A little more explanation on the muffler. I realized the the irritating
component of the engine noise was the high frequency component. By
decreasing the escape of the high frequency noise from the exhaust pipe
the exhaust note would be much more tolerable. I built this unit with NO
testing before hand and got lucky that it worked beyond my expectations.
The only critical items are the exhaust gas flow areas. I wanted the
areas to be roughly equal to minimize back pressure. Thus the formula.
The collector area was just a guess. It is used to make the transition
from th outer pipe to the exit tube. The only real important item is the
90 degree angle to assure the high frequency is reflected back down the
inner pipe. Obviously not all is but enough is to make the exhaust noise
much more pleasant. It gets hidden in the prop tip noise. The low
frequency component is not attenuated at all, but is is in the range of
the prop noise.  My installation is mounted down stream of a turbo. This
situation may improve the effectiveness of the muffler. I have not
tested it with out the turbo. I used 321 SS to make the unit but it is
down stream of a turbo and the exhaust temp is quite a bit lower than a
non turbo-ed setup. If I were to use this on a  non turbo-ed setup  I
believe that only  super alloys would stand up to the heat,noise and
pressure pulses. Even with this setup I made sure I had a stream of
cooling air flowing over the muffler. Test running on the ground the
cone metal never got hot enough to turn the metal black. It did get
slightly brown. This was with only prop blast and 3/4 or greater power
levels.


This setup is probably  by no means optimized. It will take making
several with different dimensions to find the best setup. Measuring the
exhaust back pressure should be also be done. The spacing between the
inner tube and the base of the cone may make a difference in back
pressures. Think the collector sizes make little difference as long as
the volume is sufficient to contain the gases. By the time the gases
reach the exit tube they have lost some temperature and thus volume.

Chris Smale