A set of pullies is designed to increase the torque applied to the belt by increasing the size of the
driven pulley and decreasing the size of the
driving pulley.
What this does, is makes the driven components easier to drive and therefore requires less work (horsepower) from the engine to turn.
I have heard that HP gains are between 8 and 20. What it does in real life I'm not sure, but when I get the money I'm throwing a set on!
Just remember that there is a trade off here too. Your alternator will turn slower and therefore you will have less juice returning to your electrical system. So don't have a 600 watt sound system going full blast and leave your car for an hour idling, and you'll be OK.
Hope that helps!
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Driving: 1998 F-150
Far way in Edmonton==>
1995 Mustang GT