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

General engine tech -- Drag Racing to Circle Track

Moderator: Team

ptuomov
Guru
Guru
Posts: 1520
Joined: Fri Aug 07, 2009 3:52 am

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

Post by ptuomov » Mon Nov 13, 2017 9:06 am

I was going to post this in the "2017: Revisiting 4:2:1 vs 4:1 headers w merge collectors in the modern era" thread, but it's not exactly on topic and instead of polluting that thread, I am going to branch it off here. Also related to this: viewtopic.php?f=15&t=50905&start=60#p698517

Suppose that one has a very short header or exhaust manifold. Like a cast exhaust manifold for a FAST car, or a shorty header for a modern car that has "immovable" (from the regulation stand point) cats close to the exhaust ports. In a V8, this creates a high-rpm issue with 90-degree exhaust interference. For example, if the firing order is 1-3-7-2-6-5-4-8 (where passenger side bank from front to rear is 1,2,3,4 and the driver side bank from front to rear is 5,6,7,8), then 1 and 3 blow right after each other on the passenger side bank and on the other bank 5 and 6 blow right after each other. At high rpms when the engine is making a lot of power and there isn't much time for the cylinder to evacuate, the blowdown pulse from cylinder 3 takes long enough that the pressure pulse from cylinder 3's blowdown makes it to cylinder 1 at cylinder 1's overlap. This is terrible, it leads to hot burned gas staying in the combustion chamber of cylinder 1 and causes cylinder 1 to not make power and, worse, to knock.

Question: If the too short lengths are given because of the packaging constraints, is there anything that can be done with the diameters to reduce this problem? The three variables are #1 primary diameter, #3 primary diameter, and the diameter of the "secondary" or the manifold section after the flow paths of #1 and #3 joining. Can those variables be adjusted intelligently to reduce the 90-degree exhaust blowdown interference?
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

Calypso
Pro
Pro
Posts: 356
Joined: Mon Oct 17, 2005 1:38 pm

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

Post by Calypso » Mon Nov 13, 2017 11:15 am

Subscribing.

One idea I’ve wondered about is to put Venturi tubes inside primaries. Low restriction, flows poorly backwards and close to speed of sound flow to delay pulsing. Whether it would work, IDK.

User avatar
exhausted
Guru
Guru
Posts: 1065
Joined: Mon Oct 29, 2007 9:07 am
Location: Matthews, NC
Contact:

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

Post by exhausted » Mon Nov 13, 2017 11:48 am

AR interface at each head/manifold junction.
Calvin Elston
Elston Exhaust Inc.
Matthews, NC 28104
704-443-8088
Web: www.elstonheaders.com
Blog: www.exhausting101.com

DaveMcLain
Guru
Guru
Posts: 2680
Joined: Thu Jan 06, 2011 8:57 am

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

Post by DaveMcLain » Mon Nov 13, 2017 12:42 pm

I've often wondered how the interference really works. For instance if you have a typical V8 where two cylinders on each bank fire only 90 degrees apart which cylinder is effected more? The one that exhausts first or the second one? Using a Chevy V8 as an example if cylinder number 5 exhausts first. It blows down into the manifold and then only 90 degrees later cylinder 7 does the same thing. Does the pulse from number 5 reduce the blowdown from 7 to any great degree or does the pulse from number 7 mess up number 5 more because the cylinder pressure is low and the valve is already open a long way from the seat? Or since there is now a lot of volume with the piston near the bottom in cylinder 5 does it tend to not make much of a difference in the actual pressure at all?

ptuomov
Guru
Guru
Posts: 1520
Joined: Fri Aug 07, 2009 3:52 am

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

Post by ptuomov » Mon Nov 13, 2017 12:57 pm

DaveMcLain wrote:
Mon Nov 13, 2017 12:42 pm
I've often wondered how the interference really works. For instance if you have a typical V8 where two cylinders on each bank fire only 90 degrees apart which cylinder is effected more? The one that exhausts first or the second one? Using a Chevy V8 as an example if cylinder number 5 exhausts first. It blows down into the manifold and then only 90 degrees later cylinder 7 does the same thing. Does the pulse from number 5 reduce the blowdown from 7 to any great degree or does the pulse from number 7 mess up number 5 more because the cylinder pressure is low and the valve is already open a long way from the seat? Or since there is now a lot of volume with the piston near the bottom in cylinder 5 does it tend to not make much of a difference in the actual pressure at all?
My understanding is that, typically, the cylinder firing first gets the worse of it. Let's specifically consider the 90-degree exhaust blowdown interference at high rpms. The first firing cylinder will be at overlap when the second firing cylinder's blowdown pulse arrives at the first cylinder's exhaust valve. So the first cylinder doesn't evacuate properly. Furthermore, the first cylinder has high pressure when the intake valve opens and thus the intake flow doesn't get started during overlap (not in the right direction anyway!). So at high rpm, the cylinder firing first hurts a lot more.

