The only way you can tell if a carb spacer works is by trial and error. I personally would say that a open hole phenolic spacer would work best on a performer R.P.M. air gap. Sometime's a four hole spacer screw's up the fuel distribution in a dual plane manifold. But every situation is different. They are cheap enough , play around with them untill you feel you have the right combination . You can pick up a little performance with them. They also keep the carb a little cooler which is always a good thing.