The first exhaust collector in 4-2-1

[ptuomov]

Suppose that you have a 360-degree 4-2-1 header. Suppose further that space constraints dictate a lot of things about primary lengths. There is step in the primaries where it needs to be and possibly also in the secondaries, again in the right position. In this setup, should the first collector that connects the two 360-degree-spaced pulses from primaries into the secondary go up at all in diameter? That is, should the say the two 1.5” primary ends connect to a 1.5” secondary beginning? Or should the secondary start be larger in diameter? Note that this is specifically for a 360 degree 4-2-1 header in a four cylinder engine with conventional race camshafts. The answer has practical relevance in a little project.

[modok]

should the secondary start be larger in diameter?

Usually yes, but sometimes no
It would depend on the ratio of lengths and the cam duration and maybe something about head flow.

360 paired tri-y can hit a limitation when each Y hits the second harmonic
Smaller the secondary is, the lower that frequency will be all else the same

[ptuomov]

should the secondary start be larger in diameter?

Usually yes, but sometimes no
It would depend on the ratio of lengths and the cam duration and maybe something about head flow.

360 paired tri-y can hit a limitation when each Y hits the second harmonic
Smaller the secondary is, the lower that frequency will be all else the same

Please elaborate. I’ve got 255-265 degrees or something duration at the seat and maybe 230 degrees at 0.05” on the exhaust. Exhaust port flows a ton so I’m regulating the flow with the first primary section (smaller than the port).

What exactly are the wave events that would make one want to have a large diameter secondary?

My thinking is that, with conventional cam durations, from just flow perspective of pressure loss and friction, there’s really no need for the secondaries to be larger than primaries. But maybe there’s some wave tuning reason to slow down the flow in the secondary pipes. Just can’t right now think of anything obvious.

[modok]

I'm not sure how to elaborate, it's just is how it works.
I can say that if you thought it was different intake vs exhaust, well, it isn't. This particular aspect of it is the same.
Go faster than the second harmonic than you are trying to make it run faster than it naturally does.

Now the duration......that matters because it isn't a perfect sine wave with 50% forward and 50% backward. as duration goes up the outflow time becomes more than 50% of the time, and the lets say,...springback period in the middle is less. , so, more duration there is, the sooner you would hit the second harmonic. What IS that? like a pendulum, or swing. Exceeding the second harmonic is, well, like it's pushing the swing while it's still coming towards you.
The most ideal time to be pushing on the pendulum is when it just stopped or while it's going away from you.

IMo if the exhaust flow ratio is too high, it is better to reduce the duration to suit. But that's just my own pet theory.

[ptuomov]

You've got to be patient with me, I am new to this normally aspirated exhaust stuff!

What do you mean by second harmonic?

As a starting point, what I have are (for the sake of argument) optimally stepped zoomie independent exhaust pipes. The cross-sectional areas expand by right amount at right distances from the exhaust valve. Now, I want to use the effect that the early piston engine airplane designers discovered, namely that by connecting these exhaust pipes with collectors I can utilize the otherwise wasted energy in the exhaust gas of one cylinder to create a low-pressure conditions in other cylinders. Therefore, I'm connecting the pipes 1-4 and 2-3 which fire 360 degrees apart first and later the 1-4 and 2-3 secondaries also. The problem with connecting the pipes together in order to generate suction is that it also allows the pulses to migrate between cylinders and that sometimes badly screws up the overlap between cylinders. Therefore, the primary + secondary lengths have to be at least certain length for the engine to work well at a given rpm without harmful 180-degree interference. Is this correct so far?

What I don't understand is the following two things:
- What determines the optimal split between the combined primary and secondary length into primary and secondary components? I understand the minimum length for the two combined is determined by the desired power band low end rpm and 180 degree exhaust blowdown interference. However, how far from the exhaust port should the primaries connect into the secondary?
- When the primaries connect to secondaries, why should the cross-sectional area go up at all relative to the optimally stepped zoomie independent pipes? There's not flow overlap with 360-degree pulse spacing and 250 or so degree cams.

