Chuck,
    I agree that hard mounting the exhaust without some flex between it and
the headers is asking for fatigue failure.  I purposely left the joints
between header and mufflers on the loose side.  However, based on the
vibration transmitted into the cockpit, its clear that there was not
sufficient isolation.  I had intended to use rubber exhaust donuts to reduce
the vibration but did not. I am going to experiment with two 18" lengths of
0.035 SS with multiple holes drilled in the bottom half of the tube and the
end squeezed down to a fishtail 1/2" in width.  I don't expect it to be as
quite as the arrangement of a "Round" muffler under the cowl, but it will be
lighter.  A book on Exhaust and Intake systems indicate there is some merit
to this configuration, so I thought I would give it a try.

Ed Anderson

Ed, I don't know a lot about exhaust tuning (it's really a black art), but I've
read and talked to others about good general design, and I used to do a lot of
car/boat racing. Generally, the only people who us straight pipes are
supercharged engines. The reason you want to use a muffler/collector is to
assist in the reduction of back pressure when the exhaust port opens. This will
actually increase hp, plus the engine is significantly quieter. It was explained
to me that you trying to line up tennis balls (pulses) rolling thru the exhaust
pipe. An individual  18" pipe will be specifically tuned for a certain narrow
rpm band, wheras a reasonably designed muffler collector will have a much
broader band and will make more hp throughout the band.

Using a rubber isolator will probably work much better than a loose metal
restraint, but you'll still transmit vibration into the fuselage because you
need to make it rigid enough to stop (dampen) a fatique failure. Mark Sedikas
who is an aircraft certified welder is constantly repairing SS exhaust systems
for Lyco and Cont, and they run 100-200 F less temp than a rotary, so I'm a big
supporter of Inconel which is a one time job and a safe method.  I hope I'm not
be a pain, but an exhaust failure can be catastrophic.

Chuck Harbert

 
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