the muffler isnt the only noise producing thing. props are noisey too. I run my engines at home to break in sometimes and i run the exhaust from a header into a 1 inch dia tube and then put that tube in a 5 gallon gas can. it cuts all the exhaust noise out. thing is the engine is still loud. prop, engine and vibration all making noise. to cut the exhaust you need to expand and cool the exhuast. to do this you will end up with a heavy muffler.