Here's a little more detail on why a cross over pipe is desirable:
The torque-boosting effect of exhaust pipes arises as follows:
As the exhaust valve(s) of a cylinder open, a positive wave of pressure travels down the pipe. When it comes to any point of expansion (such as when it joins a collectors pipe, or when it reaches atmosphere), that wave of expansion also expands back up the pipe. If it arrives in the cylinder during valve overlap (exhaust not yet quite closed, intakes just beginning to open), it draws out residual exhaust gas in the combustion space and can even propagate into the intake system, causing intake flow to begin even before the piston has moved downward on its intake stroke.
Because positive and negative waves alternate in the pipe, at some lower rpm it is a positive wave that arrives at the cylinder during overlap. This reduces torque at this rpm by stuffing more exhaust gas back into the cylinder and into the intake ducts, thereby delaying the intake process. In motorcycles with carburetors, the positive wave also causes an over-rich condition, as the carburetor adds fuel to flowing air no matter which way it moves.
By providing another point of expansion, crossover tubes—which are often quite small—can broaden the rpm range over which the negative wave arrives during overlap, thereby improving cylinder filling and power.
So, it seems that both torque and power are improved. Extra free HP and torque just for using a cross over pipe!
