I don't think I am losing any RPM's from the exhaust extention. It's inside diameter is twice the muffler outlet. One thing I did that seemed to make a big difference is I drilled four 1/16th " holes on the perimeter of the center baffle section which has one little hole in the center. It almost doubles the flow and also allows the unburned oil to run out the exhaust. With an 8-4 wood prop I'm getting 14,700 rpm static which seem real good. I'll be looking for aluminum to replace the copper as it's about 3oz. extra, and I'm at 2.75 lbs. now. With fuel I'll be at 3 lbs. which seems high for a .15 engine. The Trexler wheels are 3 oz. also, so may look into something lighter there too.[X(]

RS