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femno

Rb26 Low Mount Tubular Manifolds - Do They Make Any Difference?

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Weeeee a big math session and I missed out!!! lol 

I assumed it was a wording issue as opposed to actually literally referring to mass flow past a given point - by definition a flow rate is past a certain point, having a pipe length involved in that wouldn't make sense unless as @GTSBoy was essentially covering... you are taking into account external factors which could influence the flow.  I think friction/pressure ratios etc are beyond the cope of this conversation.  Well I thought so before haha.

I interpreted the statement (and I still feel that this is more relevant to this topic tbh) as effectively the rate that the manifold could be filled based on the current flow rate of the exhaust, I guess an equation to describe it could be something like "FillRate = volumetricflowrate / manifold volume".   The reason I feel that is more relevant in this situation is that the gas flow through a manifold is extremely transient, when you are talking a 3>1 manifold on one of these things you are going to have a VERY inconsistent concentration of gas through the manifold so when a new EVO event happening you aren't going to displace gas at the turbo flange at the same rate the gas exits the exhaust port.

I feel like this kind of thing is key to building drive pressure which will have a very significant effect on response - which I feel is one of the major things we are trying to optimise with this kind of thing.

Edited by Lithium
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50 minutes ago, Ben26 said:

If the pressure differential was 0, wouldn't that also mean the flow rate is 0? That's what that equation says. 

I'm happy to be wrong and learn something new, but going by that equation and researching pipe flow vs length, thats what im reading!

With regard to that equation, yes, but that equation doesn't work in this scenario is the issue. 

The exhaust manifold with a turbo in it is quite a complicated area. 

To use your above formula you could look at the exhaust as a whole, but it still doesn't take everything into account, and as such still doesn't work. 

The biggest thing with a lot of those formulas is that you need to simplify things down to a constant value, for something that is constantly changing. 

When they teach this stuff at Uni, it's "this is the rules!!!... By the way, you can only use the rules in the minutest amount of calculations because of... "

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5 minutes ago, Lithium said:

Weeeee a big math session and I missed out!!! lol 

I assumed it was a wording issue as opposed to actually literally referring to mass flow past a given point - by definition a flow rate is past a certain point, having a pipe length involved in that wouldn't make sense unless as @GTSBoy was essentially covering... you are taking into account external factors which could influence the flow.  I think friction/pressure ratios etc are beyond the cope of this conversation.  Well I thought so before haha.

I interpreted the statement (and I still feel that this is more relevant to this topic tbh) as effectively the that the manifold could be filled based on the current flow rate of the exhaust, I guess an equation to describe it could be something like "FillRate = volumetricflowrate / volume".   The reason I feel that is more relevant in this situation is that the gas flow through a manifold is extremely transient, when you are talking a 3>1 manifold on one of these things you are going to have a VERY inconsistent concentration of gas through the manifold so when a new EVO event happening you aren't going to displace gas at the turbo flange at the same rate the gas exits the exhaust port.

I feel like this kind of thing is key to building drive pressure which will have a very significant effect on response - which I feel is one of the major things we are trying to optimise with this kind of thing.

I'm interested to know, on a turbo car, once spooled, how much advantage is an equal length runner since we have such a huge restriction at the turbo... 

I feel the equal/tuned length is best for spool up and NA engines, but how far does it affect a turbo once on full boost? 

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7 minutes ago, MBS206 said:

I'm interested to know, on a turbo car, once spooled, how much advantage is an equal length runner since we have such a huge restriction at the turbo... 

I feel the equal/tuned length is best for spool up and NA engines, but how far does it affect a turbo once on full boost? 

I am still thinking on it, but at this stage I think it's wildly overestimated in many situations UNLESS the lengths and/or the amount of pulses you are collecting can result in significant interference at the collector.  When you have a group of 3cylinders merging into one on a 6cylinder you probably almost have to try to make that an issue.  The path they follow and the merge itself are probably more significant factors, and it just ends up convenient that making a nice merge involves bringing the separate runners around from different directions and bringing it in evenly.

Basically, I would put equal length as a pretty high priority if there was going to be any chance of high pressure pulse overlap at the collection point,  otherwise I'd lax quite a fair bit and focus more on making the exhaust maintain as much energy as possible between the exhaust port and the turbine - which really is why we try and avoid interference at the collector in the first place.

Edited by Lithium
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35 minutes ago, Dose Pipe Sutututu said:

I read all that and my conclusion is that a proper equal length twin scroll manifold, single turbo with a divided rear housing and a pair of external waste gates will be better.

Lol truth (aside maybe for the need for twin gates), however quite a few ideas being bandied about here apply to designing any manifold.   

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23 minutes ago, Lithium said:

I am still thinking on it, but at this stage I think it's wildly overestimated in many situations UNLESS the lengths and/or the amount of pulses you are collecting can result in significant interference at the collector.  When you have a group of 3cylinders merging into one on a 6cylinder you probably almost have to try to make that an issue.  The path they follow and the merge itself are probably more significant factors, and it just ends up convenient that making a nice merge involves bringing the separate runners around from different directions and bringing it in evenly.

