whats the difference between overdrive and underdrive pullies?

 

There really is no such thing as an "overdrive" pulley.

What do pullies do? They drive things, right? They turn acessories....things you need like a power steering pump, or alternator, or ac compressor. When you "underdrive" an acessory, you just don't drive it as much. This frees up some hp to make the car go forward. So, there is no such animal as an "over drive" pulley UNLESS, for some reason, you would want to install a pulley to make your ac compressor work harder etc...

 

http://www.3.8mustang.com/performanc...viewprod&num=5



this is what i mean, is that made just for superchargers?

 

its just smaller so it takes less power to run the supercharger

 

Bingo! I sort of forgot about blowers. Yes, with a blower, you can change pulley size to drive the blower more....as in "overdriving" it to make more compression.

Pointless on a car w/o a blower.