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Our unknown underdrive pulley squeezes in between the factory harmonic damper and the crank hub. The damper is reinstalled so that it can continue to control crankshaft harmonics. Please take the time to reinstall the damper into its original position by properly clocking it where it belongs. If it's off, the crank position sensor will pick up bad harmonics, and your OptiSpark ignition system may act up with a cylinder misfire that will make your car feel like a slouch.