That's pretty good info/feedback...they're generalizing but accurate at the same time (hmm that's contradictory? lol)
It's a speculation game...X-pipes are proven to produce better exhaust gas scavenging effects than H-pipes BUT then the exhaust system must be tuned and flow in such a way for a scavenging effect to take place under high RPM.
For most street cars in the 350-400ci range, running modest cams, catalysts, mufflers, you don't see X-pipes yielding HUGE gains over H-pipes...from all my research/experience on the subject the difference in say, for example the LS3, between an H-pipe and a (properly tuned) X-pipe system might be up to ~7-10whp (prob closer to 5 IMO)...I'm painting some broad strokes here but really in most ppl's applications the difference is fairly small.
Start adding boost, cams, entering the world of big-block displacement then, yeah, an X-pipe is the way to go all day long...
That being said, in a V8 application, to me there is NO comparison in sound - a properly located H-pipe sounds WAAAAYYYYY better than an X-pipe, and to me/some I'd rather leave (an arguable) 3-5whp on the table and not sound like a ricey/raspy foreign V8...
Now, as for V6s - I do agree an equalizing H-pipe would give a V6 a noticeably deeper tone but could contribute to increased 'drone' depending on a number of other factors (muffler type, pipe size, crossover location, etc...). But if you're looking for that, and understand it's not going to sound like a V8 no matter what, an H-pipe would likely yield the most tone change (deeper)...depending on muffler selection of course.
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