Don Terrill’s Speed-Talk

When I had a system built circa 1989 for a BBC dually with stock manifolds, 3" pipes and Walker Dynomax mufflers, I painted stripes of several different rated Tempilaq heat indicator paints down the pipes. The left bank crossed over to the right side, so the resulting indicated hot spots weren't in the same plane, but were both ~40" from the outlets. I had the parallel pipe section boxed in to form an hourglass-shaped connecting passage, open at the front end to the left bank and at the rear to the right. Just for interest I added a butterfly valve, lever and operating cable to allow opening and closing of the connector pipe. It changed the exhaust note considerably and seemed a couple of dB quieter open but was not detectable by the butt dyno.

ptuomov wrote: ↑Mon Oct 09, 2017 1:26 pm Pipemax is one program that suggests H-pipe locations. Those are related to the "optimal" collector lengths in the sense that the full-diameter cross-over looks a lot like collector end to the pulses. The same way the optimal collector length is not zero, the optimal location of the H-pipe isn't at the primary ends either. That all said, the optimal H-pipe location as prescribed by Pipemax is going to be pretty close to the header primary tube ends.

That's what is confusing about the H pipe to me...how does it look to the exhaust pulses and how does that change over RPM? I'm trying to understand the physics behind why the H pipe works and where it should be placed.

*Begin theory rambling*

The collector has to deal with the un-even firing order and the double pulse that happens. Thinking of a small block chevy with a firing order of 18436572.....the drivers side collector looks like 1XX3X57X....and the passenger side looks like X84X6XX2. The numbers are the exhaust pulses and the X's are pauses. You have the times where the pulses are separated by two pauses. You have times where the pulses are separated by one pause. You have times when there is a not a pause between pulses. Now the when the pulses are separated by one pause, that is a truly unique situation. The double pulse and the double pause share the same timing...but the actual event....the pulse and pause are inverse of one another. A double pulse is always followed by a pause then a pulse. A double pause is always followed by a pulse then a pause. Same timing, but inverse events. So seems like there's TWO frequencies that you would need to tune for. The collector has to be tuned to TWO frequencies, at any given engine RPM.

Thinking a little more simply, the double pulse happens on one collector when the double pause is happening on the other collector. If the collector is tuned for the single spaced pulsed, it would not be ideal for that double pulse which means there may be a pressure spike in the collector because it's not optimized for pulses at that frequency. The double pulse is twice the frequency the collector is tuned for. The H pipe is a great place for that pressure spike to go, especially when there's plenty of space for that pressure spike in the other collector where nothing is happening. That doesn't really tell us anything about optimally placing it though,

At the point in the collector where the H pipe intersects, obviously the pulses would see an increase in cross sectional area...sort of like the end of a collector. Or maybe it is as if you increased the tubing size at that point...maybe like a stepped tube header....multiple negative pressure wave reflections at different frequencies. So if you have a tuned length collector that gives a negative pressure wave reflection at a certain frequency, does the H pipe enhance it further by providing another negative pressure wave reflection at a different frequency? The same reasons a stepped tube header needs the tubing size increase at certain points is also the same reason the H pipe has to be at a certain point in the collector.

But even then, it seems like the effect would be variable. It's not like a stepped tube header where the increase is cross sectional area is straight ahead. I think that since it is a super sharp 90 degree turn, the pulses are going to react differently based on pressure and speed when they hit the intersection. For example at high RPM, I'm guessing that the pulses tend to blast right past the H pipe intersection. It's also not like a stepped tube header, because the increase in area doesn't stay constant...it shrinks back down again after the H intersection. Obviously the concept of stepped headers doesn't apply completely, but I think it helps understand the underlying concept of what is happening.

In theory, you'd place the H pipe at the point in the collector, where the collector would end IF it were the proper tuned length for the double pulse. The H pipe provides increased cross sectional area at a critical point, and gives a low pressure area for that double pulse to go. But, because of the 90 degree turn, the return to the original cross sectional area after the H pipe, and the fact that the pressure varies in the H pipe based on the other collector, it also preserves the original tuning of the collector. Maybe.

So with all that said, it seems that any H pipe placed in the collectors will help. I can't see how it would hurt unless it was excessively large or placed too far up so that it made the effective collector length too short. But it does make sense that there would be an optimal place to put it for maximum benefit.

*End theory rambling*

If you've read this far I'd like to hear any opinions on whether my thinking so far is valid, or if you have your own theories on this topic. I don't feel that all I've said is 100% correct but I do feel like I understand the concept a little better. I haven't been able to find any deep theories on why H pipes work and where to place them in the collectors. I will buy Pipemax in the end to make the final dimensional decisions for my exhaust but I would like to understand this better.

Perhaps the most important aspect of H-pipes is that to my knowledge, no one has ever reported more noise or less power when using one.

