what you can do for four strokes, is an old motorcycle tuners trick. which has been proven to work best for naturally aspirated engines in almost all situations. what you do is get a good length of decent sized exhaust tubing from the engine and spray a line of paint down it. then you run the engine for a good length of time (a couple of flights would be ideal) then you will notice the paint has burnt for a certain amount of length from the motor. you then cut the exhaust pipe off where it has stopped burning off, this should provide better power than any exhaust system. of course planes are a different ballgame, different rpm ranges.. it sure would be loud unmuffled too!