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Date: Fri, 03 Dec 2004 08:47:16 -0700
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: New Muffler Design
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>  I have the smaller 1988 cast iron manifold,
>and including pipe, my total exhaust weight is 12 pounds.  What appeals to
>me most is that it is compact, simple, and I don't have to worry about thin
>walled pipes glowing red inside my cowling, as the cast iron manifold
>absorbs a lot of heat that would otherwise turn exhaust pipes red hot.

         The noise reduction with thick headers is real. Thin headers on 
cars make a noticeable increase in the noise level. They make a lot of 
high-pitched "ping-hiss" sort of noise. (At the instant you first 
disconnect an air tool, it makes this sort of sound.) It is like you can 
hear the valves opening through the thin-wall header pipes. The pipe itself 
also rings with each exhaust pulse.

         A thick-walled pipe only delays the temperature rise and spreads 
out any hot spots. It does not reduce the final, steady-state, average 
surface temperature. The thin-wall pipe will indeed have more prominent hot 
spots. These hot spots will appear on the outside radius of the bends as 
they leave the engine, where you can see them easily.

         It is probably be a good idea to use thick-walled pipe for the 
first several inches, and then transition to thin-walled pipe. I don't 
know, but I suspect that the stock manifold is quite a bit thicker than it 
needs to be, however.