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

[user-23911]

It's the waves that are the energy which is passed on to the turbine.
Doubling the velocity quadruples the energy therefor most of the energy is in the peaks not the average.

[ptuomov]

http://papers.sae.org/2012-01-1170/

The paper in the above link describes the exhaust manifolds of problem of a 90-degree crossplane V8 very well. The solution, however, is not very innovative. They recommend longer tube 4-1 headers. I think you don’t really need all the software and sensor to guess that long tube 4-1 headers solve exhaust interference problems.

[ptuomov]

Coyote 5.0L exhaust manifold is one example of innovative design that picked up power despite of being very compact.

I think LS7 factory manifold may be another example.

http://www.corvetteactioncenter.com/spe ... ine16.html

The Final Push

As the LS7 development closed on deadlines forced by GM's intent to debut the engine in the 2006 Z06, the engine's power output had plateaued somewhat below the 500-hp goal set by that pesky senior management.

"For a while it looked like 480 was going to be the number," Corvette Chief, Tadge Juechter recalled in a Spring 2012 interview with the CAC, "but then, they pulled the rabbit out of the hat at the end. This was on the eve of production! I'm talking about the end of '04. We're already ordering material to build our nonsaleable validation vehicles at the plant so, we're holding up the presses at that point, trying to decide--'Ok, guys, what is the horsepower?' and remember, it was, also, the first engine in the auto industry certified to (the then-new) SAE standard. That was another level of rigor that we had to go though. The very last part released for production was the Z06 badge which said '505-hp'. In fact, we were thinking we'd have to go to production without any number on there because we didn't know what it was going to be."

Tadge Juechter had plenty to smile about when we met with him for this article in the Spring of 2012. He knew the C7 was going to be "big" and he'd just introduced the LS7-powered, 2013 427 Convertible for the C6's final model year.

In this final six-months or so, the first of two key advances, which came late in the program, was 8-10-hp gained with the sophisticated exhaust manifold design discussed earlier. This manifold was costly for a production engine. When we talked with Bill Nichols, the Corvette Powertrain Integration Manager, he told us how this part came about. "We weren't quite there yet. I went to the exhaust manifold engineer and said, 'Do you have any ideas? Is there anything more we can do?' He said, 'I've been working on an idea for a while, let me get back to you.' A week or so later he called me saying, 'We've got some manifolds here, we think, by analysis, are going to do the trick.'"

A key design feature contributing to the 9-hp gain from the LS7 exhaust manifold is this "four-in-a-row" collector.

"There were challenges," Nichols continued. "We had a port that was pretty big. We still had to do all the right mixing to be able to measure pre-converter oxygen. We had to fit them to a catalytic converter that didn't have a lot of packaging space to move in. They came up with a design which could work, but it was not cheap. It's a significant ingredient of the cost of that engine. The carrot was, a significant improvement in flow and restriction. Those manifolds ended up being worth about nine horsepower."

Corvette Powertrain guru, Bill Nichols' story about how the exhaust manifold was part of GMs last ditch efforts to get 500-hp out of the LS7 is the stuff which adds to the engine's mystique. The same manifold was used three years later on the LS9.

So my question to you is why does the LS7 manifold work so well? Is it because of the all four primary pipes blowing in the same direction?

[cjperformance]

Coyote 5.0L exhaust manifold is one example of innovative design that picked up power despite of being very compact.

I think LS7 factory manifold may be another example.

http://www.corvetteactioncenter.com/spe ... ine16.html

The Final Push

As the LS7 development closed on deadlines forced by GM's intent to debut the engine in the 2006 Z06, the engine's power output had plateaued somewhat below the 500-hp goal set by that pesky senior management.

"For a while it looked like 480 was going to be the number," Corvette Chief, Tadge Juechter recalled in a Spring 2012 interview with the CAC, "but then, they pulled the rabbit out of the hat at the end. This was on the eve of production! I'm talking about the end of '04. We're already ordering material to build our nonsaleable validation vehicles at the plant so, we're holding up the presses at that point, trying to decide--'Ok, guys, what is the horsepower?' and remember, it was, also, the first engine in the auto industry certified to (the then-new) SAE standard. That was another level of rigor that we had to go though. The very last part released for production was the Z06 badge which said '505-hp'. In fact, we were thinking we'd have to go to production without any number on there because we didn't know what it was going to be."

Tadge Juechter had plenty to smile about when we met with him for this article in the Spring of 2012. He knew the C7 was going to be "big" and he'd just introduced the LS7-powered, 2013 427 Convertible for the C6's final model year.

In this final six-months or so, the first of two key advances, which came late in the program, was 8-10-hp gained with the sophisticated exhaust manifold design discussed earlier. This manifold was costly for a production engine. When we talked with Bill Nichols, the Corvette Powertrain Integration Manager, he told us how this part came about. "We weren't quite there yet. I went to the exhaust manifold engineer and said, 'Do you have any ideas? Is there anything more we can do?' He said, 'I've been working on an idea for a while, let me get back to you.' A week or so later he called me saying, 'We've got some manifolds here, we think, by analysis, are going to do the trick.'"

