Underdrive Pulley Kits & Systems

An underdrive pulley is a belt-driven "wheel" on the front of an engine that is different in diameter from the factory size. It can also be made of a different material, such as aluminum, instead of steel. An underdrive pulley kit is a matched set of pulleys that are a different diameter than the factory proportions. Engine pulleys provide the transmission of power from the crankshaft to accessories such as the alternator, water pump, power steering pump, air conditioning compressor, and smog pump in order to make them function.

Because an underdrive pulley is a different size than the factory diameter, the speed of the belt-driven component changes. An underdrive pulley or pulley kit slows down the speed of the driven component. This creates less work on the engine, freeing up horsepower that is then transferred to the transmission and drivetrain. Essentially, underdrive pulleys move horsepower that is used to power the front belt-driven accessories to the drivetrain. The drive pulley, which is on the crankshaft, speeds up accessories when it is larger in diameter, and slows down accessories when it is smaller in diameter. On the contrary, the driven accessory pulley size is the opposite. The larger the driven pulley is, the slower it turns the accessory and the smaller it is, the faster it spins. Underdrive pulleys are tested at different diameters until the correct size for both the drive and driven pulley are under-driven up until the point at which the accessories do not function properly. For example, the water pump is slowed down until it doesn't spin enough to keep an engine cool, or the alternator is slowed down to the point just before it doesn't keep the battery charged at idle. This is how to maximize the transfer of horsepower from the front of the engine to the rear and to improve the performance of your vehicle at the rear wheels.

There are two different measurements needed for an automotive drive belt. One is the width, which is the number of grooves on a serpentine belt pulley, or the width in inches or millimeters on a v-belt. The other measurement is length. One method is to take a string and run it around the crank and accessory pulleys in the same path as the belt you will be using and then measure the length of the string. While this can get you close on serpentine pulley setups, it may not be the correct length as the string goes all the way down into the groove on v-belt setups. One popular solution is to get the size belt that matches the measurement as well as a couple of different sizes both shorter and longer. This helps reduce the number of trips/orders for drive belts. Then test fit a belt and choose the one that puts the accessory in the middle of the adjustment. The incorrect sizes can then be returned.

A JEGS expert is ready to answer your questions and help you find the underdrive pulley, pulley shim kit, or dress-up conversion kit that you require for your specific application.