Why? Exhaust crossover pipe?

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

Tom

 

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
cylinder.

Mark Hickey

 

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.

 

"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

 

Increases torque, quiets exhaust.

 

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
carbs.
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.