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How to fight exhaust reversion when the primaries are way too short

General engine tech -- Drag Racing to Circle Track

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Re: How to fight exhaust reversion when the primaries are way too short

Post by cjperformance » Wed Nov 22, 2017 8:11 pm

Does your sim allow for manifold shape or just a generic firing pattern? (You may have already saud this?)
Do you have a pic of the actual exhaust manifold you are using? Are you limited by space or rules?
Do you know what firing order/pattern koenigsegg are using? It looks like with thier unequal length exhaust manifold (also small diameter for HP they make) that the cyl that fires 90* after the leading cylinder has the short pipe therefore the quickest path to a large volume (velocity & pulse degredation , as i suggested i
on your generic hooker manifold pic in using a small pipe for #1 & a large short dump for #3).
I agree that their pressure ratio I to E is not going to solve everything but that with a small uni directional manifold 'possibly'(till i know FO/Pattern) using the short pipe for the trailing cylinder is AT LEAST enough to mask the 90 deg issue to the point where they can tune to high HP with reliability. Im sure they will be using seperate ignition, fuel mapping/timing and possibly valve timing for each cylinder.
Be interesting to see what intake runner length Koenigsegg use, are they using a runner length that is short enough to allow any exhaust gas reversion to share plenum space so as to mix and share around surrounding cylinders to reduce egr effect in a given cylinder or a runner that will contain any exhaust reversion and tune just the affected cylinders to suit. Also i wonder if they had tested quad turbos, im sure they would have, possibly saving that 'up the sleeve' for later!
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Re: How to fight exhaust reversion when the primaries are way too short

Post by ptuomov » Wed Nov 22, 2017 8:12 pm

modok wrote:
Wed Nov 22, 2017 8:08 pm
yeah man, not the whole cam just that lobe. is that not possible using the sim?
Not yet.

But with this manifold geometry, you don’t want to advanced #1. You want to retard #1. You also want to advance #3 so it blows its wad into the #1’s exhaust port and cylinder before #1’s intake valve opens.

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Re: How to fight exhaust reversion when the primaries are way too short

Post by modok » Wed Nov 22, 2017 8:13 pm

I don't know which number, but I think you are right. Of the "90 apart pair" certainly advance the exhaust opening of the one and retard the intake opening of the other. This gives more TIME for the pressure to go down before opening the intake valve of the suffering cylinder.
Last edited by modok on Wed Nov 22, 2017 8:32 pm, edited 1 time in total.

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Re: How to fight exhaust reversion when the primaries are way too short

Post by Cubic_Cleveland » Wed Nov 22, 2017 8:17 pm

ptuomov wrote:
Wed Nov 22, 2017 7:57 am
Somehow that exhaust manifold with unequal length short primaries works very well even though the pulses are all wrong. I'm guessing that it's something related to the primaries exiting parallel at the collector, like in the LS7 "four-in-a-row" collector.
For sure there is more going on than I originally thought about. Does the first and fourth primary take such a longer tighter path entering into a merge style collector work far better? Or would those two pipes taking a shorter, ‘swoopier’, path and no merge collector work just a well but not fit for packaging reasons??

I’m thinking the merge collector works better, and the longer pipes are just what’s needed for the angle to work. But, maybe unequal lengths is a function not a byproduct. I really have no idea! :lol:

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Re: How to fight exhaust reversion when the primaries are way too short

