Well thanks to Jody's help, I'm now getting 7400 RPM on a Xoar 22x8 with the stock exhaust, 3-4 gallons of Pennz 32:1. This thing is scary powerful. The velocity stack totally made it run more consistant no matter what attitude the plane is in. It's clear as day that the mixture stays the same no matter what I'm doing. Incredible. The metering diaphragm vent mod helped, then the velocity stack cleaned it up even more. This engine rocks. I mean seriously...I'm getting almost pipe power out of a stock exhaust. It pulls my 17#, 10 ounce Yak through uplines like it's weightless.