A key design feature contributing to the 9-hp gain from the LS7 exhaust manifold is this "four-in-a-row" collector.

"There were challenges," Nichols continued. "We had a port that was pretty big. We still had to do all the right mixing to be able to measure pre-converter oxygen. We had to fit them to a catalytic converter that didn't have a lot of packaging space to move in. They came up with a design which could work, but it was not cheap. It's a significant ingredient of the cost of that engine. The carrot was, a significant improvement in flow and restriction. Those manifolds ended up being worth about nine horsepower."

Corvette Powertrain guru, Bill Nichols' story about how the exhaust manifold was part of GMs last ditch efforts to get 500-hp out of the LS7 is the stuff which adds to the engine's mystique. The same manifold was used three years later on the LS9.

So my question to you is why does the LS7 manifold work so well? Is it because of the all four primary pipes blowing in the same direction?

What does the LS7 exhaust look like downstream on the manifolds?

[StockerChevy]

re...

Unread post by StockerChevy » Thu Nov 16, 2017 1:42 pm

Some of the FAST racers under cam a big compression motor with wider LSA and retard the camshaft a considerable amount. You would be surprised at the outputs and rpm band the motor will work at. The intake valve has to flow well at low lifts. There are other tricks as well but a big cubed Hemi racer runs 840 horsepower through dual exhaust 2.5 inch pipes through decent exhaust manifolds.

Jim

I went a similar route as this to some degree recently in a na application. Ended up there by way of the cam drill. Have to agree it was a bit surprising.. At times this sort of n/a combo almost reminds you of being in a turbo car. Great power band. So I got to wondering about how to incorporate a turbo to expand on this. I suppose its just cam events in relation to compression and rpm that changed but reversion issues all but disappeared along with and it just seemed to have potential for more.. I was using a wider lsa cam with same in and ex duration. 112 icl. Maybe StokerChevy has some ideas on how to apply this sort of thing to your situation. He did offer this up in regard your post.

I would run a wider LSA or run smaller exhaust duration if more compression is off the table. I would rather have a less exhaust evacuated cylinder than one with reversion taking place. You might want to consider just doing this on cylinders that you think are exposed to the reversion if you think the performance is compromised too much.

[cjperformance]

Being a DOHC setup (i think im correct there) , does your cam drive and piston to valve clearance etc allow you enough adjustment to play with lsa a lot?

[gottago]

How is everyone measuring or diagnosing reversion?

[modok]

If half or more cylinders are doing it at he same time can tell from the signs, can tell from the AFR probably rich, and more required advance, the shape of the torque curve, and even the sound. Swap headers and see what happens, what changed, what moved?
In the case of the v8 I don't know the true nature of the problem.
No data, but, Reasonable suspicion?? Something is happening there but I don't know how much when. Might be good thing to play with in a SIM

[user-23911]

How is everyone measuring or diagnosing reversion?

As per normal.....guesswork.

can tell from the AFR probably rich,

Lean


There's exhaust reversion, there's also intake reversion.
With exhaust reversion with pipes too short, air will enter the exhaust exit, flow backwards to the intake manifold on overlap.......AFR becomes too lean.You lose idle vacuum.
Intake reversion, the intake is too short, you'll spit fuel and air out of the throttle.

[user-23911]

With exhaust reversion with pipes too short, air will enter the exhaust exit, flow backwards to the intake manifold on overlap.......AFR becomes too lean.You lose idle vacuum.

That's when the overall length is too short too.
Otherwise when air can't flow backwards, it's just exhaust that flows backwards and you get internal EGR.
You still lose vacuum and need to open the throttle a bit more to get more air and fuel in.That loses more vacuum.

[modok]

At idle that's probably true, I was more interested in what happens with the throttle open.

[joespanova]

How is everyone measuring or diagnosing reversion?

As per normal.....guesswork.

can tell from the AFR probably rich,

Lean


There's exhaust reversion, there's also intake reversion.
With exhaust reversion with pipes too short, air will enter the exhaust exit, flow backwards to the intake manifold on overlap.......AFR becomes too lean.You lose idle vacuum.
Intake reversion, the intake is too short, you'll spit fuel and air out of the throttle.

So , based on this I should lengthen the primaries on my headers , because I have reversion in the intake ports. Or , change camshaft to one with wider lobe centers.

[amcenthusiast]

Reading through this thread this morning give me a 'good thought for the day' -something to ponder, so I'd like to say thanks to all the contributors -thanks for the brain food everybody.
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My mind's been locked on to understand Bernoulli's Principal more and more lately, which I currently believe has more to do with induction, gas flow through the valves and exhaust system design than is ordinarily considered by most mainstream 'high performance' technical writers.