Post by ptuomov » Wed Nov 22, 2017 8:47 pm

cjperformance wrote:
Wed Nov 22, 2017 8:11 pm
Does your sim allow for manifold shape or just a generic firing pattern? (You may have already saud this?)
Do you have a pic of the actual exhaust manifold you are using? Are you limited by space or rules?
Do you know what firing order/pattern koenigsegg are using? It looks like with thier unequal length exhaust manifold (also small diameter for HP they make) that the cyl that fires 90* after the leading cylinder has the short pipe therefore the quickest path to a large volume (velocity & pulse degredation , as i suggested i
on your generic hooker manifold pic in using a small pipe for #1 & a large short dump for #3).
I agree that their pressure ratio I to E is not going to solve everything but that with a small uni directional manifold 'possibly'(till i know FO/Pattern) using the short pipe for the trailing cylinder is AT LEAST enough to mask the 90 deg issue to the point where they can tune to high HP with reliability. Im sure they will be using seperate ignition, fuel mapping/timing and possibly valve timing for each cylinder.
Be interesting to see what intake runner length Koenigsegg use, are they using a runner length that is short enough to allow any exhaust gas reversion to share plenum space so as to mix and share around surrounding cylinders to reduce egr effect in a given cylinder or a runner that will contain any exhaust reversion and tune just the affected cylinders to suit. Also i wonder if they had tested quad turbos, im sure they would have, possibly saving that 'up the sleeve' for later!
Vannik can comment on the simulator. I don't fully understand it. My understanding is that there are some empirical approximate formulas of how pressure and flow behave at collectors/junctions. It's difficult to make very accurate by my understanding, and this is why I believe expensive commercial packages offer the option of painstakingly modeling one component in three dimensions while keeping the rest one dimensional.

The current exhaust manifold looks like this.

Outside:

Image

Inside (the casting sand core that gets broken and shaken out):

Image

I've played with the pipe diameters in the 1D software until cows come home and changing the diameters don't seem to be making much of a difference and none of it for good. The diameters that were cast into the manifold were thought out pretty carefully.

If the pipes are too short to achieve complete separation, minimizing the distance between the 90-degree apart firing cylinders is the second best option.

I don't think the pipe lengths in that Koenigsegg manifold achieve anything too sophisticated in terms of pulse tuning the interference issues, because they appear to have the same configuration on both sides. It has to be at that collector where they solve the interference problem.

Koenigsegg intake manifold runners are very straight and short. I doubt that at the peak power or peak torque they have exhaust going into the intake ports, let alone the intake plenum. With a 1000hp pump gas 5.0L motor that would be a death sentence. Somehow that motor is evacuating the combustion chambers well.

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Re: How to fight exhaust reversion when the primaries are way too short

Post by cjperformance » Wed Nov 22, 2017 9:25 pm

Nice casting and campactness of the manifold . I can guess you must be very tight for room.
Good note on the other side K-seg manifold, if it is indeed the same then yes that kills that firing pattern thought! Which brings me back to a combination of pressure ratio and valve timing to get such good cylinder evacuation.
Any time i have made (not cast, but fabricated) a turbo manifold i have always gone for tight short small diameter stuff with no regard to runner lengths, just get it to the turbo as quick and as hot as possible. Also typically space governs shape to the biggest extent.
Some live pressure tracing would be fantastic for this application (i know thats not an option for you).
Have you run any sims on your intake manifold/runner pressures?
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Re: How to fight exhaust reversion when the primaries are way too short

Post by amcenthusiast » Wed Nov 22, 2017 11:12 pm

Obviously the computer program is not a know-it-all like me!

Hey; I even know how to work cows!

There are more forms of energy in the exhaust gas vapors than kinetic energy (pressure)

Hence there are many other forms of potential energy than can be discussed.

Sonic, acoustic, elastic, heat...

Mechanical? Such as wrist pin offset with regard for piston motion around TDC?

Would a rod ratio change on the affected cylinder fight reversion for better or for worse?

If there was faster piston movement around TDC would that be better? Or would slower movement be better?

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Re: How to fight exhaust reversion when the primaries are way too short

Post by modok » Wed Nov 22, 2017 11:27 pm

Changing those parameters won't make a LOT of difference, some, but not much.

Well, I was meaning to mention at some point, on a similar issue I eventually decided that adjusting the compression ratio to suit the VE of each cylinder was more practical than trying to make the VE equal with a looney camshaft.

AND, if that's not strange enough, you could retard the intake closing point to suit the VE to match the CR, which brings you back again.

In this case tweaking the CR could be a double edged sword as well, as dropping the CR of the poorly scavenged cylinder will make it scavenge even worse, so that's not a promising direction, but if it being more knock prone is found to be a problem then you could intentionally reduce it's VE by a number of methods, and leave the CR the same or higher.

