According to all the research I have done, x pipe is best, h pipe is better than nothing and can save space in cramped areas, and true duals are best in the minds of red necks that are stuck on 1960's ideas. Do a search of exhaust scavenging. To be honest most people use too large diameter exhaust pipe and to short header collectors. Now 180 degree headers are a totally different story with x or h pipes.
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