(We have cams that are fixed by the rules so changing the exhaust duration isn't possible. Furthermore, playing with the simulator hasn't shown a clear preference for reducing exhaust duration vs. reducing the primary pipe size. It seems that the four stroke racing engine could have a quite small exhaust valve and primary and long exhaust duration without reducing power, making room for a larger intake valve. But that's a digression)

I'm not sure how to elaborate, it's just is how it works.
I can say that if you thought it was different intake vs exhaust, well, it isn't. This particular aspect of it is the same.
Go faster than the second harmonic than you are trying to make it run faster than it naturally does.

Now the duration......that matters because it isn't a perfect sine wave with 50% forward and 50% backward. as duration goes up the outflow time becomes more than 50% of the time, and the lets say,...springback period in the middle is less. , so, more duration there is, the sooner you would hit the second harmonic. What IS that? like a pendulum, or swing. Exceeding the second harmonic is, well, like it's pushing the swing while it's still coming towards you.
The most ideal time to be pushing on the pendulum is when it just stopped or while it's going away from you.

IMo if the exhaust flow ratio is too high, it is better to reduce the duration to suit. But that's just my own pet theory.

[ptuomov]

Relatedly, why does this page say that primaries and secondaries should be about the same length?

https://www.musclecardiy.com/performanc ... t-headers/



13-7. For four-cylinder applications below 7,800 rpm, a 4-2-1 system (as seen here) gives better results than a 4-1 system. The primary and secondary lengths need to be about the same and generally run around the 13- to 18-inch mark.

[modok]

I think the fastest way to see the difference would be to look at the pressure traces of the different arrangements of Y
Just one cylinder test would be enough to see what happens

If your simulation cannot do this then perhaps you can get one that does, or maybe I do it, but I need to mow the lawn and so forth also

[ptuomov]

I think the fastest way to see the difference would be to look at the pressure traces of the different arrangements of Y. Just one cylinder test would be enough to see what happens. If your simulation cannot do this then perhaps you can get one that does, or maybe I do it, but I need to mow the lawn and so forth also

I've been simulating these combos for a couple of months now with EngMod4T. Furthermore, what works in the simulator seems to be working in real life too when put into metal.

So far, what the simulator seems to be liking is:
- starting primaries small
- many small expansion steps on the way to the final collector
- the first y that makes the primaries and secondaries not too different in length
- the cross-sectional areas not expanding much at all in any of the y (beyond how the single zoom pipe would step up in diameter)

[modok]

Looks like you are headed for an important discovery of one form or another. I'm not going to try to steer you different.

But far as your quesion.....
IMo make a one cylinder model with straight pipe vs y pipes of different sizes all the same total length..... and compare the pressure trace graphs, and see how frequency and harmonics works.
With a basic formula you can calculate the frequency or "period".... of a Helmholtz resonator, and you have one for a straight pipes.

What is the frequency of a pretzel?
No formula for that
you can find out thanks to the power of computers

It does follow basic trends, for example
If the outlet path out of the pretzel is bigger, or shorter, then you have a shorter period,
If the volume of the pretzel is larger relative to the outlet path then you have a longer period.

[ptuomov]

After playing with a four cylinder simulation with independent zoomies, dual 360pdegree 4-2 system, and 360-degree 4-2-1 system, I've come to the following tentative conclusions (thinking out loud here):

When the primary diameter off the head is the limiting factor on power at peak power rpm, the zoomies seem to be not too far of from the best header system for peak power. They have all sort of torque holes at lower rpms, however.

In the 4-2 dual system and 4-2-1 system and below peak power rpm, the primary lengths (and diameters) have to be such that the side branch resonator return pulse from the other cylinder's closed exhaust valve arrives in this cylinder's exhaust port either before the intake valve has opened or after the exhaust valve has closed. So either short primaries or long primaries work, but there's a middle length that sometimes doesn't work very well, at least not at rpms below peak power. So far, the simulator is liking the short primaries, and those fortunately fit in this engine with its 80mm bore spacing.

From the flow friction perspective, I don't see any evidence that one would need a step up in size at the first Y when combining 360-degree separated pulses. The only reason for changing the pipe sizes there at that first 2-1 are related to some sort of resonant tuning, I believe. According to the simulator, even the final 2-1 collector outlet/choke of a four cylinder engine can be really small compared to the stepped up primary pipes.