At low rpms this 90-degree exhaust blowdown interference of overlap doesn't happen, because the pulse from the second cylinder makes it to the first cylinder before the overlap period. The pulse in the second cylinder's exhaust valve has already decayed by the time that the second cylinder's intake valve opens. So at low rpms, the really bad stuff from 90-degree exhaust blowdown interference is avoided.

At low rpms, the bad stuff happens to the cylinders that have another cylinder firing 180 degrees after it in the same bank. And there are four of those cylinders, two per bank.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

NewbVetteGuy
Member
Member
Posts: 196
Joined: Sun Aug 07, 2016 4:11 pm

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

Post by NewbVetteGuy » Mon Nov 13, 2017 1:03 pm

I could be remembering incorrectly, but I thought on the Anti-reversion thread I remember someone posting a link to Hytech / John Grudynski's site that included anti-reversion chambers designed to be a step in a headers' primary tubes and that they were $25 a piece...


If you're talking about a custom shorty header, couldn't you just go with a starting primary pipe on the small side for your application and then step up one size at say 7" from the head using the Grudynski / Fueling-style antireversion chambers and still fit it all into a shorty application?


Just throwing an idea out there.


Adam

NewbVetteGuy
Member
Member
Posts: 196
Joined: Sun Aug 07, 2016 4:11 pm

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

Post by NewbVetteGuy » Mon Nov 13, 2017 1:07 pm

exhausted wrote:
Mon Nov 13, 2017 11:48 am
AR interface at each head/manifold junction.
Any pictures of what one of these looks like?

I'm obviously a super newb at all this, but the only option I've seen at the head/manifold or head/header junction is Vizard / Cyclone-style cones; is this what you're referring to or something else entirely? Does this mean that you need to up-size the starting diameter of the primary pipe to not lose power from the restriction from the reduced diameter of the cone?


Adam

ptuomov
Guru
Guru
Posts: 1520
Joined: Fri Aug 07, 2009 3:52 am

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

Post by ptuomov » Mon Nov 13, 2017 1:07 pm

NewbVetteGuy wrote:
Mon Nov 13, 2017 1:03 pm
I could be remembering incorrectly, but I thought on the Anti-reversion thread I remember someone posting a link to Hytech / John Grudynski's site that included anti-reversion chambers designed to be a step in a headers' primary tubes and that they were $25 a piece... If you're talking about a custom shorty header, couldn't you just go with a starting primary pipe on the small side for your application and then step up one size at say 7" from the head using the Grudynski / Fueling-style antireversion chambers and still fit it all into a shorty application?
Suppose this is all that fits:

Image

Suppose further that only things that you can change to improve the manifold is inside diameters at various points of the manifold.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

DaveMcLain
Guru
Guru
Posts: 2680
Joined: Thu Jan 06, 2011 8:57 am

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

Post by DaveMcLain » Mon Nov 13, 2017 1:15 pm

ptuomov wrote:
Mon Nov 13, 2017 12:57 pm
DaveMcLain wrote:
Mon Nov 13, 2017 12:42 pm
I've often wondered how the interference really works. For instance if you have a typical V8 where two cylinders on each bank fire only 90 degrees apart which cylinder is effected more? The one that exhausts first or the second one? Using a Chevy V8 as an example if cylinder number 5 exhausts first. It blows down into the manifold and then only 90 degrees later cylinder 7 does the same thing. Does the pulse from number 5 reduce the blowdown from 7 to any great degree or does the pulse from number 7 mess up number 5 more because the cylinder pressure is low and the valve is already open a long way from the seat? Or since there is now a lot of volume with the piston near the bottom in cylinder 5 does it tend to not make much of a difference in the actual pressure at all?
My understanding is that, typically, the cylinder firing first gets the worse of it. Let's specifically consider the 90-degree exhaust blowdown interference at high rpms. The first firing cylinder will be at overlap when the second firing cylinder's blowdown pulse arrives at the first cylinder's exhaust valve. So the first cylinder doesn't evacuate properly. Furthermore, the first cylinder has high pressure when the intake valve opens and thus the intake flow doesn't get started during overlap (not in the right direction anyway!). So at high rpm, the cylinder firing first hurts a lot more.

