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Exhaust Design: Firing order considerations

RYU

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I need some help thinking about the effects of the cylinder firing order in terms maximizing scavenging effects at the header collector and further down stream in the exhaust. If both header banks dump into a mixing/collection chamber, say 1 cubic feet in volume, will I run the risk of blowing exhaust gasses from one bank into the other bank essentially incurring a reverse the flow?

For example, if one piston is in it's exhaust stroke in bank 1 and another piston in bank 2 is in it's intake stroke will I run into a potential problem with exhaust wanting to "blowback" into bank 2? I believe its dependent on the firing order which is 1-4-2-5-3-6 for the NSX.

This doesn't seem to be a problem in a correct X-pipe designs that flow well but in looking at the stock exhaust design (thanks for the image trevvvvvv!) Honda combines exhaust gasses from both banks in a mixing chamber albeit it there's a lot more crap going on to dissipate pressures and the stock exhaust flows poorly.

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You might be wondering why i'm going thru this. I'm not 100% happy with the exhaust options out there. My current design calls for a mixing chamber to reduce weight. If this "blowback" issue becomes a problem then I will have to separate each bank into it's own muffer (not ideal for weight concerns).
 
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If i'm understanding your concern, I think you may not be stating it correctly. Exhaust gases should never enter back into the combustion chamber of another cylinder via the intake valves. I believe the only exception to this is the EGR system.

But your concern about maximizing cylinder fill by using the scavenging effect of the exhaust flow from other cylinders, while possibly too esoteric for many (including me), is real.

I think to really understand what's going on with the exhaust pulses of the engine, one would need advanced data acquisition and flow modeling software. Guesses, assumptions, estimations and extrapolations can be made but when designing at that level it'll be relatively expensive to experiment and difficult to measure the results.

I've seen people make gains by degreeing cams and playing with the valve overlap which, among other things, can optimize the scavenging effects of the exhaust gases. I don't know how effective they are on NSXs, but you may find that more substantial gains can be found by getting adjustable cam gears and spending some time on a dyno versus designing an exhaust to try to maximize flow through and out of the engine. Once purchased and installed, it becomes very easy to tune, compared to an exhaust.

Good luck, though. I'm interested to see your results

J
 
If i'm understanding your concern, I think you may not be stating it correctly. Exhaust gases should never enter back into the combustion chamber of another cylinder via the intake valves. I believe the only exception to this is the EGR system.

But your concern about maximizing cylinder fill by using the scavenging effect of the exhaust flow from other cylinders, while possibly too esoteric for many (including me), is real.

I think to really understand what's going on with the exhaust pulses of the engine, one would need advanced data acquisition and flow modeling software. Guesses, assumptions, estimations and extrapolations can be made but when designing at that level it'll be relatively expensive to experiment and difficult to measure the results.

I've seen people make gains by degreeing cams and playing with the valve overlap which, among other things, can optimize the scavenging effects of the exhaust gases. I don't know how effective they are on NSXs, but you may find that more substantial gains can be found by getting adjustable cam gears and spending some time on a dyno versus designing an exhaust to try to maximize flow through and out of the engine. Once purchased and installed, it becomes very easy to tune, compared to an exhaust.

Good luck, though. I'm interested to see your results

J
You could very well be right. It isn't easy for me to articulate what i'm trying to ask. A lot of it has to do with not knowing the proper terminology. Conceptually, my concern is perfectly clear in my head :)

I agree that the valves will stop any backflow into the combustion chamber. I'm concerned about backflow up through the collector back up the headers. Changes in pressure tells me this likely won't happen but to maximize scavenging (the phenomena of when air travels past a open orifice and wants to scavenge/suck/pull from air behind it) probably won't happen? I don't mind if there is a net 0 scavenging effect. I just do not want a negative scavenging effect situation where air wants to travel back up the headers due to the firing order between the first bank pistons and the rear bank pistons. Looking at the firing order I think this isn't a concern but I'd like some reassurance.

I can tell you this though.. the 3 into 1 header design will have some scavenging effect IF the headers are equal length. This is why I like the Taitec JGTC parallel exhaust design because each bank is kept independent. However, it doesn't sound all that great. In essense, i'm asking what if theoretically speaking this was a 6 into 1 header design? In concept this my question.

I'll draw a diagram tomorrow.

Appreciate your thoughts guys.. even if it's just to tell me i'm overthinking this.
 
I think what you're describing are what the performance industry used to call tuned headers. I believe that nowadays the industry lots of times calls a header a tuned header to make it sound special when really they're not.

My understanding is that a tuned header is developed in such a way that the scavenging effect is maximized - the exhaust pulses of one combustion chamber actually suck the exhaust gases from adjacent ports. This is done by carefully designing the primaries to not only be equal length but also of a specific length. This specific proper length places an exhaust pulse of one combustion chamber at the collector when the exhaust valves of another combustion chamber are open. This phenomenon is exactly the thing you're describing with the air rushing by an orifice.

I don't know if the equal length headers for the NSX have had this tuning done. They may have, but I've never heard anyone say they were and (possibly because I can't read Japanese) I've never read it anywhere either.

I wonder if it makes the NSX sound better if it has tuned headers.

J
 
I had not given the length of the headers enough thought. I think I will keep the discussion simple and assume equal length headers at some given total length. It's definitely a great point and perhaps length maximizes performance in a certain RPM range.

The exhaust flow depicted via the orange arrows is the phenomena i'm worried about. Does this make sense?

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btw.. this is a rough rendition of the Sorcery F1 exhaust for the NSX. It's center exit so pardon the odd orientation. I do not know if this is how the sorcery looks inside. Its just an example for illustration purposes.
 

