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Effect of exhaust size/shape on note

Discussion in 'Modifications and Projects' started by Yellow Meanie, Jan 3, 2009.

  1. Not sure if this should be in general or tech, but here's the question anyway.

    I'm wondering what effect different shapes and sizes of slip-on exhausts will have on the note of the exhaust.
    eg: Will an oval shaped exhaust sound different to a round exhaust?
    Will a larger diameter exhaust have a deeper note than a smaller one?
    And what effect would the length have?

    I'm not interested in what people think "looks better", or "sounds better", I've already made my mind up there.

    I would just like to know how the change in pipe would change the sound.


    Here's the two pipes in question:


  2. Its going to depend a lot more on what's inside than the external shape. You need to listen to them!
  3. The mole's got it in one.

    Just out of interest, I've got a Mivv pipe on my Zed. It's the full sized carbon oval shape (with the baffle removed). At < 4k rpm it's got a really deep note, but anything above that it doesn't stay deep, it sorta roars more.. It's not loud compared to a Leo Vince, but that's not always a bad thing.
  4. slickncghia's has a series of amplifiers built in, and several of the worlds largest brass bands performing inside....

    I believe it has more to do with the internals.
  5. Just like mum always told you, "It's what's inside that counts."
  6. my 2 cents:
    coming from an audio engineers point of view,

    the bigger the space that the air resonates off (the inside of the tube) whether it be wider or longer.. will lengthen the harmonic wave length.. eg.. making it lower in pitch.. deeper.. also keep in mind it will make it sound less tight... but that could be countered with baffling...

    also.. (keep in mind I'm not at all mechanically minded)
    I've seen exhausts were they are dual.. eg 2 sitting right next to each other.. this brings an interesting phenomenon known as the coupling effect, assuming individually they both have the same pitch as a single exhaust.. putting 2 side by side will exaggerate the and introduce lower harmonics.. once again making it sound deeper... and also louder. (assuming they are in phase)

    Its the same reason a choir sounds like a single voice and louder than if they were not bunched together... and why you can get more low end out of 4 10'' speakers than a 18'' speaker.

    this will not work for dual exhausts were there is one pipe on each side with space in between.. for it to work they need to be close together
  7. Or what's not inside.

    The Staintune is a large, empty container. It is a straight-thru, no internal baffle muffler.

    There is steel wool packing in the inner edges. The exit baffle thus determines the volume of noise and its depth.

    The exit baffle is about 24mm in diameter, and gives a great sounding but gentle note.

    Remove the baffle and the sound is too loud inside a helmet, and somewhat loud for passers-by. This hole is about 52mm.

    Remember, this muffler has no internal baffles - it is just straight-thru.

    The shape of the muffler and the exit baffle has little effect on sound, until it gets smaller. That then starts to give you that anaemic sound that most J bikes come with from standard.

    There is one more thing that governs the sound - the length of the exit baffle. This can extend inside the muffler, and on the staintune is around 120mm for the inner, removable baffle, and somewhat less when the baffle is removed.


    Trevor G
  8. Ultra-basic finding by me playing with diff exhausts on an old Kawasaki Zephyr 750 found that longer pipes (overall length from engine exhaust port to exit of exhaust system) generally gives you deeper (lower) exhaust tone.
    Going for wider 'internal diameter' pipes for a given length also lends itself to a lower tone in sound.

    A basic comparison is that alot of aftermarket Harley pipes are short, so their tone is a relitivly high "blat", wheres a majority of ducati aftermarket exhausts tend to have longer serpentine system that gives a lower "bop" sound.
    (And yep, there are a million other differences in the 2x examples I've given, but i think I got the message across).

    Oh, & if the shape of the end 'can' is different, but has a similar volume, then you will not get a disernable change in engine note. Most aftermarket exhausts are straight-through perforated core set-ups so the determining factor is the internal diameter of the pipe as apart from the extrnal shape or size of the end 'can'.
  9. Thanks Guys - a wealth of info as usual :grin:

    Marx - you got it! The "bop" is what I'm after. Cheers :LOL:
  10. Yeah, I agree. I had a yoshi that gave a muted "bopping" sound that I got bored of. So I cut off 13cm and now it produces a louder, slightly higher pitched, idle and more of a "blatt" sound on WOT.

    For what it’s worth I have found that my baffle (which was not specifically designed for the yoshi) gives a crappy idle note, but a more dynamic note when under power.
  11. I'm not sure the analogy holds directly, but thinking about the pipes of a pipe organ might be instructive. As it turns out, from a physics point of view, it's only the length of the pipe that is relevant to the pitch of the note, not the diameter. The reason that the longer pipes on a pipe organ are also fatter is that low sounds sound quieter to our ears than high sounds, and they have to crank out more real volume (sound energy) to sound the same to us.

    So if that's the case, a longer pipe is going to be deeper, and a fatter pipe is going to be louder. The cross-section should be largely irrelevant between an oval (or tri-oval) and a circle, though it's possible you'd get some interesting effects with a square or triangle.

    It's more complicated because we're talking about exhaust gases from an internal combustion engine and porting and jetting and tuning and so on in terms of performance, but the basic physics of the sound is fairly straightforward.
  12. So, obviously the sound is coming from the engine (duh), and each different style of engine has it's own sound, in this case a V-twin.

    As I am changing the can and not the rest of the system, would I be right in assuming that there will not be a huge change? Just louder (especially with baffle removed) and a crisper note?

    Each different can is made differently, so I suppose it has it's own way of muffling the engine noise. But how much difference can there really be?
  13. The only answer left, is Seven.
  14. Interesting :)

    FYI each cylinder is out of phase and they start their cycle consecutively in a specific order, producing exhaust pulses at different times. (Though length of pipe etc effects what arrives out of the end when etc.)
  15. in most cases, the bike's exhaust goes into a collector, then splits back into 2 (or more) pipes, thus the pulses coming out of the pipes are in time with each other. a lot of I4's with two tail pipes, the system is 4-2-1-2, same with the triumph street triple's 3-1-2, etc.