At low rpms this 90-degree exhaust blowdown interference of overlap doesn't happen, because the pulse from the second cylinder makes it to the first cylinder before the overlap period. The pulse in the second cylinder's exhaust valve has already decayed by the time that the second cylinder's intake valve opens. So at low rpms, the really bad stuff from 90-degree exhaust blowdown interference is avoided.

At low rpms, the bad stuff happens to the cylinders that have another cylinder firing 180 degrees after it in the same bank. And there are four of those cylinders, two per bank.
What makes me wonder about it all is how when I've done back to back testing on engines built for boats I've found that going from a set of tubular headers to a set of water cooled manifolds that while horsepower is reduced mid range torque is where the real hit takes place. With a 434 Chevy making about 520 horsepower at about 5500rpm it dropped about 15 horsepower but close to 50lbs/ft down toward the torque peak at about 4000rpm.

ptuomov
Guru
Guru
Posts: 1520
Joined: Fri Aug 07, 2009 3:52 am

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

Post by ptuomov » Mon Nov 13, 2017 1:19 pm

DaveMcLain wrote:
Mon Nov 13, 2017 1:15 pm
ptuomov wrote:
Mon Nov 13, 2017 12:57 pm
DaveMcLain wrote:
Mon Nov 13, 2017 12:42 pm
I've often wondered how the interference really works. For instance if you have a typical V8 where two cylinders on each bank fire only 90 degrees apart which cylinder is effected more? The one that exhausts first or the second one? Using a Chevy V8 as an example if cylinder number 5 exhausts first. It blows down into the manifold and then only 90 degrees later cylinder 7 does the same thing. Does the pulse from number 5 reduce the blowdown from 7 to any great degree or does the pulse from number 7 mess up number 5 more because the cylinder pressure is low and the valve is already open a long way from the seat? Or since there is now a lot of volume with the piston near the bottom in cylinder 5 does it tend to not make much of a difference in the actual pressure at all?
My understanding is that, typically, the cylinder firing first gets the worse of it. Let's specifically consider the 90-degree exhaust blowdown interference at high rpms. The first firing cylinder will be at overlap when the second firing cylinder's blowdown pulse arrives at the first cylinder's exhaust valve. So the first cylinder doesn't evacuate properly. Furthermore, the first cylinder has high pressure when the intake valve opens and thus the intake flow doesn't get started during overlap (not in the right direction anyway!). So at high rpm, the cylinder firing first hurts a lot more.

At low rpms this 90-degree exhaust blowdown interference of overlap doesn't happen, because the pulse from the second cylinder makes it to the first cylinder before the overlap period. The pulse in the second cylinder's exhaust valve has already decayed by the time that the second cylinder's intake valve opens. So at low rpms, the really bad stuff from 90-degree exhaust blowdown interference is avoided.

At low rpms, the bad stuff happens to the cylinders that have another cylinder firing 180 degrees after it in the same bank. And there are four of those cylinders, two per bank.
What makes me wonder about it all is how when I've done back to back testing on engines built for boats I've found that going from a set of tubular headers to a set of water cooled manifolds that while horsepower is reduced mid range torque is where the real hit takes place. With a 434 Chevy making about 520 horsepower at about 5500rpm it dropped about 15 horsepower but close to 50lbs/ft down toward the torque peak at about 4000rpm.
I don't want to pretend to be an expert, but I think this is due to 180-degree exhaust blowdown interference. Longer pipes of any kind help, as the below Mazda graphic shows:

Image
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

DaveMcLain
Guru
Guru
Posts: 2680
Joined: Thu Jan 06, 2011 8:57 am

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

Post by DaveMcLain » Mon Nov 13, 2017 1:22 pm

I think you're right. I would say that it just simply hurts more down around by the torque peak simply because that's where the engine's VE is the highest. As the speed increases power goes up because you have more pulses but those pulses are weaker.
Last edited by DaveMcLain on Mon Nov 13, 2017 1:45 pm, edited 1 time in total.