More generally, the 180-degree pulse separation of a four cylinder engine seems to like a small final choke without any penalty at peak power rpm. Something like in this photo that has 2.125" choke after merging four 2" primaries that start at 1.75" and then step up to that 2". That's only an about 13% flow area increase from combining four pipes:

[modok]

I think that is all plausible and could prove to be true. It's not always true.
The fact that you don't see such systems winning any races means it is "a road less traveled" but still possible, restricted class exhaust systems just ARE weird.

I remember one case I investigated with simulation was kind of a "truck" engine, a conventional header was tried and it wasn't great.....
Usually what a header does is provide strong negative pressure over the valve overlap period, at the expense of some pumping loss...and it was doing that, but at the top of the rpm range this engine did not have enough valve overlap to take advantage of it. The port pressure was nice and low at TDC but at that point in rpms the valve was already in critical flow, so, it just didn't work.
So no matter the size of the port....the valve overlap was not enough to let the header work at high rpms....so instead I guess what was needed was some header that boosted mid rpms and yet provided minimal pumping loss at high rpms. And maybe that could be done a few different ways but in any case into WEIRD territory. Super long tubes dancing a fancy dance, or tapers which bounce in a certain way back and forth with the cylinder.


Most of my OWN quests had the restrictions of... the muffler is part of the system, and I am limited on how short the primaries can be, but valve timing is open. So that colors my experience. But whatever this is.... is coloring yours too.
So the way harmonics don't seem to be a factor is either due to system being naturally too short for it to matter, or because you haven't made it long enough to see yet. In most exhaust systems..... harmonics are a huge factor, and elephant you have to work with or work around.

[ptuomov]

Thinking of restrictions, here’s the following idea for four cylinder engines. Take the engine bore spacing and multiply it with the redline rpm. Lower the number the better suited a 360-degree 4-2-1 header is. Higher the number, the better suited a 4-1 system is. Since I’m working with a 80mm bore so sing and 9300rpm electronically limited engine speed, it’s fertile ground for a 4-2-1 system, in my opinion.

[n2omike]

4-2-1 and 'choke' collectors both work on the same principal... and some people overthink or underthink it.
They both remove excess 'dead' area from the exhaust.

4-2-1 relies on firing order. They pair cylinders that fire as far away from each other in the firing order as possible. That way both pulses don't try and fit through the merge at the same time. They trade using the same area.

A collector with a choke has pulses going through one at a time with no excess area for flow speed to decrease.

Pick your poison. lol

Both Hooker and Flowmaster sold the 4-2-1 collectors at one time. The Hooker units were actually pretty nice. Since they installed on existing headers, you had to pay attention to the firing order in order to install them as efficiently as possible and avoid roadblocks by not merging adjacent firing cylinders.

[hoffman900]

After playing with a four cylinder simulation with independent zoomies, dual 360pdegree 4-2 system, and 360-degree 4-2-1 system, I've come to the following tentative conclusions (thinking out loud here):

When the primary diameter off the head is the limiting factor on power at peak power rpm, the zoomies seem to be not too far of from the best header system for peak power. They have all sort of torque holes at lower rpms, however.

In the 4-2 dual system and 4-2-1 system and below peak power rpm, the primary lengths (and diameters) have to be such that the side branch resonator return pulse from the other cylinder's closed exhaust valve arrives in this cylinder's exhaust port either before the intake valve has opened or after the exhaust valve has closed. So either short primaries or long primaries work, but there's a middle length that sometimes doesn't work very well, at least not at rpms below peak power. So far, the simulator is liking the short primaries, and those fortunately fit in this engine with its 80mm bore spacing.

From the flow friction perspective, I don't see any evidence that one would need a step up in size at the first Y when combining 360-degree separated pulses. The only reason for changing the pipe sizes there at that first 2-1 are related to some sort of resonant tuning, I believe. According to the simulator, even the final 2-1 collector outlet/choke of a four cylinder engine can be really small compared to the stepped up primary pipes.

