[yotatechie04]

The amount of horsepower varies with each different kind of aftermarket part that you put on your car. Aftermarket drive pulleys lighten the load on the engine, and assist in the creation of more power. Most underdrive pulleys that are available give a 10+ HP gain. In order to find out the information that you're craving, you'll need to do a bit of research on different brands, and different types of aftermarket parts. Best of luck to you, man.