ptuomov
Guru
Guru
Posts: 1520
Joined: Fri Aug 07, 2009 3:52 am

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

Post by ptuomov » Mon Nov 13, 2017 1:32 pm

DaveMcLain wrote:
Mon Nov 13, 2017 1:22 pm
I think you're right. I would say that it just simple hurts more down around by the torque peak simply because that's where the engine's VE is the highest. As the speed increases power goes up because you have more pulses but those pulses are weaker.
I believe it's a slightly different and more complex thing going on. Depending on the bore spacing and cam timing, there's a rpm range at which the cast exhaust manifold is going to run pretty close to the long tube header. Not better, and not as well, but almost as well. That's the rpm at which the 180-degree blowdown interference is no longer a problem because the 180-degree separated pulse isn't fast enough to make it to the victim cylinder's overlap and the 90-degree blowdown interference is not yet a problem because the 90-degree separated pulse resolves itself before the victim cylinder's intake valve opens. There's a cast manifold sweet spot there somewhere around 5500 rpm or so for big block engines, a little higher for small block engines. Much above or much below, the cast manifold shits the bed. That's what I believe, but I'm not sure if I'm right.
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

ptuomov
Guru
Guru
Posts: 1520
Joined: Fri Aug 07, 2009 3:52 am

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

Post by ptuomov » Mon Nov 13, 2017 4:22 pm

This is one of my favorite exhaust design articles, which explains how the Ford Coyote designed decided to think outside the box in a space-constrained situation:
Exhaust

An area where the Coyote breaks from the modular pack is its pulse-separated, tubular headers. While hardly the first tubular Ford headers, these intelligently tuned manifolds represent a deep commitment to making power. Doggedly designed, protected from both axe-wielding finance men and dent-prone assembly plants, then nurtured by patient calibration engineers, these headers visibly represent the willing-to-bleed-for-it dedication the Coyote team had toward making power.

Technically, Coyote headers are a short Tri-Y design complicated by the Coyote firing order differing from other Blue Oval V-8s and the need to package the catalytic converters close to the engine. We'll let Adam Christian, the team member who designed these headers, as well as the prototype builder who welded up the prototypes in his home garage, tell the Coyote exhaust manifold story as he told it to us.

Adam started by showing us some test results of the current Mustang GT header. "Here's a comparison of a standard cast manifold like on the Three-Valve 4.6 today, which is a nice design. It was hard to beat those manifolds, actually."

"Headers only give you torque, right? That's in general. And while we wanted torque, we needed to sell the manifolds on power. We had already beat our torque target, so advertised power [was the goal]," Adam explained. "When I left racing [Ford Racing], I told the guy, 'I'm going back to production and I'm taking two things with me: headers and valve lofting.' And at least we got one of them into [the Coyote]. We almost loft-it's really close! We basically go to zero force over the nose, but it doesn't actually come unglued."

"So basically the benefits of the tubular headers in a nutshell is about 15 lb-ft and 6 hp," Adam added. "The thing that you'll notice is, you know what a set of Tri-Y headers are supposed to look like-a simple side and a complex side. On all previous Ford engines, the complex side is always on the driver side. This engine is swapped because the firing order is changed. The complex side should be on the passenger side, which is nice for the steering-shaft packaging and everything. What you'll see on these headers though is that we look like we don't know what we're doing, and they are actually simple connectivity on both sides-front pairs, rear pairs, both banks."

"The reason is that you have to have the catalysts very close to the engine-they have to light off-and when you have that kind of length and you try to separate the 90-degree cylinders, which is what you pick for connectivity, you don't have enough length. What ends up happening is you take the blow-down pulse that occurs in the second cylinder, and you push its pulse into the overlap period of that first cylinder, and you actually destroy the volumetric efficiency," Adam continued. "You've helped the pumping because you've moved that pulse out of the pumping portion of that cylinder, but you've hurt its Vol-F [volumetric efficiency] and the net result is zero; you don't get anything for it. And if you look at [Brand T], they're made that way. [Brand C] tends to do just straight-up manifolds. They're nice manifolds, but just straight up."

