You might have heard of an old trick in the world of automotive performance that uses special pulleys to somehow pick up more horsepower from an otherwise stock motor. This might seem a little too good to be true, but actually it is a proven technique that you could apply to your engine in order to increase performance.



How does it work? Well, keep in mind that your engine is providing power to more than just the wheels of your car. For example, the alternator, the air conditioning compressor, and the power steering pump are all connected to the engine through a belt and pulley system. Each of those accessories has a small pulley that gets rotated by your serpentine belt that big belt located at the front of the engine. These pulleys take a small amount of horsepower from the engine in order to keep turning and working while you are driving. This is known as parasitic drag.

Now, normally this is not an issue. After all, the amount of energy it takes to turn two or three small pulleys is in the 5 to 7 horsepower range. No big deal, right? Well, when you are trying to squeeze every last drop of performance out of your engine, you should grab whatever you can. If this horsepower is there for the taking, why not scoop it up?

The way to eliminate a substantial portion of this parasitic drag is to install special pulleys that take less energy to turn yet are still capable of running your accessories. These are called underdrive pulleys. The most obvious difference between an underdrive pulley and a standard pulley is that it has a smaller diameter. This makes it so that the pulley does not turn as fast as the stock pulley, meaning that less energy is used in the motion. More importantly, these pulleys are also much lighter than a stock pulley, and the reduced mass makes them far easier to rotate. It is this last feature which gives you the biggest performance gains. Usually you can access about 3 to 5 free horsepower with these devices.

I know what you are thinking if the pulleys are not spinning as fast as they used to, doesnt this mean that my accessories are not going to get the power that they need to work properly? Yes and no. It is usually safe to underdrive your accessories by about 20 percent. If you go below this point, you might start to notice issues like an alternator which does not generate enough electricity to keep the engine running, or a power steering pump that you have to fight against in order to turn the wheels. These issues, when they appear, usually occur only at idle. This is why the 20 percent limit is recommended on a street car. If you are running primarily on the track, where you will see wide open throttle most of the time, then you can take the chance of running a more aggressive set of pulleys that will reduce parasitic drag by a greater amount.