best answer so far...being a fabricator by trade ive built a few exhausts and even my own mufflers.i can tell you this,on my setup a cross over helps low end vs no crossover at all.i dont have a dyno to verify what i think,but top end W.O.T. felt about the same on all the systems.the H pipe seems to have a flatter torque cure and the x seems to have more mid range torque and be a little quieter.but id be willing to bet if i changed cams to something with a narrower lsa then my current "best" system may not be "best" any more.i think it really depends on the setup.like putting a mandrel bent 3" x pipe with race bullets on a stock 273 or a 400hp 360.youll see better results with one than the other.
