Stupid question about crossover pipes

[martialstax]

What is the purpose of crossover pipes, are they required on all new motorcycles, and if so, why?

I guess I should have called this "Stupid questions about crossover pipes.

[Pat Kelly]

Not a stupid question.

The crossover pipe sends exhaust pulses from one side to the other. These exhaust pulses draw the next pulse in the fireing order along (exhaust scavaging).
It's not necessesary to have a crossover pipe but it is supposed to help with mid-range torque.

[martialstax]

Thanks for the answer. Since it appears that every bike on the market seems to have one, I sort of thought they were government mandated. Especially considering the ridiculous lengths some manufacturers were going to include them, like the awful crossover pipes Harley had on their Sportsters for several years.

[Dennyz]

The cross-over is also for sound control. It provides a path for out of phase pulses from one side of the engine to cancel with the pulses from the other side. Can reduce "popping" on deceleration also.

The result is usually a smoother (softer) idle tone.