Alex and I visited Tracy today and the subject of muffler came up.
 Here is some info on our coax muffler.

 The best muffler for a rotary is 3 to 8 feet long about 4 to 5
 inches in diameter.  That gives the lowest aero drag for a given
 silencing.  Lowest frontal area.  Mufflers work by the exhaust
 gases rubbing on the metal surface losing kinetic energy to skin
 friction and losing heat to the metal surfaces.  The cooler the
 gases in a pipe the lower the gas speed.  When high speed gas
 exits a pipe there is a sudden expansion that causes the noise.
 The idea here is to cool the gases before they get to the end of
 the inner pipe.

 The inner tube is best done in .045 Inconnel but there is a
 trade off with thickness.  If you make the inner tube 2 inches
 in diameter and use 321 SS make it .065 thick with a bunch of
 holes drilled in it.  The outer tube can be .030" thick 321 SS
 as it will always run cooler.  Also make the front of the
 muffler look like this.

 Don't drill any holes for the first 8 to 10 inches. The slip
 stream will blow cooling air down between the coaxial tubes.  At
 the back end make the inner tube 6 inches shorter than the outer
 tube.  That will act as an ejector to draw cooling air between
 the two tubes.  Patent applied for:)

 A 2 inch diameter SS tube .065 thick 72 inches long will weigh
 about 8.8 pounds.

 A 2 inch diameter SS tube .065 thick 72 inches long will weigh
 about 8.8 pounds.

 A 4 inch diameter SS tube .030 thick 72 inches long will weigh
 about 7.50 pounds.

 A 2.5 inch inner tube and a 5 and 1/2 inch outer tube may give a
 little less back pressure for a weight trade off.

 Tracy, if you get this reply with "OK".

 Paul Lamar


Paul, how does this compare with the fishtail configuration?

Paul Lennemann

The fish tail works but it is heavy. One must beef up those large flat surfaced as they vibrate and fatigue the metal. I would guess the tone would be higher as well.


Paul Lamar