Expert: Paul Soderman - 1/21/2002

Question
Dear Paul,
Do you have any thoughts on design of a simple muffler system for my engine.  It is a 100 cc boxer style two-stroke twin cylinder gas powered engine that turns a 26" prop about 6500RPMs.  This engine flys a 1/3 scale Extra 300S aerobatic plane weighing 28 pounds with unlimited vertical climb performance, but it is noisy.  Any ideas will be much appreciated.
Thanks,
Bill eich

Bill Eich

Answer
My first thought would be to look for a commercial muffler.  There must be designs on the market for lawn mowers and chain saws (maybe not) or other two-stroke machines.  Without knowing much about that size engine, I assume the mufflers are part passive and part reactive.  Passive mufflers have fiberglass wrapped porous ducts the exhaust must pass through. Length of duct is critical. Reactive mufflers have side branches, area changes and convoluted passage ways designed to reflect the fundamental tones of a two-stroke engine.  Of course, your challenge is to make it light weight and streamlined.  Back pressure is also a concern.  Acoustics of Ducts and Mufflers by M. Munjal (John Wiley and sons, 1987) may help, though it is somewhat mathematical in nature.  There are other muffler design books with more empirical examples.  Good luck