i have a 99 gsr motor and i want to taake out the a\c pully and put a new one in some one tell me the differnce form under and overdrive pullys and which one would be better for higher rpm

 

Most underdrive pulley are poorly balanced and could damage your crank or it's bearings.

I've never heard of overdrive pulleys but underdrive pulleys ad a larger pulley to the dynamo and a smaller one somewhere else to keep the belt from getting slack.
The larger pulley causes the dynamo to turn slower and draw less power from the engine. So I guess overdrive pulley do the opposite. but why?
Maybe for a larger dynamo on a big audioinstall.