Pulleys help reduce the parasitic loads on the front of the engine by slowing everything down using a smaller balancer pulley and larger accessory pulleys. The only disadvantage is that they can cause the alternator to spin slow enough at idle to not keep the battery fully charged, especially with high loads such as headlights or high current stereo equipment. This problem is easy to overcome by raising the idle around 50-75 RPM.
The gains from pulley are claimed at about 6-8 HP and 1/2 MPG. I think the mileage is a little optimistic but the HP gains are pretty accurate.
Hope this helps.
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Bill Cohron - The Mad Doctor  
Power Hungry Performance - The ORIGINAL in Ford performance tuning... Since 1996!
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