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

Post by Calypso » Sun Dec 01, 2019 7:57 am

ptuomov wrote:
Tue Nov 28, 2017 8:23 am

What’s unknown to me is how the various merge angles etc influence the issue. Koenigsegg and LS7 manifolds seem to suggest that collector with parallel inlets/flow paths and no choke that is directed towards the cat or the turbine somehow works well.
Old, but good thread...

Wondering if parallelelish primary exits prevents the interfering pulse and flow to almost directly hitting the suffering cylinder. High flow velocity at primary exit point also would mean low pressure communicated to the suffering cylinder. Königsegg collector case, 90deg cylinders would be close together, so it might be beneficial. LS case, because of the design constraints, a big area collecting point would at least delay the pressure raise in the collector and adjacent suffering primary.

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

Post by ptuomov » Sun Dec 01, 2019 11:10 am

Calypso wrote:
Sun Dec 01, 2019 7:57 am
ptuomov wrote:
Tue Nov 28, 2017 8:23 am

What’s unknown to me is how the various merge angles etc influence the issue. Koenigsegg and LS7 manifolds seem to suggest that collector with parallel inlets/flow paths and no choke that is directed towards the cat or the turbine somehow works well.
Old, but good thread...

Wondering if parallelelish primary exits prevents the interfering pulse and flow to almost directly hitting the suffering cylinder. High flow velocity at primary exit point also would mean low pressure communicated to the suffering cylinder. Königsegg collector case, 90deg cylinders would be close together, so it might be beneficial. LS case, because of the design constraints, a big area collecting point would at least delay the pressure raise in the collector and adjacent suffering primary.
Yeah, I never really understood why it works. It’s counterintuitive to me that an expansion chamber before the turbine would somehow help more than just a larger turbine.

I still like the Coyote still arrangement of maximizing separation between 180-degree pulses and minimizing the separation between 90-degree pulses while increasing the cross sectional area after the two 90-degree pulses combine.

I can tell you that the exhaust pulse interference is real in the log turbo manifold and cross plane V8. Every cylinder seems to work a little differently, including required valve seated spring loads. When the exhaust manifold pressures approach 3bar absolute, these seem to be a big deal.

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

Post by Calypso » Sun Dec 01, 2019 4:12 pm

Curiously Coyote seems to have a firing order of 1-5-4-8-6-3-7-2. So there is only one instance of firing adjacently in the same bank: 8-6 ? And the distance between 8-6 in the manifold seems lengthy. What am I missing?

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

Post by ptuomov » Sun Dec 01, 2019 4:42 pm

Calypso wrote:
Sun Dec 01, 2019 4:12 pm
Curiously Coyote seems to have a firing order of 1-5-4-8-6-3-7-2. So there is only one instance of firing adjacently in the same bank: 8-6 ? And the distance between 8-6 in the manifold seems lengthy. What am I missing?
I don’t have the coyote numbering convention memorized, but both banks must always have one 90-degree victim and two 180-degree victims. Where the 90-degree victim is next to the offender, coyote strategy of merging those as early as possible is easy. When they are not right next to each other, bore spacing starts mattering — modular ford at 100mm isn’t too far.

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

Post by Calypso » Sun Dec 01, 2019 11:24 pm

ptuomov wrote:
Sun Dec 01, 2019 4:42 pm
Calypso wrote:
Sun Dec 01, 2019 4:12 pm
Curiously Coyote seems to have a firing order of 1-5-4-8-6-3-7-2. So there is only one instance of firing adjacently in the same bank: 8-6 ? And the distance between 8-6 in the manifold seems lengthy. What am I missing?
I don’t have the coyote numbering convention memorized, but both banks must always have one 90-degree victim and two 180-degree victims. Where the 90-degree victim is next to the offender, coyote strategy of merging those as early as possible is easy. When they are not right next to each other, bore spacing starts mattering — modular ford at 100mm isn’t too far.
I quess they must have changed the cylinder numbering scheme from the cam in block engines...

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

Post by BLSTIC » Mon Dec 02, 2019 3:10 am

The ls7 style collector resembles both an anti reversion cone and a pulse terminator box. Isn't it effectively running "zoomies" with back pressure at this point? It looks like it works because the CSA change is so massive the pulses can't really interfere with other cylinders.

Or did I get that wrong?

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

Post by ptuomov » Mon Dec 02, 2019 6:22 pm

BLSTIC wrote:
Mon Dec 02, 2019 3:10 am
The ls7 style collector resembles both an anti reversion cone and a pulse terminator box. Isn't it effectively running "zoomies" with back pressure at this point? It looks like it works because the CSA change is so massive the pulses can't really interfere with other cylinders. Or did I get that wrong?
I never fully understood that collector, so I'd also be grateful if someone in the know would explain to me why that hydroformed exhaust system works better than the cheaper alternatives. My hunch is that it has something to do with the catalytic converter opening area.

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

Post by BLSTIC » Mon Dec 02, 2019 7:08 pm

It looks like it works because the sharp change in area and having an exhaust plenum means that the blowdown pulse from one cylinder going up the other pipes is reduced by a *lot*. Also the short length of pipe means that any restrictions from running a smallish ID are minimised, so there's still some inertia scavenging that's able to happen.

The pipes being parallel when they end would discourage any pulses from directly being able to hit the "inlet" of another pipe.

Unless I saw it wrong the other Koenigsegg collectors looked pretty conventional and I can't figure out how they would be special. Maybe they aren't concerned because they are far enough below the limits or have little overlap anyway. They do mention being small diameter. Does this higher gas speed interfere much with ability of the blowdown pulse of the other cylinder to affect scavenging?

These technical discussions are interesting. I look back on my early time watching dad play with cars and think about how things could have been better with what I know now...

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

Post by BLSTIC » Sun Dec 08, 2019 6:20 am

This probably won't help your exact situation, but what would happen if a merge spike were to extend into the turbine housing, like you were using the turbine housing as the outside of the collector? Or even just the tube from the offending cylinder poking down well past the flange?

Would this do a better job of directing the blowdown pulse through the turbine and end up with less returning? In the latter case of a single tube extending into the housing it would change direction and *then* be split into three.

Another thought I just had: Does messing with the area ratios of each join/merge affect where the returning pulse goes? As in could you hide the overlap cylinder from the blowdown cylinder by making the other two cylinders branch off first (looking at it from the turbine end, so from the point of view of the blowdown pulse) with much larger diameter tubes than is necessary for the gas flow? I'll use my awesome paint skills to draw it in the morning if I'm not making sense

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