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

[ptuomov]

Another observation about “chokes”. There’s some relatively recent research on combining interfering pulses in one of those earlier referenced SAE papers. The problem is combining two sizes with interfering pulses. Their solution is to choke the pipe BEFORE the problem collector and then have an expansion, not venturi, in the collector where the pipes merge with zero angle. Like in the LS7 four-in-a-row collector, but the primaries necking down a little bit at their ends before they open into the plenum-style collector. Very counterintuitive, if you don’t consider pulse interference.

The only way I can make any sense of this is that the pulse interference is a much bigger problem with very short primaries that you can and should compromise on everything else to mitigate that problem. But I’m not sure, of course.

[swampbuggy]

Calvin, can not thank you enough for sharing SO much of your knowledge!! Thanks Mark H.

[ptuomov]

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?

Port and primary are 40mm or 1.577” and turbine housing inlet is 50mm or 1.967”. So that’s expected 500hp worth of exhaust flow going thru a 2” pipe. When multiple cylinders primaries combine, the size is in between of the two. Inside the turbine, it gets more complicated, but I trust the turbine designer (those guys are not dummies) getting it right.

—-

LS7 style collector would go up 80mm or 3.15” if fed with four 40mm or 1.577” primaries, which is contrary to all header design rules I’ve ever read on speedtalk! Those LS7 manifolds have a very high per-unit cost as they are hydroformed, so I’m guessing everything in them is as the designer intended and per the histories I’ve everything in the LS7 project was done to get the engine over 500hp mark. But why?

[ptuomov]

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.

I feel (let’s make it known that this is not me knowing anything) that in my exhaust the supersonic blast waves are more relevant for muffler and turboback exhaust design for nice sound than to pre-turbo exhaust manifold design and making power. This car will have pre-mufflers in the test pipes that are intended to quiet down the supersonic blast waves escaping thru the wastegate circuit. I’ve got a thread on that (“making a muffler”) where I contradict myself every five pages and generally go in circles. People who use professional software at OEM exhaust component suppliers would likely find that thread amusing and full of unintentional comedy.

I think that this pulse interference issue is conceptually pretty simple. Exhaust valve for one cylinder opens an blows a violent high pressure wave. That wave goes everywhere that is geometrically open when the wave gets there, including other cylinders’ combustion chambers if their exhaust valves are open. If this happens at valve overlap, it’s a tragedy. With very short primaries, there aren’t many good and obvious solutions.

[ptuomov]

So now Holley/Hooker is also making those LS7 style shoorties! Look at the collector, it’s like a plenum! Log manifold lessons indeed...

BBC340D9-E94D-4A5F-9BA3-B40977FD879B.jpeg

Also, look at the factory cast LT1 exhaust manifolds from 2014. The collector is again a plenum, a cast iron version of the LS7/LS9 manifolds. They’ve clearly decided at Chevy that this is the way to do a shorty header.

[exhausted]

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?

ok, 1.6ID primary will be pushing it to make 1000/8. I know a couple 410 Sprint Car engines with 1.75" OD off the head that may approach that but 188OD is normal for me on 400inch PS engines. (1100+). They step 3 times in 18inches though...

Well there are a lot of caveats like I said. The LS system is designed with tailpipes, Cats, Mufflers etc. I would ask if they looked at some kind of a choke but mass-producing a nice venturi is touchy? and short of that a questionable endeavor. The Cats probably do a good bit of atmosphere protection? I would also ask if they tried any kind of a AR device at the junction of the 4 tubes. Yes they probably left some torque on the table.

[exhausted]

At some point here the observation is that a "plenum" on the exhaust side of a engine after a bit of "blowdown Length" does the same thing a plenum on the intake side...It dampens and attenuates pressure waves...What a AR device would look like in thos LS aftermarket manifolds would be the same as shearplates in intake manifolds...

We are getting close here to my talk about IR intake engines needing a exhaust collector volume, (Plenum). I know that that plenum can then be choked before atmosphere, ...Cats?

