What are pulleys and why do they make horsepower?
Underdrive pulleys do exactly what the term implies, they slow down the speed at which the accessories drive. They accomplish this by using different size pulleys on the crank, water pump, and alternator. The alternator and water pump are run a slower speeds while the crank spins more freely. Think of switching gears in your 10 speed bicycle from the big gear in front and the little gear in back to the little gear in front and big gear in back. This slows down the tire and makes it easier for you to turn the pedals. This allows the engine to rev up much more freely and is usually good for 8-10 horsepower.
