You remember all that stuff we've discussed about backpressure vs. delta-pressure and the idea of exhaust scavenging? See the sticky at the top of the forum if you don't...
Well, in your 4 cyl, you can scavenge between cylinders since they are right next to each other; but in a V8, the firing order jumps back and forth between banks (each side of four cylinders is a bank). If I have a dual exahust on a V8, each bank has a completely seperate exhaust route. By adding the crossover pipe, I now get that scavenging wave from one bank across to the other.
X pipes are generally considered a little better from a standpoint of turbulence, but H pipes work almost as well and are a LOT easier to route and install. They generally boost mid-range power significantly; but they may not affect the peak, high-RPM power much at all. Most people try to get them as close as possible to the collector to push the effect as high in the rpm band as possible.
But both the H- and X-pipes work (and they do work!) by allowing us to exhaust scavenge across banks.