[ptuomov]

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?

ok, 1.6ID primary will be pushing it to make 1000/8. I know a couple 410 Sprint Car engines with 1.75" OD off the head that may approach that but 188OD is normal for me on 400inch PS engines. (1100+). They step 3 times in 18inches though...

Well there are a lot of caveats like I said. The LS system is designed with tailpipes, Cats, Mufflers etc. I would ask if they looked at some kind of a choke but mass-producing a nice venturi is touchy? and short of that a questionable endeavor. The Cats probably do a good bit of atmosphere protection? I would also ask if they tried any kind of a AR device at the junction of the 4 tubes. Yes they probably left some torque on the table.

The exhaust manifold is running at 2x-3x atmospheric pressure (cycle average), so it’ll all fit they there quite nicely. Just saying that it’s probably not grossly oversized!

The LS7/LS9 exhaust is hydroformed and incredibly expensive to make (by oem standards). This makes me believe that they got exactly what they wanted, as hydroforming can deliver whatever shape. This is what they wanted.

The cats on that Corvette flow a ton and the downpipes from that “plenum header” are dual 3” pipes. The mufflers I believe have valves to straight pipe the exhaust out when need arises. My guess is that the exhaust system has very little restriction in that car.

[ptuomov]

I get the sense that the need to place the cats (or the turbo) very close to the exhaust ports has produced some genuine innovation on the exhaust side over the last 15 years. In particular, for the cross plane V8.

[exhausted]

I get the sense that the need to place the cats (or the turbo) very close to the exhaust ports has produced some genuine innovation on the exhaust side over the last 15 years. In particular, for the cross plane V8.

Interesting to be sure.
huh, I often wonder what air intake temp the factory uses for their HP declarations...

[ptuomov]

I get the sense that the need to place the cats (or the turbo) very close to the exhaust ports has produced some genuine innovation on the exhaust side over the last 15 years. In particular, for the cross plane V8.

Interesting to be sure.
huh, I often wonder what air intake temp the factory uses for their HP declarations...

LS7 Z06 Corvette was the first GM car, I think, that was independently rated for power by an outside test facility per SAE standard. The story goes the 505hp placard was the last part designed and made for that engine.

[MadBill]

..
Interesting to be sure.
huh, I often wonder what air intake temp the factory uses for their HP declarations...

I believe it is now 77°F. The current test and engine specs. are very closely specified; when the current standard came in some years back, certain engines 'lost' as much as 20 HP, others gained nearly the same...

[modok]

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?

Port and primary are 40mm or 1.577” and turbine housing inlet is 50mm or 1.967”.

By inlet you mean.... at the flange? I was asking about inside the turbo housing.

[modok]

At some point here the observation is that a "plenum" on the exhaust side of a engine after a bit of "blowdown Length" does the same thing a plenum on the intake side...It dampens and attenuates pressure waves...What a AR device would look like in thos LS aftermarket manifolds would be the same as shearplates in intake manifolds...

We are getting close here to my talk about IR intake engines needing a exhaust collector volume, (Plenum). I know that that plenum can then be choked before atmosphere, ...Cats?

There may be a situation that exists where the turbo can be placed close to the head and far from the head and work ok, but in the middle will work worse. I found a similar case with the intake side with a turbo or blower

The turbine is probably functioning both as a centrifugal turbine and ALSO as a simple impulse turbine.
It does seem most here are aware of how unsteady the exhaust gas flow actually is, with very high flows during the peak of blowdown, and also some periods with very little action.
Energy being velocity squared, it may be seen that averaging the flow would result in less energy. A header that was had enough volume to make the flow going into the turbo more steady would actually reduce the available energy to run the turbine, but the turbines ARE tested at steady flows, and likely more efficient at steady flows, so we make it steady and get the turbo efficient at that flow it might make up for the loss. Both ways can work. As an impulse turbine you want to minimize volume and velocity loss from the exhaust port to the turbo.
Assuming the turbine is a steady state flow device you'd want constant steady flow into the turbo.
It may end up in a state where you want as short/tight as possible to preserve energy, OR place the turbo where the choke would be best placed in a conventional header.... so the header functions to efficiently couple unsteady flow INTO steady flow for the turbine. In the middle between those two choices could be worse than either, if you reduce the peaks of flow but still don't approach steady flow nor good pulse timing for the engine. I have not heard any discussion of this.

