting the operation of the engine. This is particularly important in the case of the vehicle's alternator, where unacceptable changes in operational speeds can cause serious performance issues, especially at low engine speeds. Weight saving in power pulley manufacture is generally achieved by including material saving cut-outs and profiles not included on manufacturer parts. Specially developed lightweight, high-strength alloys are also used in the production of the pulleys as an additional weight-saving measure.
Although fitting a power pulley set to a vehicle is generally successful at achieving an increase in available power, their use is often considered to be more dangerous than beneficial. The exclusion of manufacturer features such as harmonic dampers in an attempt to save weight on main drive pulleys may cause premature failure of various engine components. Incorrect pulley diameters resulting in excessively slow alternator or water pump speeds can also have serious performance and part-longevity implications. It is therefore best to fit power pulley sets with a proven track record for the relevant automobile model.