"This literally was a morning-shower epiphany thing ... you don't know the amount of work [it was] to push that exhaust flange down as far as it is. The catalysts are short. They're actually stacked on top of each other. The bricks have no separation between them at all, they're just crammed together. They touch; there's no cat monitor in-between. Usually there is a HEGO in-between and we don't have it," Adam said. "So we had pushed the package as far as we could and there just wasn't enough length to get it to work, and then I thought, 'What if we just don't try to pair the 90-degree cylinders? What if we just try to bring them together as much as possible?' And that's what you see, particularly the right bank; right-bank cylinders 1 and 2 come right together, and those two fire right on top of each other. You see the secondary pipe is actually bigger than the rest-that's to take the larger blow-down of those two."

"So we've separated the 180-degree cylinders because we have enough length that we have fixed the Vol-F on all those cylinders so they scream. And the 90-degree pairs are also happy in terms of volumetric efficiency-but they have a pumping hit. So that's the best trade-off; basically, if you have to be that short, this is the type you want to have," he said.

"I have to hurry up and apply for a patent on these, 'cause no one else builds them this way," Adam confessed. "Our peak Vol-F, which is at peak torque, is 110 [percent]. It depends on the dyno cell, right, but we've hit as high as 110, 108, so it's pretty impressive. And at peak power we're pretty close to 100. I don't know, typically 98, 99 [percent]."

Certainly the end result is impressive. "Torque is almost 400 lb-ft out of 5.0 liters; no one else comes close. And it's these type of things that help-the intake runner lengths, the port volumes-because we could have gone with a super-short intake and sold out all the torque to go for peak power. It's those small details, the TiVCT, those are the things that let us get that kind of torque," Adam elaborated.

Or, in the words of Gary Liimatta, "This is a really, good engine, but it is the culmination of a many, many, many small details all pointing in the right direction. The successes we've had are by very hard work."

Some of the hard work in places far from the exhaust paid off in delivering these headers as production pieces. Asked how a stainless steel tubular header compares to a cast-iron exhaust manifold in cost, Mike Harrison spoke right up. "To run a fabricated tubular header on a production engine is a decision that's not taken lightly, and we revisited it on a number of occasions."

Politically, headers are highly visible targets to the cost-cutters. "They are more than double the cost of a cast manifold," explained Mike. "At 6 hp it's hard to justify, but we wanted to build the best 5.0-liter engine out there. And sitting with the [management] team and educating them on the details, defending them ... but we set up cost targets early in the program and we hit the cost targets, so there really was no leg for the more senior management team to stand on. In fact, if I had been overrunning my costs, I would have had to give something up and [the headers] would have been it. [But] we were able to contain this within the overall cost target of the engine, so we were able to deliver our metrics, and, you know, that helped."

Durability is another tubular-header concern. "The problem is the manifolds grow with heat and this pipe tries to pull the short one right out of the collector," explained Adam. "Those rear, short primaries need to twist, so it grows to the rear." These durability concerns led to some of the otherwise non-optimal intersections around the Coyote headers collectors.

Another issue is getting a tube header to work at the vehicle assembly plant, where the tools are huge and time is precious. The header, "has to be durable, work, and can be assembled [with workable] decking zones and tool paths," said Adam. Coyote engines are fitted to Mustangs from the bottom at the plant and are the widest part of the engine, so tucking them close to the engine was important.
http://www.mustangandfords.com/parts/m5 ... te-engine/
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

User avatar
modok
Guru
Guru
Posts: 1297
Joined: Sun Jun 06, 2010 1:50 am

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

Post by modok » Mon Nov 13, 2017 10:07 pm

If the system is restrictive the 90 degree interference might be a problem at higher rpms
At lower rpms 180 degree interference is likely a greater problem.
Glen Urban

ptuomov
Guru
Guru
Posts: 1520
Joined: Fri Aug 07, 2009 3:52 am

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

Post by ptuomov » Tue Nov 14, 2017 7:45 am

modok wrote:
Mon Nov 13, 2017 10:07 pm
If the system is restrictive the 90 degree interference might be a problem at higher rpms
At lower rpms 180 degree interference is likely a greater problem.
What do you mean by restrictive?

And any ideas how to cure the 180 and 90 degree interference problems in a short exhaust manifold?
[b]Paradigms often shift without the clutch[/b] -- [url]https://www.youtube.com/watch?v=cxn-LxwsrnU[/url]

Post Reply