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Midpipe thoughts

Discussion in 'Modifications and Projects' started by Bonzee, Sep 17, 2015.

  1. Hey guys
    I want to eliminate the midpipe inbetween the cat and the slip on.
    Will this increase the noise of the exhaust and will i have a loss of power due to less backpressure?

    Img attached

  2. The noise is likely not to increase dramatically in Db, however the pipe length decrease may increase the pitch of the tone.

    If you're keeping the CAT in place the back-pressure difference will be trivial.
  3. Since that section of pipe is straight-through, losing it won't make a lot of difference to either noise or back-pressure. Not sure how you'd do it though, the outlet from the cat comes out at an angle so you'd need a pretty sharp bend back to get the slip-on close enough to mount on the pillion peg bracket.

    If you really wanted more noise and less length, I'd look at something like an Ixil Hyperflow slip-on. If it was just less visual length, there's probably room to make that midpipe shorter by a couple of inches. Depends on what you're trying to achieve.
  4. if it is tuned length, the length of single pipe after the two pipes join is part of the harmonic tuning of the exhaust.
    typically length of the (secondary) midpipe would be same length as the primary pipes (or is it half? I forget..).
    taking it off means you don't get any beneficial scavenging between the primary pipes.

    does it matter? probably not, but may shift torque curve around a little (probably not that much in a low power bike)

    edit: any loss of power is not due to lack of back-pressure.. but lack of anti-back-pressure :p

    backpressure is a high rpm bandaid for a poor cam choice (way too much overlap).. ;)
  5. Thanks BitSarBitSar for the info. Yeah im keeping the cat, just wanting to lose the midpipe. Ill be sure to listen out for a change in tone.

    hyperspexhyperspex Yeah that was one of the things i was inspecting/measuring. It will turn out to be a homemade bracket,
    Yeah its definitely a visual outcome im after. Just less length...it is so loud already!
    The other option is shorting the midpipe like you said. Thanks bud!

    oldcorollasoldcorollas you make a good point. That was another issue i was concerned about. Im not overly keen on it affecting its torque.
    Its a honda cb400sf so im not sure if its going to affect it all that much. I guess its a matter of trial and error.
    Thankyou for that piece of vital info, we learn something new everyday ;-)

    Thanks for your help guys!
  6. Another thought, if your muffler is basically a straight through pipe of similar diameter to the midpipe, and only has mild restriction at the end..
    Then the muffler can act as the midpipe (from a tuning perspective), so may not lose any power at all
  7. Yeah the slip on is kinda a straight through but with a baffle that doesnt do a whole lot. The entrance of the slip on is of similar diameter but then it opens up quite a bit. I had ride with it like this and i did notice it felt quicker off the mark but a bit sluggish climbing revs. Does that seem right to you or is it all in my mind..
    Heres how it looks.

  8. that's a CB400?
    looking here the 4-2 junction is just before the frame bends up at the rear? so the distance from 4-2 to 2-1 junctions is not very long.. 20cm? 30 at most?
    so if primary pipe to secondary pipe ratio (length) is around 2.5:1 or 3:1 (ish), then the length of final collector (your midpipe) is less likely to have as large an effect...

    so difference may just be in your mind :p and there's probably buggerall difference

    to see if there is major difference you'd have to dyno, or find a flat safe bit of road and measure time between certain revs (or video speedo and calc an acceleration curve from that... but.. effort..)
  9. Is that a big fcuk-off nail in your rear tyre?
    • Funny Funny x 1
  10. oldcorollasoldcorollas Yeah its a CB400.
    Wow, you got all kinds of dimensions! Haha. Yeah your probably right, all in my head. I definitely notice a change in tone though...Its not as gargly as it was before.
    But im still happy with the way its performing. My only concern now is how far its hanging out the side! Do you think i could get pinned for that?
    It doesnt actually go further than my bars.

    BitSarBitSar Haha yeah i was thinking the same thing when i saw the pic. But its just a shiny part on the tyre.
    • Like Like x 1
  11. LOL :D

    "primary" is the first pipe from the engine, in your case, there are 4 pipes
    "secondary" is where two of them join together, at the 4-2 join (4 to 2)
    and "collector" is the final single pipe where all the pipes are collected together.. the 2-1 join

    4-2-1 means 4 pipes, to 2 pipes, to 1 pipe
    a 4-1 manifold means 4 pipes go directly into 1 pipe

    the ratio of the lengths will change how the exhaust scavenging and sound waves are shared between the cylinders at different rpm. but it's complicated..
    basically, you can emphasise the effect in a particular rpm range, but possibly to the detriment of a different rpm range.
    but bikes have quite a wide range of RPM to work in, so the quite long primary and short secondary is a compromise to widen the rev range the exhaust works well in

    you can see it a bit more clearly here
  12. Christ...impressive oldcorollasoldcorollas ! Thankyou for the lesson.
    What do you do for a living sir?
  13. #13 oldcorollas, Sep 20, 2015
    Last edited: Sep 20, 2015
    nothing at all to do with this, and I'm not even toughing on details above.. that's just starting points... ;)

    if you want a really good insight into tuning of intake/exhaust systems, here would be the place to start

    Scientific design of intake and exhaust systems

    this is an oldish book, but great for the basic theories, although there is maybe more understanding of the processes today.
    it's partly harmonic tuning (like a wind instrument), and partly momentum of the gases (velocity and mass), and also timing of when positive and negative reflections (of the exhaust pulses) reach the valves, to either help or hinder gas escaping from cylinder..

    so there are multiple processes going on, and each can be tuned for a particular engine and the characteristics you want.
    the theory is or can be quite complicated, but the things you actually change are simple :)

    on the engine side, you have cylinder volume/displacement, rpm, and cam timing
    on the manifold side you have pipe diameters and lengths, and the order in which they join
    • Winner Winner x 1
  14. Wow, very interesting read. I appreciate you putting in the time to help me out like this!
    Im going to have a read of this during the week.
    Thanks oldcorollasoldcorollas :]