underdrive pulley
 

ive been looking at a bunch of stangs and reading there mods list. a lot of them have an under drive pulley. i know where it goes on the engine but what does it do.

I read that too
 

All I know is that they add like 8 to 10 more hp I wanna get some to but my friend told me that it drains tha batt I don't trust him to much though cuz he lies allot but yea its a pretty good mod for like 80 bux for Mac under drive pulleys

The diamater of the pullies are different. This is why the alternater might not charge fully, it will turn less than with the regular pulley. I had this problem at night with the lights and radio on. A local speed shop changed out the alt pulley and over drived it so it would fully charge. In short they turn your accessories less, which takes power.
Hope that helps

This might be a little late, but an underdrive pulley does a few things. It increases HP a little, increases gas millage, and reduces wear on your belt driven accessories. Because the pulley slows down the belt, it's recomended that you get a different alternator that can handle the slower belt.

Hope this helps. :)