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post #15 of (permalink) Old 10-11-2015
Norm Peterson
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Originally Posted by GT500MUSTANGFAN View Post
I've tested both, there is no "Crossing Flow" on an H-Pipe. It's a straight connection pipe to either side that allows flow to balance as needed.
If there is no crossing flow you are losing all of the benefit of having both mufflers share the total peak flow (which as I said before is larger than any single cylinder's peak flow). Even a brief reduction in flow resistance contributes to a slight power increase. We aren't talking about any big increase here, probably just a handful of HP and ft-lbs.

The X-Pipe's converge point leads to back pressure that doesn't allow the exhaust to balance freely, thus imbalance is more of a possibility. The X-Pipe's converge point "Forces" the flows to meet, the H-Pipe's connector "Allows" it, so saying the X-Pipe is 'gentler' isn't true all all.

I can't think of a better word to describe how much better the small direction changes in an X are than the two sharp, right-angle turns that flow across an H must make. And you have to have some flow across the H if you're going to get any backpressure reductions from it.

If you cannot share the total flow between your two mufflers, there will be two brief periods where this flow might instantaneously be 40% higher (due to the firing order). If you force that flow to mostly go through only one muffler, its backpressure would briefly be nearly double the peak backpressure at other times. Share it equally and that peak backpressure drops down to about half (and all of the single-cylinder peaks will lose some backpressure as well). Even though this occurs over only two parts of the 720 complete cycle, it's there to be taken advantage of. You do have to be thinking on a millisecond time frame level here.

It's why H-Pipes are great for those that run on the track & generally spend most of the time at higher speeds, balanced/quicker/easier exhaust flow. However X-Pipes have that back pressure which is ideal for low end applications, such as from a stop. Amercainmuscle has their description backwards, they claim H-Pipes are better suited for low end.

I know what an 'H' is. It does balance the pressure spikes (sort of). But that's not quite the same thing as equalizing flow, and you can hear the difference between straight duals, H, and X.. Partially equalizing the spikes takes some of the harshness out of the exhaust tone - damps the rumble a little, while equalizing the flow eliminates virtually all of the choppy low frequency rumble that is characteristic of crossplane V8 engines.

An X would only result in the extra restriction you speak of if the piping length where the flows are merged is not large enough in terms of total inside area. Sure, you can design a bad X just like you can design a bad H, with either one using tubing that's too small for the job. But let's at least assume that everybody knows how to do a good job here, and not credit the H guys for using full-diameter pipe for theirs while claiming that the X guys aren't sharp enough to go bigger through the merge, OK? My guess, 20% or a little bigger diameter through the X for a big single pipe or pairs of exhaust line diameter pipes that do not overlap too much with the dividing portions of the walls removed is in the ballpark.

Just so you know, I worked up a very unique X-type of crossover for a very specific set of circumstances. Gentle bends going in and out and everything. Being somewhat familiar with fluid flow topics (I was employed as an engineer working with piping systems for most of my career - now retired), I just borrowed some of the tech I had to understand on the job to my build.

Just because . . . the cylinder numbering happens to be for a SBC but the plots end up the same


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