More generally, the 180-degree pulse separation of a four cylinder engine seems to like a small final choke without any penalty at peak power rpm. Something like in this photo that has 2.125" choke after merging four 2" primaries that start at 1.75" and then step up to that 2". That's only an about 13% flow area increase from combining four pipes:

My sims agree with this as well and looking at Superbike exhausts from outfits like Akrapovic (who has OEM race team contracts) as well as the last NA F1 engines, seems to jive with some of this as well. Take a look at some of the exhausts of the V8 era cars, and you’ll see some of them have collectors the size, or very close to that, of their final primary diameter.

The lack of it on cars tells me that I don’t think this is something many engine builders have ventured down. Be it from pre conceived biases, lack of time / money (most race efforts seem to get to like 90% before running out of time and the rest of the budget goes towards racing the car), or whatever else. John at Hytech has been playing with a really small choked collector lately, even on bent crank V8’s. Here is what he is doing: https://www.instagram.com/p/B7_ogqFH6xW ... =copy_link

With most of the market being cornered by a few, most are just relying on whatever Burns or a competitor spits out number wise and never moves beyond that. Then the majority just copy what they see.

My hunch is there is to be something to learn from what Calvin is doing, and taking it one-step further with a collector design that isn’t Burns, but it might require a 3D printed or cast collector to control the internal collector geometry, which the F1 teams were doing. All of this would be expensive and performance to dollar ratio is likely better elsewhere on the car.

This all gets back to what Calvin and Darin Morgan have been hinting at here for 15 years now, most exhaust ports are too big. Look at a Ford Coyote exhaust port and what it is capable of vs. a Cosworth BD_ exhaust port. The latter are designed much larger, and consequently most tuners start off with huge diameter primaries because that’s what the port is.

[ptuomov]

“Take a look at some of the exhausts of the V8 era cars, and you’ll see some of them have collectors the size, or very close to that, of their final primary diameter.”

I did take a look at those, and it’s a good a good idea to do so as the 2.4L flat plane V8 exhausts are just like two separate 1.2L four bangers.

The characteristics of those exhausts are massive steps in the primary, very steep merge angles in the 4-1 collector, and collector outlets not much larger than the primary ends.

I know of one company that replicated something like that for a regular four cylinder car engine and results seem to be blowing the commercially available competing products out of the water:

https://www.boefab.com/blogs/tech/84476 ... r-shootout

We have a winner!

Everything mentioned above about the construction can be said about these older style ForcedFed spec headers. They are a work of art! A bit different than the YYY variant, the step on these 4-1 headers is more pronounced and jumps to a full two inches following the 1.75” port matched primary. Immediately, we see cary over technology from the F1 header above, e.g. a large step in the primaries and a very steep merge. These headers also contained a choke after the merge to reduce the merge volume, which should result in a more pronounced sonic reflection to augment that second low-pressure wave.

DMC tried many iterations of this header before finally settled on the surprisingly small 2 1/8” choke. Yes, 2 1/8 inch! That is as visually stunning as it is to write it. The choke appears that it must be restrictive. However, the dyno shows otherwise when compared to the others.
We did try two versions of this header. One retaining the 2 1/8” choke and the other with a 2 3/4” choke but located slightly farther up the merge, which turned out not to be a productive modification. This modification made the midrange slightly softer, which is likely due to a softer sonic reflection from the modified merge.

One very interesting bit of data about the unmodified DMC ForcedFed Spec 4-1 header is that it was the only one to be very sensitive to the cam change event at 6000rpm. All the other headers did not register much of a torque dip at the cam change. On the other hand this header (which seems to be making the most of the exhaust signal) had a significant torque dip at the cam change. While we’re quite certain that we can tune this dip out with the cam phasing, it is interesting to see how sensitive a header that is properly making use of both exhaust mass and sonic reflections to generate power can be to cam phasing when compared to the others.

The unmodified ForcedFed Spec DMC header made 355whp and 227wtq while the modified version averaged about 1whp and 1wtq more. Hardly worth the effort! More interesting is that at 7000rpm this header made 9whp and 7wtq more than the next place header. The dyno results below exhibit a much more advantageous torque curve than all the others tested! Needless to say, we are going to work hard to convince DMC to bring these back to the market for us to enjoy again!