Quieter exhaust's

I want to make a quieter exhaust box for my Allen scythe (2 stroke)
I have a rough idea of the principles involved from years of messing
about with old 2 stroke bikes when younger and appreciate that if I
restrict the exhaust too much I will lose power.
Are there any scientific principles to apply?
I guess quite a few people will have made and fitted quieter exhausts
to their engines for showing, or is the general concensus "thats how
loud it was when new so leave it alone"
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In my apprentice sheet metalworker days I had the good fortune to work alongside a guy who raced Cotton motorcycles at the TT's (Sadly lost his life there!) and watched him make many exhausts. I do remember that the principle he applied to them was that the expansion chamber must match the swept volume of the cylinder, but the silencing element didn't exist on his track bikes. He used to fit a "straight through" box to make the noise bearable when he was tuning the engines. This consisted of a perforated pipe within a box stuffed with fibreglass and it worked a treat without detriment to the exhaust flow or performance.
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Would the oil from a 2 stroke exhaust eventually clog the fibreglass? This would make it solid to sound and hence reduce it's absorption.
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John Manders
Yes it does - and quickly too! I wish I had a quid for every Japanese 80's two stroke silencer from which I have removed the disgustingly full fibreglass padding. Fine to sell them with and - one assumes - important in getting the thing through type testing procedures, but a poor idea longer term.
On a simple two stroke (especially one with a ramp piston) the best thing is a cast iron (or heavy steel made from an old air bottle etc) expansion chamber to deaden the high pitched note which exits into a fairly long pipe. The addition of a removable baffled tube is even better.
Kim Siddorn,
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J K Siddorn
Kim, What you describe sounds like the kind of design I already had in mind. I think I will use a thick walled tube and machine a removable endplate (the exhaust comes in at the side) with a perforated steel liner and pack the outer void with fibreglass, then I can replace it when it gets gunged up. I had a DT250 years ago and chopped the box open and ripped out the fibreglass and baffles, it went a lot better after but had the same tinny note to it as my allen scythe has now. Regards, Kev

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