I was thinking of getting a set of underdrive pulleys. Do they work I have heard yes and no. Any opinions?

 

I personally wouldn't buy any unless you get a real good deal on used parts. Very little if any rear wheel power will be gained. Most say its just better throttle response.

 

They produce a small power savings. ws6.com has a dyno test, showing only 0.7 peak HP, but a larger average increase in power at the wheels.

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

Depending on the underdrive ratio and whether or not you also install an underdrive alternator pulley, or try to recover alternator capacity at idle with an overdrive alternator pulley, you may see inadequate system voltage at idle with accessories on.

And, there are a couple different designs.... one that sandwiches between your stock damper/pulley and adds rotating weight, and another that totally replaces the damper/pulley. The latter is a better design, but a lot more expensive.