I'm helping out a builder working on a government sponsored project that is powered by a Mazda Renesis.   I thought the muffler was a total joke when I saw that it had an unobstructed path right through it.  There is no glass or other fiber packing in it.   It's 6" in diameter and about 20" long.   There are two of these in series on this installation but I still didn't think it would be effective.    

When I got the engine running I was astounded at how good it sounded.   The only thing that sounded half as good was Dennis H's  setup (which sounds great).   I only had it up to 3000 rpm so far but I'm encouraged enough to try one myself.  I'll get this thing tied down and do a full power runup before ordering though.

Muffler says "Aero Turbine"  on it and is made of 304 stainless.  Google it to see internal construction.  It is quite novel.

Tracy