Originally Posted by chriszeh
my understanding of it is that an H pipe allows some equalization of pressure from side to side thus reducing backpressure to a small degree but an X pipe brings the two sides together allowing scavaging of exhaust gases from the other side creating a much greater pressure drop hence freer flowing exhaust.
No, they function the same way. Just different ways of tuning the scavenging, you can really only vary the position and size of an X pipe, where you can vary the width, position and size of an H pipe. Akrapovic's Evolution (and Gt4) midpipe use both X and H pipes. I haven't seen a significant difference, I ran both the Akrapovic and Challenge mid pipes and they both work pretty similarly and sound similar too.