We spoke about underdrive pulleys as a source of raising gas mileage in the underdrive pulley section. However this mod is also a relatively cheap way to gain horsepower as well. Underdrive pulleys usually represent smaller diameter alternator, crankshaft, and other engine pulleys that are installed in place of stock diameter pulleys. The smaller pulleys serve to decrease the amount of action the drive belt has on the accessories the pulleys are attached to, including the alternator, water pump, and crankshaft. By driving the accessories at slower rpms the excess energy saved contributes to an increase in horsepower and torque. This occurs because there is less stress placed on the crankshaft through the drive belt. This allows the crank to spin more freely and allows the pistons to do their job more effectively.
Underdrive pulleys can typically raise horsepower by up to 10 hp. Since power gain from underdrive pulleys is rpm based, power gained will increase with increasing rpm. This means that you will gain more power at 2000 rpm than 1000 rpm and even more at 4000 rpm. Prices typically range between $50 and $150 for an underdrive pulley setup. The new pulley setup will also require you to buy a new drive belt of the correct size for your new system. The instruction manual that comes with the kit will usually tell you the correct size and model # of belt to purchase. Underdrive pulley companies include ASP, BBK, AEM, SLP, and March Performance to name a few.
There is one drawback to underdrive pulleys that should be mentioned here. Since they underdrive your car’s accessories you will notice a drop in performance of your alternator output, and water pump throughput as well as any other accessories that your vehicle runs. This may or may not cause your battery charging system not to work at its optimal levels in some applications. For example instead of charging to 14.7 volts your alternator might charge your battery to 14 volts or some similar value. This is a case specific issue and usually does not cause enough problems to warrant removing the pulleys. However this is something that should be taken into account when purchasing underdrive pulleys.