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Old 02-22-2003, 10:26 AM   #1
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building custom mufflers

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I wish to build a muffler for an O.S. Max .25 LA to hide it inside a cowl. Are there any guidelines for designing your own muffler? What about size of the chamber? Where does the pressure nipple go?
I'm planning to silver-soldier sheet brass (.060") together. I guess the design will be similar to a Bisson "Pitts-style", however it's going into a 1/12th scale P-40 warhawk combat airplane.
Any suggestions would be appreciated!
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Old 02-22-2003, 02:53 PM   #2
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Silver brazing brass shimstock will be fine and if you keep the brass sheet thin, it will be fairly lightweight.

For volume, I would take an average muffler for that engine and measure it. If it is a funky shape just fill with water and see how many ml you have, that is the same a cc's and then you can figure the volume you need.

Keep the pressure nipple anywhere in the main chamber, ie not on the exhaust stack going into the muffler.

As for exhaust tubes, again measure a bought muffler. You cant go wrong with a proven design!

Show us some pics when you get it done...

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Old 02-23-2003, 06:58 PM   #3
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thanx for your reply.
what you said about measuring an existing muffler and copying the chamber's dimensions is something i considered doing. the only confusing bit is that i have two O.S. .25's and one of them (the older one) has a much smaller muffler which i thought was a bit strange. how much does the size of the chamber affect the performance of the engine?
as for the exhaust tubes, i was considering 6 small ones coming out of the chamber to approximate the look of the exhaust pipes of an actual p-40 warhawk. i wonder what that will do to performance?
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Old 02-23-2003, 07:04 PM   #4
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The newer muffler may have a baffle inside to quiet it down even more, where the old one wouldn't.

Just a thought.

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Old 02-23-2003, 07:36 PM   #5
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Just a few more Ideas

Be careful with putting to many exit pipes you will still want to maintain some back pressure to use for fuel tank pressure. One way that I use to tell if my muffler design is ok is to break in the engine on a test stand with the stock muffler. Get it running perfect. Note the rpm. Put on your muffler and compare it. Up to a 500 rpm drop at top end is acceptable. If you have to increase the fuel flow at top end be careful this is an indication that the engine is running hot and you need more fuel to keep it cool. The larger the exhaust volume the quieter it should be. The volume size as long as it doesn't restrict the engine is not as important as you would think. Slim line makes a very efiecient line of 2 stroke exhaust systems that are very small but they are not that quiet. Build what will fit, alter the outlet diameter till you get it to run as close to the stock muffler both in fuel settings and rpm and live with the sound.

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