Why? Exhaust crossover pipe?

Discussion in 'Motorbike Technical Discussion' started by Tom Kutz, Sep 8, 2003.

  1. Tom Kutz

    Tom Kutz Guest

    Can anyone twll me what is the purpose of the exhaust crossover pipe. Why
    not just seperate pipes from each cylinder?

    Tom Kutz, Sep 8, 2003
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  2. Tom Kutz

    Mark Hickey Guest

    It reduces back pressure by allowing some flow out both pipes, and
    allows the designers to use smaller mufflers (they can get downright
    huge if they're connected to large cylinders and aren't
    cross-connected). The mufflers (real mufflers, not straight pipes)
    need to have a certain volume to work with a given displacement

    Mark Hickey
    Mark Hickey, Sep 8, 2003
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  3. Tom Kutz

    OH- Guest

    Even if Keybearjr already wrote about it, it seems that it needs to
    be stated again:
    Exhaust tuning is first and foremost a question of pulses (or waves),
    reflexes, resonance, damping, interferences and stuff like that.

    Good old steady state flow resistance has surprisingly little
    influence as can be seen easily by looking at the unbelievably small
    diameter of the outlet from a very powerful 2-stroke. On the other
    hand even a seemingly innocent change in pipe cross section in the
    wrong place can reflect the pressure pulses in the system and cause
    all sorts of tuning changes.

    So the role of the crossover is not to equalise pressure but to send
    pressure waves to the right place at the right time.
    OH-, Sep 8, 2003
  4. Tom Kutz

    Tom Kutz Guest

    "So the role of the crossover is not to equalize pressure but to send
    pressure waves to the right place at the right time."

    So then what you are telling me is that not only the size but the placement
    of the crossover pipe in the exhaust circuit important. But again, wouldn't
    this only effect a very narrow RPM range? And if the engine fires 180
    degrees = actually 360 degrees for a 4 cycle - doesn't one cylinder -
    exhaust - have a higher pressure and the other have a lower pressure?
    Wouldn't this then raise the pressure on the lower side? Thus, if the
    exhausting gas is suppose to be scavenged out of the exhaust circuit by not
    only the positive pressure wave traveling down the pipe but also "pulled"
    out of the combustion chamber by the lower pressure wave following
    it.wouldn't this equalizing of the negative "lower" pressure have a
    detrimental effect? And then how important a role does the placement of
    the crossover play. If it is closer to the cylinders or if it is nearer the
    termination end would be like playing a trombone. Wouldn't it again be
    tuned only to a narrow RPM range?

    Tom Kutz, Sep 9, 2003
  5. Tom Kutz

    Uncle Bill Guest

    Increases torque, quiets exhaust.
    Uncle Bill, Sep 9, 2003
  6. Tom Kutz

    OH- Guest

    Depends on how well these pulses correspond to the various other
    pulses created inside the pipe they enter. In a worst case, all these
    pulses get into resonance at the same RPM and there is no damping
    in the system. The engine would get "on the pipe" violently and at
    half that RPM it would probably blow fuel backwards out of the
    On the other hand the designer can broaden the power band by
    having the different parts of the exhaus (and intake) system tuned
    to different RPM and get some damping in there.
    It all depends on _when_ the pulse from the crossover reaches the
    exhaust port on the other cylinder. OTOH, "softening" both the
    good and the bad effects of resonance might be the aim of the
    exercise as I already said.
    Well, again, yes, but the end result depends on what the saxophone
    and trumpets do at the same time.
    BMW at least once had bikes with two crossovers, some trombone,
    and definitely not a peaky power curve.
    OH-, Sep 9, 2003
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