Engine Under Drive Pulley Kit

What Are Engine Underdrive Pulley Kits?

Underdrive pulleys increase engine output by reducing the draw of the engine's accessories by slowing them down and reducing the horsepower (HP) they use. These kits typically include replacement pulleys for the crankshaft, alternator, water pump, and power steering pump, designed to operate your vehicle's accessories at a slightly reduced speed. Underdrive is the slowing of the rate of rotation in a system, achieved by either making the crank or main (drive) pulley smaller or making the accessory (driven) pulley larger than the original diameter pulleys.

Performance Benefits and How They Work

Horsepower gains from underdrive pulleys can vary by vehicle, engine, number of accessories and the amount of underdrive (improvements of up to 5–15 HP at the wheels have been seen). Beyond just reducing accessory speed, additional and significant performance improvements can be seen by reducing the weight of the pulley versus the original pulley, with gains ranging from 3 to 6 HP per pound of weight reduced. The result is not only freed-up power but also improved throttle response and faster engine revs, similar to the effect of a lighter flywheel.

The underdrive pulley essentially uses a small diameter crankshaft pulley, decelerating the serpentine belt, with the intended effect of a reduction of parasitic power loss on the engine. This makes underdrive pulley kits particularly popular among enthusiasts building performance street cars and track vehicles, where every horsepower counts and efficiency matters.

Important Considerations

While underdrive pulleys offer significant performance gains, proper engineering is essential. Poorly engineered underdrive pulleys can cause unwanted side effects due to not spinning the alternator, power steering, and/or air conditioning fast enough, leading to low alternator voltage, weak or no power steering assist, and weak or no air conditioning effectiveness, especially at idle or low RPM. Quality kits from reputable manufacturers are designed to maintain adequate cooling and charging at all engine speeds.

Top Brands in Underdrive Pulley Kits

When shopping for engine underdrive pulley kits, choosing a trusted manufacturer ensures proper engineering and reliable performance. GFB - Go Fast Bits specializes in lightweight underdrive pulley systems crafted from aircraft-grade aluminum, offering substantial weight reduction alongside underdrive ratios optimized for street and performance applications. BBK has been a pioneer in the underdrive pulley market since the late 1980s, with precision CNC-machined aluminum kits engineered to retain proper cooling and charging while delivering bolt-on power gains across a wide range of domestic performance vehicles.