What's an underdrive pulley?

An underdrive pulley is a replacement pulley that bolts up the same as stock, but has a different size (ratio) and material specifications intended to free up lost horsepower. By increasing the diameter of each accessory pulley and decreasing the size of the crank pulley, the speed at which the accessories are turning relative to engine RPM decreases.

Under drive pulleys do not add power to the engine, they free up the power that is normally wasted on overdriven accessories. The theory is that a larger pulley will drive the accessories slower thus reducing their parasitic horsepower loss. No more power total, but more power sent to the wheels. As an additional benefit, the reduction in wear (less usage) on the accessories due to the lower duty cycles can extend the life of these parts.

For example, the alternator, power steering, water pump and air compressor rob horsepower from the engine. By replacing factory-sized crank balancer/pulley and water pump drive pulley with a resized pulleys, the accessories are slowed, resulting in more horsepower available to the drive wheels.

MOTOBLUE™ H.O. Series pulleys will increase horsepower up to 16 RWHP, and increase mileage by 1 to 2 MPG. MOTOBLUE H.O Pulleys are manufactured from billet steel, exceeding factory quality and are the only Pulley system available on the market today that are SFI approved for all racing applications.

MOTOBLUE™ H.O.UnderDrive Harmonic Balancer
• 25% underdrive SFI approved balancer pulley kit