Don't use automotive mufflers.
Make your own out out of  Inconel or 321 Stainless.
Best not to put anything inside that can  melt
and plug up the exit.  This one has been sawed
off. Not a good  idea for anybody on the ground
that might happen to be under it when it  spits
out its guts.

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This is a  fail safe muffler. If the ribs melt
and distort they are unlikely to plug  up the exit
and can be inspected by merely looking under  the
airplane.

Paul Lamar



Paul,
That is the diffuser that Powersport/Raytec put on the end of their
muffler. I don't know what the actual internals of their muffler looks like. It
was the muffler used when they flew the planes at Vans. They were reported to
be  very loud prior to putting the diffuser on the end
Bill Jepson