When you are working under your vehicle, make sure it is supported by secure jack stands. There are always reports indicating someone has been injured due to working under a car that was only supported by a jack. Please do not take any chances!
One of the best Horsepower per dollar upgrades that you can make to your Mustang is the addition of underdrive pulleys. The installation frees up between a 8 and 10 hp to the rear wheels by reduction friction drag on accessories driving by the crankshaft of the motor.
How do they work?
The alternator and the water pump are 2 accessories that are driving by the serpentine belt.Both of these units operate at a higher rpm than they really need to, basically over producing as a safety factor. The installation of underdrive pulleys replaces the stock harmonic balancer and pulley with a smaller one, and replaces the water pump and alternator with a larger one. The final result is the reduction in component rpm by about 25% therefore releasing HP directly to the driveshaft. It just doesn't have to work so hard to turn these 2 components.
This is similar to riding your 10 speed bicycle. When you switch to a lower gear, your pedals go faster but your speed slows down. It takes less work for you to pedal because you have a better mechanical advantage over the lower gear you have chosen.
The installation is not that difficult but there are some snags that you will have to deal with. The removal of the alternator pulley is difficult without an impact wrench. First things first, loosen the bolts on the water pump pulley while the serpentine belt is still in place. Do not remove it yet, you have to remove the belt first. Once the belt has been removed, direct you attention to the alternator. I removed this one completely, stopped by the local auto parts store and had them spin off the pulley and zap the new one on for me. Took all of 20 seconds. They don't advertise that they do this because of liability reasons, so check with them first to see if they are willing to do it for you.
Replace the alternator and make sure that all the electrical connections are back in place. Next, install the new water pump pulley and tighten the bolts as much as possible. I used a rubber oil filter wrench to resist the pulley during tightening.
The next step is to work on the harmonic balancer or crank shaft pulley. We chose the Steeda Pulleys as they replace the harmonic balancer completely. There are some products on the market, referred to as piggy back, but in our opinion they do not work as well. The piggy-back style are installed over the existing stock harmonic balancer and do not reduce the rolling weight of the pulley.