This to me makes the most sense for a muffler if you are
going to come out the bottom of the airplane.

Simple and durable as there is nothing to melt inside
a conventional muffler. There is a ball joint between these
pipes and the down pipes from the engine.  As I recall these are 
0.035 wall 304 stainless steel. Correct me if I am wrong Ed.
Somebody suggested a range of hole sizes might work better than
all one hole size.

If you need a bit more quiet a larger pipe positioned concentric with these 
open in the front and rear  for cooling air flow should work well. 
Nobody has tried that yet to my knowledge.

Paul Lamar
 
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