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In the case of the exhaust, I don't think your concern is warranted. It is so far downstream that the effect will be minimal when a pulse from one bank goes backwards all the way up the other exhaust pipe back through the header and back to the cylinder. By the time you get that far downstream I think there is too much energy dissipated. The pulse of the exhaust exiting right when the exhaust valve cracks open is very high energy. Much of that energy and velocity is diffused by the time you get several feet to the muffler.
 
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You are touching on a major design function in the factory exhaust of every well built car: using the pulse toward the exhaust exit of one cylinder to help pull the burned gasses from the next cylinder to fire. Years ago, when I was learning a bit about hot rodding, I studied some header designs and learned a bit about this technology. It is the reason why one did not arbitrarily lengthen or shorten an exhaust system. (Think under-doorsill exhaust exits or extending a system with long, same diameter tips.)
A great deal of science goes into the factory design. Redesigning it may save weight. Enlarging the tubes may improve breathing. The net result, however, may be a decrease in designed-in scavenging.
My $.02
 
Thanks for your thoughts gents. I was afraid I may be overthinking this. I'm simply worried about creating any type of opposing pressure where gasses are competing instead of the complete opposite; scavenging. Yes, the direction of flow (aka. free-flow) is important.

I'm also wondering if it's more beneficial to separate the mixing chamber (in my illustration) with a dividing wall. Essentially separating bank one and bank two. I'll have to think thru how that will affect the sound quality and perform some helmholtz calculations. Ideally sound improves when both banks are "mixed" in my experience.

In any case, appreciate the feedback gents. I might be good enough to proceed to the next round.
 
RYU, whos car is in your avatar? Where is that? Cool pic!!!:smile:
Dave, I can't seem to find the higher res version but basically it depicts a young girl (typical Anime) who drives a Widebody NSX. She's having problems scanning her keycard (or paying toll, or picking up a parking ticket...?) because her car is so wiiiiiiiiiiiiiide! It's rather hysterical in full res.
 
Thanks for your thoughts gents. I was afraid I may be overthinking this. I'm simply worried about creating any type of opposing pressure where gasses are competing instead of the complete opposite; scavenging. Yes, the direction of flow (aka. free-flow) is important.

I'm also wondering if it's more beneficial to separate the mixing chamber (in my illustration) with a dividing wall. Essentially separating bank one and bank two. I'll have to think thru how that will affect the sound quality and perform some helmholtz calculations. Ideally sound improves when both banks are "mixed" in my experience.

In any case, appreciate the feedback gents. I might be good enough to proceed to the next round.

I think you are hitting on the key reason the banks are mixed--to raise the exhaust sound's frequency by an octave (double). In terms of getting the higher sound, I don't think a mixing chamber will work as well as a merge collector that takes both banks and puts them into one high velocity stream first. You see this in Nissan's VQ exhaust design where there is a merge collector right where the two headers come off the engine. It is like a continuation of the header which merges the two streams into one very early, and the actual sound is muffled several feet later at the very end. That's why the VQ has that high pitched sound.
 
Are you just doing a muffler? Have you determined if the headers you are using (stock or aftermarket) are tuned themself? If the stock manifolds (either version) aren't tuned, I don't know how you could do anything at the muffler that would mess anything up...
 
Ryu I think you should tread carefully here, as it is a lot easier to screw something up than wind up with superior performance. I went down the same research road as you and ended up with the OEM muffler. There is a lot of math and testing that goes into a properly tuned ehxaust design. Most aftermarket shops don't have the resources of a major manufacturer like Honda to design and test systems until they hit the sweet spot. To do it right you need a dedicated engine dyno, a bunch of computers, a team of engineers, and months of time to fabricate different setups and test them.

The practical result of this is that all aftermarket exhausts I have heard, including popular ones like the Comptech and Tubi, have varying levels of drone. Some are better than others, but they all do it. In addition, usually you see a loss of low-end torque all over the range, but a bump of 5 to 10 hp above 6500 rpm. I believe the reason for these issues is that no one has the resources to design the entire exhaust system from front to back. Yes, you can remove "restriction" to open up some peak hp numbers, but that is just good for dyno racing and internet bragging rights, IMHO. The real restriction in the NSX exhaust track is the manifolds, not the muffler. Now, if you go FI, all bets are off. ;)

Switching from my B&B Tri-Flo (which sounds AWESOME, but too loud for the wife) to OEM, I noticed an immediate and surprising increase in torque from standing to 4500 rpm. The car is more lively off the line. Gone is the drone at 2000 to 2500 rpm. Exhaust note at WOT is similar to the B&B, but quieter. It is not noticeable, but my B&B dyno sheet says I lost about 10 hp at the top of the rpm range. I don't miss or notice it. Driving the OEM exhaust after having a high performance aftermarket unit has convinced me Honda knew what they were doing.
 
What are your thoughts on a supercharged setup as it relates to scavenging? This is assuming equal length, long tube primaries, and a straight thru exhaust.

I believe I once read scavenging becomes less of a benefit with supercharging. Could have been heresay...
 
exhaust scavenging is more appropriate at the header merge points for each bank. as the iron stock manifold have no effort for scavenging effect, i'd not worry about it at the box. i mean have you seen the stock cast iron rear manifold, it's a log. the later headers are a step in the right direction, but still not ideal.

here's what i made up as my stock box was starting to split on one edge....

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and tuneable depending upon which turbo muffler you pick. i had thrush 3 chamber boxes to prove the concept, and changed to magnaflow stainless packed ones later for a more mellow tone.

i put headers on mine too, that definitely improved the sound and increased the bottom end torque!
 
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