The idea behind underdrive pulleys is to change the revolution ratio between the accessory (driven pulley) and the crankshaft pulley (driver pulley). It works exactly like a gear ratio. If you have a 2” driver pulley connected via a belt to a 2” driven pulley, they will rotate the same number of revolutions per minute. (a 1:1 ratio). Now, if you swapped the driven pulley with a larger 4” pulley, the smaller driver pulley will have to rotate twice to complete one revolution of the larger driven pulley. Another way to look at it is that for everyone one revolution of the driver pulley, the driven pulley completes a half revolution. Thus the driven pulley is turning at half the rate. Keep in mind, that for this application, the driver pulley is actually the crankshaft pulley, which is turning at the same RPM as what is indicated on the tachometer. Thereby the crankshaft pulley is spinning like always, but the accessory pulley is now being rotated at half the rate of the crankshaft pulley at any given RPM, thus reducing power consumption.
A 3-piece underdrive kit from Ford Racing, BBK or MAC Performance all feature different-than-stock pulley sizes that are optimized for the best ratio between the crankshaft pulley and accessory pulley. The change in pulley size allows for the accessories to turn at a lesser rate and thus drawing less power to operate. See below for pictures of stock pulley sizes.
Underdrive pulleys are a very easy way to pick up some horses (5-10, at a minimum), but they can also pose a couple of problems. Because they slow down the operation of the water pump and alternator, particularly at idle, overheating (rather, above average heating due to a diminished coolant flow rate at idle) and low voltage can occur if your Foxbody already shows signs of these problems before installing underdrive pulleys. If you’re concerned about this happening to you, make sure the cooling and charging systems are in tip-top shape. If either system is less than optimal in stock form, it is likely one of the above issues will show face at idle with underdrive pulleys installed. In a way, underdrive pulleys can weed out existing problems in the electrical and cooling systems.