Underdrive pulleys

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An Underdrive pulley refers to an aftermarket crankshaft or accessory pulley (such as an alternator pulley) that is designed to drive a vehicle's accessories at a slower rate than stock. Underdrive from the crankshaft pulley means the pulley is smaller than the original pulley it replaces. Overdrive from the accessory pulley means the pulley is larger than the original pulley it replaces. Originally they were invented for race cars in the 1950s and 60s, whose engines had been modified and ran at higher RPMs for longer periods of time than the manufacturers had intended. It was therefore necessary to underdrive the belt-driven accessories (among other things like the water pump) to keep the accessories from burning out and the engines running reliably. Underdrive pulleys did not become mainstream until the mid to late 1980's with their introduction from March Performance for fuel injected 5.0L Mustangs, Camaros & Trans Ams. They also appeared for small displacement engines in the mid-nineties with the entry into the market of Unorthodox Racing.

These pulleys are marketed as performance enhancers that actually net additional engine torque and horsepower by reducing parasitic drag caused by belt-driven accessories, but more importantly by being lighter and reducing rotational mass. Horsepower gains from underdriving alone are usually minimal, 2-3 wheel horsepower depending on the engine. HP gains from being lighter can be significant, from 4-14 HP for normally aspirated engines and 12-25 HP for forced induction engines. These are some of the best HP improvements enthusiasts can find for their money. Be careful if your car is driven on the street to not have more than 15-20% underdrive. Some companies also offer stock diameter crankshaft and accessory pulleys. Poorly designed cheap pulleys or copies and knock-offs of major brands can have severe side effects because of too much underdrive of the alternator, power steering, and/or air conditioning units, which are not spinning fast enough. These lead to dips in alternator voltage, power assist, and air conditioning effectiveness may be noticeable, especially at idle. The headlights may dim when coming to a stop, or the stereo may lower in volume, for instance. Too much underdrive for a race car, this is not of concern, but for a daily driven vehicle it can be a major annoyance.

Changing the original crankshaft pulley can have negative effects if the replacement pulley is not manufactured properly. A crankshaft or accessory pulley that is not machined or balanced properly can cause severe damage. For most domestic V8s a harmonic damper is necessary to absorb crankshaft vibrations. Early to mid-year V8's kept the pulley and damper as separate components. Most late model V8s now integrate the damper and the pulley into one part. For small displacement engines found in many import and domestic vehicles (4.0L and under) the original crankshaft pulley only serves to quiet noise vibration and harshness (NVH) you hear in the occupant compartment. Although it may look like a damper on V8's engines it serves no protection function to the engine. It is the same as the baffles and resonators found in the intakes and exhausts systems of most cars today that serve to make the engine as quiet as feasibly possible. The first things most enthusiasts do is install high flow intakes and exhausts; the quieting function is eliminated.