H-pipe vs. x-pipe vs. no cross over

headers or exhaust manifolds do not factor in to X or H pipe function.

 

I've made this point before too.

Depending on the type of crossover/merge, this effect would be realized more on some designs than others, but overall is true.

Some "X" pipes simply cross over the path of the other side, while others encourage pulse gathering from one side to the other in addition to encouraging each pulse from one side to exit down both exhaust pipes after the center of the "X"...

Not all merge pipes are created equal, which may be why some see impressive gains when using one, while others see negligible gains (the "X" pipe used in the video is less than optimal IMO, because of the angle of entrance on each side in relation to the other).

Most of the benefit of an "X" pipe would be being able to use more restrictive, yet quieter mufflers due to this sharing phenomenon, particularly with the use of headers--though manifolds would see this benefit too, would probably see better gains from pulse gathering with this crossover than headers would.

Tubing size would be very important here for maximum benefit, as trying to find a good balance between flow volume and maintaining good velocity for evacuation would be imperative, and would depend heavily on intended RPM main sequence usage, corresponding to the power output within that spectrum.