Here is what the heat side muffler looks like on my Debonair. There isn't much restriction in it, just a flame cone. Lots of heat. It's not really for comparison (that's an IO-520 with 3 cylinders going through it), but for what is needed to radiate heat.
My mechanic said that he brazed screws to a pipe in an RV-6 to get this kind of heat radiation. He said that worked very well. Only in experimentals, though...
--paul
BoHeaterMuffler.JPG