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HOW THEY WORK
Freeing up power that the engine is already making is always more efficient than adding more thrugh brute force. The result is more power delivered to the wheels and much sharpened rev response. Combine with a lightweight flywheel and cam lifters to produce an even more dramatic effect! Dropping pulley mass also lessens moment of inertia -thus making it easier to accelerate and decelerate these moving parts. Finally, underdrive pulleys enhance engine performance by releasing torque and horsepower wasted by parasitic drag. Spinning belt-driven accessories more slowly reduces these avoidable losses.
Typical dyno gains with VW/Audi engine range up to 5hp at the wheels after installing a complete underdrive pulley system. However, the enhancement feels much more significant. That's not only for the driver. The engine itself experiences less stress when it has less work to do.
USAGE CONCERNS
A crankshaft or accessory pulley that is not machined or balanced properly can cause severe damage. Also, the alternator, power steering, and/or air conditioning units will produce insufficient output if not spun fast enough. These dips in alternator voltage, power assist, and air conditioning effectiveness may be noticeable -especially at idle. Headlights may dim when coming to a stop or the stereo may lower in volume, for instance. Too much underdrive in a race car is not much of a concern, but for a daily driven vehicle it can be a major annoyance.
All engines benefit from lower weight pulleys but not all are appropriate to underdrive (because electrical demands may require stock alternator output). Street cars with powerful audio systems and rallycars with lights drawing over 600W RMS, for example, will need stock diameter crank pulleys. The same is so for those running crank-driven superchargers.
