Hello 68OrangeSunshine and 64Duce, sorry if one of my earlier posts was kind of confusing, but for the record there IS only one SIZE power valve,(they did make it in a '2 stage" for some smog carbs back when though). It seems there has been some confusion between Hg (vacuum) ratings and sizes. All power valves are much larger, and flow way more, than the restriction channel they feed into. Holley uses different size power valve restriction chanels (PVRC's) that are built into the metering block, but just one SIZE power valve.
Some times if tuning is to be done right you will find out you need a bigger PVRC and smaller jets. What this does is keep you from being rich at cruise, and lean at WOT. I would also be quick to say that you need to be sure of what you are doing if you are going to mess with your PVRC's, because once you drill, there is no turning back. As far as choosing a the right vacuum rating goes, a good place to start is with a valve that is rated a little less than half the vacumm you have at cruise.
