Ed,
> Cones are the GO.
>
> Any bare edge will take a battering from the heat and shock waves. Cones
> formed back to back eliminate any edges.
>
> There must be supporting structure for the cones, I've used 1/4" solid
> round bar welded into the exhaust skin.
> George (down under)

Thought must be applied to the idea that eventually the structure will decompose, and then the pieces thus liberated, will plug the exit and render the engine a balance weight. As in the Mistral crash.

 

Perhaps the back to back cones could be installed over some cooling air holes in the central pipe.

A solid rod just aft of the cones could support the central pipe and help prevent such a blockage.

 

My version would have the entire muffler below the cowl in the airstream. Maybe there would be room for a collector before the muffler so as to maintain more engine power.

 

Lynn E. Hanover