Underdrive Crank Pulleys – Free Up Horsepower

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Pulleys - Crank, Underdrive

Underdrive crank pulleys reduce the rotational speed of engine accessories by way of a smaller-diameter drive pulley at the crankshaft. By slowing the alternator, power steering pump, water pump, and air conditioning compressor, less engine power is consumed through parasitic drag — freeing up horsepower and improving throttle response. These pulleys are a popular first step for enthusiasts building a naturally aspirated or forced induction setup.

Key characteristics of an underdrive crank pulley:

Diameter: Typically 10–25% smaller than the factory crank pulley, altering the accessory drive ratio.
Material: Most aftermarket underdrive pulleys are CNC-machined from 6061-T6 billet aluminum for strength and light weight, often hard-anodized for wear resistance.
Weight: Substantially lighter than the OEM steel or cast iron pulley, reducing rotating mass and aiding engine rev quickness.
Belt compatibility: A smaller pulley may require a shorter serpentine belt; check manufacturer recommendations.
Damper integration: Some underdrive pulleys retain a harmonic dampener or are designed as a solid piece for race applications. Ensure the design matches your engine’s requirements.

When choosing an underdrive crank pulley, verify the bolt pattern, bore size, and offset match your engine. Consider whether you need a pulley that includes a timing mark, and whether you’ll run a standard or overdriven alternator pulley to compensate for charging at idle. Our collection spans a range of diameters and fitments to suit your build.

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Pulleys - Crank, Underdrive

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