Accessory drive systems have been made more compact on Late Model V8 engines like the Ford 4.6L and the GM LS1 by machining serpentine belt grooves on the OD of the crankshaft damper. The crankshaft damper functions as a combination crank damper and crank pulley on these engines.
Under drive dampers are smaller in diameter than stock production crank dampers which slows the speed of the accessories and reduces the HP required to drive them. Since performance and racing engine applications generally run at higher RPM, the water pump, alternator and power steering pump speeds can be reduced to save HP and still meet engine cooling, electrical and vehicle steering requirements.
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The trick is to reduce the crank damper diameter without losing the torsion control capability required to prevent engine vibration and possible damage. The PRO/SPORT Under Drive dampers have been specially tuned to control crankshaft torsional vibration to stock production levels.
Dynamometer tests of the PRO/SPORT 25% under drive damper on a 2005 Mustang 4.6L 3V engine at Livernois Motorsports in Dearborn Heights, Michigan produced a 13 HP gain over a stock production damper.