MadBill wrote: ↑Mon Oct 09, 2017 5:10 pm Perhaps the most important aspect of H-pipes is that to my knowledge, no one has ever reported more noise or less power when using one.

I think that's probably the best and most to the point lesson on the H pipe that I've ever heard. Now if I could just the part of my brain that always asks "why?" to accept that...

Best use for an H pipe IME is when stepping exhaust pipe diameter down, for e.g. using 3"collectors and then a 2.5" exhaust after the collector. Run correct collector length at correct collector diameter then fit the H pipe at the end of correct collector length then reduce pipe diameter after the H. After this the exhaust can continue as a dual or be connected as a single. This method , with a good system makes the least difference in hp/et (over the 1/4 mile) exhaust on V off. But the H crossover pipe needs to be as big as possible, collector diameter is ideal.
Could ramble about exhausts all day but I'll stop there!
P.s. use pipemax, its the quickest way these days to get very good exhaust results. Be honest with spec entry into the program too!

I did a little more research on Pipemax. It seems whatever it gives you as your ideal collector length (4th harmonic), it will also tell you to put your H pipe right at the same spot. This makes sense for a full exhaust system, the H pipe is an easy way to get some sort of cross sectional area increase to try and make the collectors seem like the proper length.

So if the collectors are made to the ideal length, and they dump into the terminator boxes, then the terminator boxes are going to simulate open atmosphere which goes far beyond what an H pipe of capable of. So the only place to place the H pipe is ahead of the terminator boxes, BUT WHERE?

I think I know where. The 4th harmonic is listed as "highly recommended, best torque" and that's what the collectors/terminator boxes will take care of. Pipemax also gives the 5th harmonic which is listed as "reduced torque, more top end HP sometimes." The 5th harmonic is exactly HALF the length of the 4th harmonic. The only place that would make sense to place the H ahead of the terminator boxes would be the 5th harmonic.

That's pretty simple. Make the collectors the ideal 4th harmonic length. Place the H pipe half way between the end of the collector and the end of the primary tubes. Use an H pipe tube that is a little smaller than the collectors, as to not seriously effect the 4th harmonic tuned length.

Based on the headers I have now, the 5th harmonic H pipe would be under the transmission pan. That's probably why I've only ever seen terminator boxes and an H pipe ahead of them on paper. On my truck, it will be slightly below the frame but it will not impact ground clearance in a big way.

Ok,, read the 'reduced TQ' bit. And realise that with a full system after that , its not realistically going to be increased HP.
Deliberatly making the h pipe smaller?,,, save the welding wire, time and pipe. Once you kill a harmonic its gone.
Put the T-box (not my personal choice) or H pipe at ideal collector length.

cjperformance wrote: ↑Tue Oct 10, 2017 4:09 am Ok,, read the 'reduced TQ' bit. And realise that with a full system after that , its not realistically going to be increased HP.
Deliberatly making the h pipe smaller?,,, save the welding wire, time and pipe. Once you kill a harmonic its gone.
Put the T-box (not my personal choice) or H pipe at ideal collector length.

David Vizard recommends the H pipe ahead of the terminator boxes. He doesn't say exactly why. My guess is that he's trying to bleed off some of the pressure from the double pulse that's going to happen in each collector because of the un-even firing order. That's the only reason I can think of as to why it would still help. I guess there's the sound cancellation and decibel reduction as well.

I really would like to have a deeper understanding of the H pipe, but with space under the car typically dictating placement and that fact that they almost always help no matter what you do...I can see why there's not a lot of deeper information out there about them. Why overthink a simple thing that works?

I have a feeling I'll sleep better if I just let this one go and live by a classic Joe Dirt quote "How exactly does a posi-trac rear end on a Plymouth work? IT...JUST...DOES!" https://www.youtube.com/watch?v=1E6IfdUJn6s

GARY C wrote: ↑Wed Oct 04, 2017 8:09 pm I did this for my 79 400 4spd T/A because I had most of it and it would fit... It's 3" X dumped into 4" open pipe tapered down to 3" bullets tapered down to 2.5" tail pipes.

That pretty much describes my thinking on street car systems up to 600 horse. The taper down is important to keep the speed up and when you go up a notch just after the X and into a pair of mufflers before you taper down into another set you get great muffling without much resistance. Totally unscentific thinking, just my experience.

If the logic says that the terminator box and cross over should be placed at the same spot and serve the same function, why not push this train of thought to its logical conclusion? The logical conclusion being the optimal location for the pressure wave termination box is... nowhere in the exhaust system! We’ve seen the car factories spending billions on exhaust design and basically none of them have come up with a system that has a dedicated pressure wave termination box in the system. They do all use crossovers of some kind in the cross plane V8 exhaust, so that seems to prove out. So isn’t the logical solution for this exhaust system to put in three mufflers, generously sized H-pipe at a location that is about the calculated collector length, and the last muffler combining the both flow paths the second time?