[exhausted]

At some point here the observation is that a "plenum" on the exhaust side of a engine after a bit of "blowdown Length" does the same thing a plenum on the intake side...It dampens and attenuates pressure waves...What a AR device would look like in thos LS aftermarket manifolds would be the same as shearplates in intake manifolds...

We are getting close here to my talk about IR intake engines needing a exhaust collector volume, (Plenum). I know that that plenum can then be choked before atmosphere, ...Cats?

There may be a situation that exists where the turbo can be placed close to the head and far from the head and work ok, but in the middle will work worse. I found a similar case with the intake side with a turbo or blower

The turbine is probably functioning both as a centrifugal turbine and ALSO as a simple impulse turbine.
It does seem most here are aware of how unsteady the exhaust gas flow actually is, with very high flows during the peak of blowdown, and also some periods with very little action.
Energy being velocity squared, it may be seen that averaging the flow would result in less energy. A header that was had enough volume to make the flow going into the turbo more steady would actually reduce the available energy to run the turbine, but the turbines ARE tested at steady flows, and likely more efficient at steady flows, so we make it steady and get the turbo efficient at that flow it might make up for the loss. Both ways can work. As an impulse turbine you want to minimize volume and velocity loss from the exhaust port to the turbo.
Assuming the turbine is a steady state flow device you'd want constant steady flow into the turbo.
It may end up in a state where you want as short/tight as possible to preserve energy, OR place the turbo where the choke would be best placed in a conventional header.... so the header functions to efficiently couple unsteady flow INTO steady flow for the turbine. In the middle between those two choices could be worse than either, if you reduce the peaks of flow but still don't approach steady flow nor good pulse timing for the engine. I have not heard any discussion of this.

Ok, makes sense but who the hell is going to make two completely different setups to determine any difference much less measure it, A dyno can measure static load power but drivablility even for racing is what the real issue here is I think which means you have to build it. Some want to build it in a computer. Is that actually easier>\? takes one hell of a computer and software to do that...anyway, alas, we are stuck guessing.
The panacea of boredom and birth of "hey, watch this...

Anyway this whole thing is about controlling a very rapidly heated MASS that has been let out of a confined space and has created pressure waves as well. We can attenuate the pressure waves but until all the heat energy has been dissipated through the various mechanisms the question is how much of the available energy have we harnessed to augment the engines performance.

The folks who are only concerned about pressure wave tuning wind up with very large tubes because they are not trying to conserve momentum energy or heat energy. And they also find out that because they did the former they have to contain the pressure wave in the tubes for a long time in order to protect the other cylinders in the manifold from the positive waves going the wrong way at the lower rpms of the desired band. So when I look at a common conventional header, I shake my head. A ubiquitous testimony to the simpler task of building intake manifolds.
I built my first set of Cup headers in a race shop. Spent a lot of time in the engine and dyno rooms. I have said before but I can remember all the intake manifolds hanging on walls and in engine carts, discarded, cut up etc but there was only one dirty old set of headers in the dyno room usually under the bench. A loud statement of something, i"ll leave that to you.

Pressure wave tuning is obviously real but there remains is a lot more we can do.IMO (Sunday mornings can get a bit dour, sorry.) I appreciate speed talk forum. I hope my musings and time can help other folks who have the energy and desire to push forward in exhaust systems can gain, from the things I have seen and done, a more complete understanding of exhaust systems.