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Re: How to fight exhaust reversion when the primaries are way too short

Post by ptuomov » Thu Nov 23, 2017 6:53 am

amcenthusiast wrote:
Wed Nov 22, 2017 11:12 pm
Obviously the computer program is not a know-it-all like me! Hey; I even know how to work cows!
Is that what you're doing now in this thread?
amcenthusiast wrote:
Wed Nov 22, 2017 11:12 pm
There are more forms of energy in the exhaust gas vapors than kinetic energy (pressure). Hence there are many other forms of potential energy than can be discussed. Sonic, acoustic, elastic, heat... Mechanical?
The software does the total enthalpy calculation. That's pretty comprehensive, no?
amcenthusiast wrote:
Wed Nov 22, 2017 11:12 pm
Such as wrist pin offset with regard for piston motion around TDC? Would a rod ratio change on the affected cylinder fight reversion for better or for worse? If there was faster piston movement around TDC would that be better? Or would slower movement be better?
Koenigsegg designers think that this matters to some extent. They think their wrist pint offset, rod to stroke ratio, etc. help a little bit with the knock. The bulge in the Koenigsegg piston dish center is mostly to make room for the rod small end, by my guess. In any case, in the presence of unsolved exhaust blowdown interference issue, this is like rearranging the deck chairs on the Titanic.

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Re: How to fight exhaust reversion when the primaries are way too short

Post by ptuomov » Thu Nov 23, 2017 9:28 am

This is the cast iron version of the Coyote pulse combination manifold. Anyone run these on anything, I think they are for F-150 truck?
6214C3A7-53C6-4BD7-8ADD-F50CEBA3426B.jpeg
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Re: How to fight exhaust reversion when the primaries are way too short

Post by amcenthusiast » Thu Nov 23, 2017 10:26 am

Thanks for your sense of humor and please accept my apology for any negative or unhealthy negative criticism.

OK; there's an appropriate term but one that is commonly ignored: entropy; molecular disorder -first Law of Thermodynamics.

So a component part of exhaust manifold design would be attempting to manipulate molecular disorder (entropy) to increase system efficiency.

In my own defense, yes at first it may seem dumb (like a muffler bearing) to suggest magnetism because there is very little magnetic particulate matter in exhaust vapor, but a magnet could be an overlooked potential energy source to control or help re-establish molecular order in the exhaust system passagways. Here's a quote form Georgia State University's "Hyperphysics" website:

"In sound applications, a resonant frequency is a natural frequency of vibration determined by the physical parameters of the vibrating object. This same basic idea of physically determined natural frequencies applies throughout physics in mechanics, electricity and magnetism, and even throughout the realm of modern physics."

...that's where my small scale logic came from anyway...

May I still suggest how consideration for constructive and destructive interference may be helpful, where we know now that a sonic boom is caused by molecular friction (causing additional sound wave vibration) up to a point in time where re-constructed sound waves cause the dramatic 'boom' sound.

Since exhaust gas speeds approach, match or exceed speed of sound, there could be a similar re-constructed 'sonic boom' or 'bad' shock wave happening in the exhaust outlet passageway of the one cylinder showing reversion?

(reversion in one or two cylinders, and not all, may indicate undesirable sonic constructive interference which plausibly could be altered by changing the resonant frequency of those passageways?)

The molecular density of the selected material to make the exhaust manifold is also a factor. Eg: mild steel vs. stainless steel vs aluminum vs cast iron as this too will affect resonant frequency of the system.

Hey; at least I got you thinking? and thanks again for tolerating the' alternative" ideas.

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Re: How to fight exhaust reversion when the primaries are way too short

