I've only run the Poulan 42cc with the original muffler completely stock. I've taken it apart though and the exhaust goes into a hard metal "cage" with holes in it. This cage is also wrapped with the spark screen and the cage gives support when tightening the muffler to the engine. Once I get it running on gas/glow, I'll try running it again with the stock muffler and then with the spark screen removed. These are the only components inside the muffler shell.