I experimented with several exhaust configurations to satisfy my curiosity. All tests were done within a few hours using Byron's 15% with 16% synthetic oil, and a brand new OS A3 plug. The prop used for testing was a balanced APC 10x6. The numbers are from memory...I don't have my notes in front of me at the moment.

TT .46 pro
with Mac's 1140 pipe and 2590 header: over 16,000 RPM
(header cut to 3 1/2")

with Mac's one-piece muffler: 15,200
with Stock muffler (baffle removed): 14,000

I am using this engine on my own design Diamond Dust - like plane. I will most likely be using an APC 9x8 or 9x7 with the tuned pipe.

FYI, the Mac's one-piece mufflers are half the weight of the stock mufflers and very quiet.

-Tom