Post by exhausted » Fri Nov 24, 2017 11:52 pm

Wow, you fellows have beat it up pretty good. Lots of interesting thoughts about how to fix a common problem when there is no room.
A few thoughts.
Low rpm power is reduced by pressure wave interference and high rpm power is more related to particle flow restrictions. Its quite a mess.
I was concerned when someone suggests varying tube diameters or making them bigger to "slow" things down. The fellow who talked about Bernouli's principles is getting close. IMO
Having short headers makes it a lot easier to make power as the less time you spend flowing gases through tubing means less flow resistance and that is how to make power...It also means that you can use smaller tubing without losing power and the higher particle velocity helps fight reversion. This is where I lose a lot of people, it is hard to convince folks but smaller is better certainly in this discussion as nothing kills torque more than short big headers. Not having room to make tubes longer means you can and have to make them smaller diameter. It is not my fault that oem ports/valves are larger than they need to be and this can be a great detriment to getting headers smaller, the fix to make more torque is already compromised.Does anyone remember my talks about "blowdown length"? Its not my fault that ports are too large and velocities are slower in most engines. I will always push my customers if I can into smaller ports and valves then what they have, (there are caveats) Shorter headers need and can be smaller and valves and ports also and your torque will be better and still good up top.
Another factor to all this is Final choke diameters. For the same reasons it is good to have as high a velocity near the head at "blowdown", The higher the velocity that can be maintained through the final orifice to atmosphere is also used to draw the entire system down in advance of atmosphere. This is a flow issue, Bernouli depression stuff. There is another aspect to this that gets some peoples dander up but the final choke/venturi in a header is a orifice between the cylinders and the atmosphere. The smaller you can get the hole the longer it takes for atmospheric pressure to try and equalize at a given rpm. The smaller the hole the lower rpm that the header can protect the engine from atmospheric waves traveling back in.
A properly designed final collector venturi can flow the same amount of air through a smaller hole than a poorly designed collector and that smaller hole will have a larger depression which will lift both ends of ones power curve. The closer a final venturi is to the engine the more energy is available in the exhaust flow, (once again short tubes) and the smaller the hole can be yet still flowing enough to make power. This is also why Erland likes to get turbos as close to the engine as possible. :D

I will unveil a secret, (at least I have never mentioned it) If I have a header that has a longer than desired primary I will attempt to pair it with a shorter tube so the length of half the combined length is closer to the desired length. 23+14=37 and 18.5+18.5=37, the pressure wave from either one has to travel the same distance to the other paired cylinder. It also has the same volume if you want to talk about "chambers". This refers to 421 designs.

When you look at the LS headers, they are very smooth and I also suspect they are close in cross section area to the ports in the head, ie, flow well but not too big. If you go smaller diameter you can not violate flow rules. (Well you never should :) ) I've never seen the Coyote tubular headers but it sounded like they worked to keep diameters as small as possible and you always have to enlarge tube diameters when two tubes merge.
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Web: www.elstonheaders.com
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Re: How to fight exhaust reversion when the primaries are way too short

Post by modok » Sat Nov 25, 2017 12:50 am

If you can maintain a constant velocity of the particle stream from the port MCSA to the "choke" of the turbo, it can be very efficient. Slowing down the flow and speeding it up again can be more losses than the frictional losses of the high velocity. How big is the passageway in the turbine housing right before it's introduced to the blades? How does that compare to the port?

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Re: How to fight exhaust reversion when the primaries are way too short

Post by ptuomov » Sat Nov 25, 2017 9:36 am

In my case, the primaries are 40mm or 1.577” inside diameter and the plan is to push about 1000hp/8 thru each of them. Of course, pressure and density matter, too, but I don’t think the ports or primaries are currently large. They are the same size that the stock 320hpnornally aspirated 5L engine has from the factory.

Exhausted, the LS7 four-in-a-row collector that gave the last prescious +10hp for the design team to take the engine above 500hp has the crossectional area at the collector that is more than four times the cross sectional area of individual primary. It just like the four pipes would continue except the inner walls removed. No choke whatsoever and, given the pulsed nature of the flow, a significant slowdown in the mean gas velocity. Since then, many of Chevy’s cast manifolds have mimicked that expansion, the primaries seem to exhaust into a plenum. If I understood what you wrote above, this is exactly the opposite of what you are recommending. Did they leave hp on the table, or are the LS7 primaries so short (pulse interference problem not solved) that the usual header design wisdom doesn’t apply?

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Re: How to fight exhaust reversion when the primaries are way too short

Post by ptuomov » Sat Nov 25, 2017 9:45 am

“I've never seen the Coyote tubular headers but it sounded like they worked to keep diameters as small as possible and you always have to enlarge tube diameters when two tubes merge.”

The Coyote 4-2-1 header employs two tube sizes. The big tube looks like it’s one pipe size up from the small one. When the “simple” side merges the two cylinders that fire 90 degrees apart, they step up the pipe size for that secondary. Every other pipe in both sides is of the small size, the final exit of course being larger. The complex side does the conventional 4-2-1 pairing without any change in the pipe size between secondaries and primaries.

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