The % amount associated with the pulley is the reduction amount correct....?

 

A "25%" pulley will reduce the speed of all the belt driven accessories by 25%. The diameter is also 25% smaller.

 

so a 20% pulley would create more power than a 15% pulley?

 

Theoretically yes, it should free up more power.

 

But you aren't talking about much in the way of gains. Since the LT1 water pump is gear driven, and not belt driven, there isn't much to be saved. The reduction in the amount of energy absorbed to accelerate the rotating mass (HP loss) will be proportional to the square of the RPM. So a 20% pulley, in theory, should produce 4X the gains of a 10% pulley. Look at the savings on this dyno test - pulley % not shown.

http://www.ws6.com/mod-2.htm

The potential loss of charging capacity at idle isn't worth it, IMHO.

 

Personally, I like my underdrive pulleys. There is definitely some charging issues at idle for the moment, but I'm working around that. Bang for the buck; probably not worth it.. however, I like it and I'm going to roll with it.