Gas engine exhaust extensions and back pressure

Awl --
Recently, in asking about pipe diam/length and pressure drops for natural gas, I was surprised at how significantly delivered gas volume drops with length. For example, for 3/4" pipe, the cfh drops from about 350 to 250 to 200 for run lengths of 10, 20, and 30 feet. I"m wondering if this is even correct.... from http://www.propane-generators.com/natural-gas-chart.htm
If this is indeed correct, I could assume the same is true of exhaust back pressure with pipe length. The flange on the muffler is 1.5" OD (proly 1 3/8 id), and I want to put a 25 ft flexible exhaust extension on it, like in http://www.napaonline.com/Catalog/CatalogItemDetail.aspx?R=EXH40002_0306020506
and the Q is, what id tubing would be practical to use? The tubing is given by id, so 1.5" should hose clamp to my 1.5" flange. But, I can go larger, if back pressure will be an issue, and just machine a bushing. The motor is a 22 hp Honda GX690, twin cylinder, with a single muffler.
Opinions on the "right diameter" for this run? Should I use a high temp gasket material along with a hose clamp? I'll be monitoring CO.
--
EA



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-snip- Don't know the answer- but I'm interested in it-- Be sure you keep a.h.r updated.
My instincts would be to put a tach on the machine with the old muffler and check it again when the new one is hooked up. If it is the same- forget about it.
My problem with flex-anything is that it often takes more to fasten it in place than it would take to use rigid stuff in the first place. --especially with steel exhaust which is likely to get hot enough to start fires along it's length.
If this is going into a fixed place and won't be moved around, I'd stop at a muffler shop and see what they'd charge for a section of smooth pipe. [bent up if you dare- or with a couple 'L's]
Jim
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response limited to rec.autos.tech, alt.home.repair

If only it were that simple.
http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/viewall.html
I think you might want to Google "exhaust scavenging". -----
- gpsman
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The big difference in exhaust systems is the volume of air reduces rapidly with distance down the pipe as the exhaust cools off. On intake, there is no temperature differential as the propane/nat gas comes thru the pipe, but the exhaust cools rapidly so the diameter of the pipe, if constant, has varying effects on the air flow. Keep us posted on whatever you find out.
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