For this reason, I find more satisfactory information on the subject, on the websites that teach 'the theory': Physics and Thermodynamics websites.

One thing I've noticed, that I find disturbing is an apparent lack of regard for using the appropriate terminology for the topic under discussion.

Eg: How to upgrade oil system? < That's a trick question for it's inappropriate terminology; the term 'upgrade' is not hydraulic system jargon. This is like using the word 'gallop' in a discussion about human anatomy (humans don't 'gallop' anymore than we replace outdated programs, memory boards and transistor circuits on the inner working parts of an engine's oiling system)

It's not about 'hairsplitting', but it's about 'speaking the correct language'.

-When we speak the correct language, now we are able to discuss the topic with more accuracy.

I read a quote last night on the Georgia State University 'HyperPhysics' website that reinforces the concept of using the correct terminology:

"Thermodynamics is like a song, when we make a change to say it simple we make it wrong."

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.htm
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Are we not talking about thermodynamic principals in order to discuss the original question asked here?

Exhaust heat energy transfer is not applicable to this topic?

I think it is!
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From another perspective, consider the definition of 'sophistry':

"A subtle, tricky, superficially plausible, but generally fallacious method of reasoning."

Are we not grown weary by so many 'superficially plausible' methods of reasoning?

Hence, sophistry is evidenced by using 'superficial' terminology.

Wherefore, I believe this exhaust system oriented question can become better if we move towards using the appropriate thermodynamic terminology.
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I would argue there is reason to believe Bernoulli's Principal is in operation on the exhaust side because of the gas flow moving through the constriction of the 'valve curtain' area.

-because the valve curtain area has an annular venturi shape.

One of the more commonly misunderstood component parts of why Bernoulli's Principal works is the introduction of potential elastic energy into the kinetic energy of the air column; due the compressible nature of a gas, a vacuum is formed by a continuous flow through the venturi shape.

Hence, energy is added to the molecular movement through the venturi by harnessing the potential energy of the gas' elasticity; how the gas stretches through the venturi shape imparts 'work' energy into the system -molecular momentum then begins to pull the air column through the venturi; to initiate 'suction' by using the elastic potential energy of the gas.

...how to say it right! ...I'm not a trained Thermodynamics technical writer; just enjoying the knowledge as a very curious observer!

So I think, Bernoulli's Principal for a gas passing through a venturi is applicable to this discussion.
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Could the exhaust manifold design benefit by the addition of a venturi shape immediately before or immediately after the the turbine housing area?

When we analyze the known scientific phenomena pertaining to the air column dynamics before and after the venturi device, how would that interplay with the molecular energy introduced by the turbine blades?

Should attempts to straighten molecular flow occur before or after the turbine... or where would it be beneficial to introduce venturi vacuum (to use potential elastic energy to help reduce 'reversion')
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There is definitely more 'heat' related energy transfer to consider on the exhaust side!
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There is definitely more 'constructive and destructive interference' sonic wave activity up for consideration as well: how does the interplay of a venturi shape affect constructive and destructive sonic wave activity?
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An added on tube shaped chamber could be used to affect constructive or destructive interference?

-Physics of sound wave dynamics in tube...

[ptuomov]

What does the LS7 exhaust look like downstream on the manifolds?

It dumps into a cat. The cat is a bit of a restriction always, so in my "close enough for government work" thinking, it's a little like a large turbine!

[ptuomov]

I would run a wider LSA or run smaller exhaust duration if more compression is off the table. I would rather have a less exhaust evacuated cylinder than one with reversion taking place. You might want to consider just doing this on cylinders that you think are exposed to the reversion if you think the performance is compromised too much.

Given that I'm running on pump gas and with low compression, I really want to empty that combustion chamber from hot exhaust gasses.

I tend to agree on the overlap. If the gas flow goes in the wrong direction, shut the valve! Filling the intake runner with hot exhaust gas isn't going to help make things better. It's going to make things worse.

In terms of the shorter exhaust duration, in my opinion, more complicated. Definitely helps at low rpms to reduce the 180-degree blowdown interference. Now so sure at high rpms when the problem is 90-degree exhaust blowdown interference. I guess it depends on whether the exhaust manifold design strategy is to separate the 90-degree pulses (like in the LS7 manifold) or to lay them on top of each other (as on one side of the Coyote 5.0L manifold).

Why does the LS7 manifold work so well? is it because the four-in-a-row collector aims all the short primaries in the same direction? That's the sort of effect that the 1D software (Vannik's) I use isn't really accurate with, you'd need some sort of 3D software to model that. Why isn't the LS7 manifold killed in the top end by the 90-degree exhaust blowdown interference?

This is another "too short" cross-plane V8 exhaust manifold that works really well on pump gas, yet simple 1D simulation would say it would be really hurt by the 90-degree exhaust blowdown interference:



It has the same feature as the LS7 manifold, namely that the primaries are pointed in the same direction before the paths merge. Maybe that's a clue?