The holes should be bent so that the aft side is in and the outside,
forward, is bent out, Virg

Finn Lassen wrote:

I don't get it. Wouldn't the exterior air tend to pressurize the muffler
and thus increasing back pressure?

The drawing Ed sent seems to be opposite Virg's description.

Finn

Ed Anderson wrote:

Finn,

With the sketch, I showed the exhaust pulse coming down the tube (from the
right).  A portion of the pulse gas  is deflected by the rear bent-in
portion of the hole and forced out (toward the slipstream) through the hole.
The forward edge of the hole bent outward probably creates a small negative
pressure behind it due to the outside airflow which also helps to drag a bit
of the exhaust gas out through the hole.

  The "aft side" of the hole (the down stream part of the hole) is bent "in"
(that is into the center of the tube).  The "outside forward" portion of the
hole (or the edge of the hole furthest upstream) is bent out (toward the
airstream).  I believe that is what my sketch shows.

  With this bending of the circumference of the holes, it is less likely
that exterior air would tend to pressurize the muffler as the portion of the
hole edge bent out into the airstream would tend to create a small negative
pressure area behind the holes lip - thus tending to pull exhaust gas out
through the hole.

  At least that is the way it appears to me, FWIW.

Did you get the photos I sent you??

Ed

 
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