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Improving muffler design


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I'm posting this on behalf of a friend so...

Currently looking to make a Briggs & Stratton 4stroke 1 cyclinder engine quieter as part of my final year uni project.

Despite it having a low spec muffler, I am looking at different muffler designs to add to the engine


Perhaps a ‘glasspack’ as they are relatively easy/cheap to make and can reduce the noise considerably. Although struggling to find any specs for them, for example…is there any benefit for having a certain amount of holes in the perforated inner material, or a desired length?

Thank you!

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There's a lot of theory behind it do with with the length of the box vs the length of the sound waves you're trying to dampen. The length and shape of the tail-pipe after the box can have quite a drastic effect too. It's all down to resonance really. There must be plenty of journals on it surely, have a look on your uni's e-library. The info's definitely out there, but I'm not 100% where to look.

Edited by RobinJI
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Does it need to breathe freely? If you increase exhaust back pressure it will affect carburation.

Possibly have a look at reactive silencers, essentially at low revs it dampens the sound, as gas flow increases it shortcuts and reduces the back pressure in order to allow more flow. I have such a silencer on my supermoto and I took some photos when I last repacked it, will see if I still have them somewhere. You would need to tune the silencer to suit the engine and expected revs.

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I did a little research on this for Uni, but not really enough to give a decent answer.

The size of holes does affect which wavelengths are 'silenced'. Also, if you make the silencer wider than the exhaust on the engine, then theoretically some sound waves will be reflected (due to sudden expansion or something like that) back down the exhaust helping with the damping effect.

A simple 'glasspack' is a good shout in my books. Just don't do what we did and try to drill all the holes yourself, better to buy some already perforated material!

Please note, I have no idea what I'm talking about.

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Yeah, I've also looked into it a little, but not enough to be confident telling you what to do with regards to something that'll effect your uni grades! (Hence my cop-out answer of 'read up on it').

There's a fair bit of maths behind the theory, but it's all relatively understandable. The engine is giving off sound at certain calculable wave lengths and pitches, and you want a silencer that'll break these frequencies down to and avoid resonance. There's also considerations to do with harmonics for making the sound 'pleasant' so it's more bearable, In the same way a piano note's a lot less annoying than white noise at the same volume. It's not just the silencers to think about, any bends will cause a reflection and resulting change of the sound waves, as will differing lengths of straight pipe. A well tuned system should get away with a relatively small and high-flowing silencer.

As I said, there's a lot of info out there on it, just a case of sifting through it and making sure you stick to the reliable sources.

(PS, it's not a muffler unless you're from 'murica :P)

Edited by RobinJI
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