"Del Johnson, North Point Solutions, Inc" wrote:

Great discussions on the subject of exhaust tuning.

One of the emails contained information on the work of a builder that was
installing tuned muffler system in using a Subaru engine.  It included a
reference and picture of a 1/2" tube into the collector that was used to
supply vacuum for the instruments.  Unfortunately I can't locate that email
in my outlook list to include it in this reply.  I would be interested in
finding out more about the vacuum tap on the exhaust system and how
effective it is.  Could the person that supplied that email contact his
friend that is building the Subaru engine/airplane and get more details.

Also, I was wondering what might be the effect of placing a spiral insert
into the primary exhaust pipes?  It would seem like it would force the
exhaust to travel a longer distance in a fixed length pipe in comparison to
the exhaust path in the same pipe without the insert.  Could this work to
effectively shorten the length of the exhaust primary pipes in order to keep
it inside of the cowling.  At least one exhaust manufacturer uses spiral
inserts to quiet the exhaust noises.  How effective is this approach to
taming the exhaust noise?  If it can lead to shorter tuned exhaust systems
while at the same time reducing exhaust noise it might be worth trying.
Finally, how might you calculate the exhaust path length in a spiral insert
system?

Assuming you can make it out of a material that will live it should
quiet the exhaust. Like any muffler it is not free in terms of back
pressure. The friction of the 200 MPH gases against the inside walls
results in some back pressure and consequently some HP loss.

All mufflers work the same way on a micro level. Break up
the wave into small parts and rub those parts against the metal
in the exhaust system. In that way the noise energy is converted 
to heat energy.

Paul Lamar
 
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