question about stock motorcycle headers.

[Parker]

I was making our flanges for our manifold yesterday and noticed something about the stock manifold that kinda confused me. Between the #2 and 3 primarys, there is a crossover pipe. I have never seen crossovers other than after the collector, and never on a four cylinder, and was just wondering why they are put there? It seems like disturbing the flow in this area would be slightly counter-productive to making power on those two cylinders.

[Parker]

I was making our flanges for our manifold yesterday and noticed something about the stock manifold that kinda confused me. Between the #2 and 3 primarys, there is a crossover pipe. I have never seen crossovers other than after the collector, and never on a four cylinder, and was just wondering why they are put there? It seems like disturbing the flow in this area would be slightly counter-productive to making power on those two cylinders.

[Chris Allbee]

I'm not 100% sure (not even 80%), but just speculating that it might be to even out exhaust pulses. maybe? if they were able to remove any huge pulses it might reduce the exhaust noise a little. Like i said i'm not positive on this, but on these bikes the manufacturers would only sacrifice performance if it was for some legal reason (emissions, noise, etc.)

[VFR750R]

That same exhaust probably had primary collectors for 1-2 and 3-4. This is not an optimum setup for a 4 cylinder. By adding the crossover it helps even out the exhaust waves in the collectors.

I haven't quite figured out why most manufacturers don't have 1-4 and 2-3. I guess it's to keep the powerband wider and for ground clearance, but it definitely hurts peak torque and hp.