Thanks CF but I'm trying to build a compact light weight muffler that just gets the sound comfortable. Who wants to drive a silent go kart, bike, etc? He bought some exhaust pipe to create a second chamber around the current muffler. We'll test it soon.
In case anyone else missed it (the picture uses inches, mm, and cm), I've seen this muffler design before and have been thinking of making one for a while, but just noticed that it is 4 to 5 feet long (125-150cm). That's over 1/3 the length of my plane, not very practical for me.