Basically, I would put equal length as a pretty high priority if there was going to be any chance of high pressure pulse overlap at the collection point,  otherwise I'd lax quite a fair bit and focus more on making the exhaust maintain as much energy as possible between the exhaust port and the turbine - which really is why we try and avoid interference at the collector in the first place.

I think we may be on the same wave length. 

When you do tuned length for an NA engine, the primaries and secondary lengths are huge in comparison to what we do for a turbo manifold, so it's possible we're even putting high pressure pulses rebounded at an exhaust valve when an opening event occurs which is bad! 

I'd love to do some testing of "equal tuned length" vs some smooth flowing but unequal length. 

Alas I'd want to do it with an EGT sensor per cylinder, and back to back on a dyno and I don't have money to build it, yet alone dyno time costs! 

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12 minutes ago, MBS206 said:

I'd love to do some testing of "equal tuned length" vs some smooth flowing but unequal length. 

Definitely on a similar wavelength.

Look up "pulse converters" in regards to manifold design - especially in patent applications etc.  There is some irony in the amount of effort that the aftermarket have put into making pretty tubular manifolds when OE manufacturers have highly qualified people with massive financial backing designing things, and decide they know better.  There is naturally often space/reliability/other factors often at play, but then there is still more method behind the madness which is factoring performance into things as well than credit is given - at face value it may look too agricultural but actually the reasoning can be brilliant.  There is quite a bit of data out there in SAE papers etc if you dig deep enough.

Also, while pondering on this kind of thing - check out the AMS AlphaX exhaust manifolds which are used for >1900whp R35 GTRs....

 

Image result for ams exhaust manifold r35

 

Edited by Lithium
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Obviously the subject is huge, but there are a couple of points to remember when pondering runner lengths, pulse tuning and pulse interference.

1. On an NA tuned length manifold, the lengths of the runners are "tuned" to get the pulses to come through the collector in a tidy order over a relatively narrow range of revs. They have to be long (like many inches for each of the primary and secondary pipes) in order to actually get the tuning in the desired rpm band.

2. The tuning means that you get a little extra low pressure at the exhaust port that has an opening valve.

3. When you are not in that rev range, you don't get that. You will actually end up with a little extra positive pressure on the port at certain rpm ranges. Yet the engine still works.

4. Turbo manifolds are obviously a lot shorter. So....their rpm "tuning" would be for much much higher revs, if they were tuned at all. But they're not really "tuned", because the engines don't run at those revs.

5. So the primary concern over any "equal length" runner business should be to try to get the pulses to arrive at the turbo in a polite order across as wide a range of revs as possible. That way they don't pile up on each other if a pair of pulses arrive at overlapping time.

I suspect the only way to get 5 to work is with equal length pipes, or maybe, just maybe, pipes with integer divisor lengths. But I wouldn't bank on it (the integer divisor length bit). There's a few more points to make about that.

6. The raw gas velocity in the runners can be more or less calculated simply from the total (actual, hot, pressurised) flow rate and the cross-sectional area of the runner. Of course, because it is pulsatory, the peak velocity is going to be higher, blah blah blah. But you can work out the order of magnitude of the velocity. And when you do, you find that the travel time from port to turbo is really really really small. So if you have different runner lengths at all, you are going to struggle to get a non-interfering collection of the pulses.

7. Keep in mind that the static pressure of the manifold (once up on boost) is pretty steady. The exhaust pulses might be happening, but the whole manifold is up at 15 or 20 or 30 psi. Clearly, the exit flow from the manifold out through the turbine and the exhaust is nowhere near as pulsatory as all the above discussion makes out. There is a large base flow rate and the pulses just vary the top X% of the flow.

I'll be f**ked if I'm going to try to do any of the maths for all of that though! Don't need the head pain.

8. I think the conclusion has been reached and discussed so many times in the past that it probably doesn't need reiteration. But just in case.....the tidiness of the pipework is more important than the lengths in a turbo manifold. The whole concept of length tuning is not relevant at the short lengths being considered. Even trying to arrange for equal length pipes is fraught with the question..."exactly what is the length of this pipe anyway?". By that I mean, where are you measuring from? The valve, the port lip? Where are you measuring to? The start of the "collector"? The turbo flange? The end of the nozzle in the turbine housing? What line are you measuring along? The centreline of the pipe? What about where you have two pipes merging on an angle? etc etc etc bastard math blah blah blah.

Make 'em short and sweet is as good a guideline as any other. If you could make them equal length, you will at least avoid the issue of the pulses piling up. But the cast 3 cylinder manifolds seldom have the middle runner as long as the outers....so that's out the window.

 

And, one last thought.....On an NA manifold the little bit of suction that you get at the port from tuned pulses doing their things at the collector is a reasonably large effect - because the manifold pressure is not very high. The suction represents a larger fraction of the base pressure, has a noticeable effect. In a turbo manifold, with the running pressure being so high, the pressure pulses turning up at the ports, whether positive or negative (relative to the base pressure) are much smaller, relative to the base pressure, so their contributions to adding or subtracting flow through the port are reduced. Still there, but not really in the realms of "free power" like they are on NAs.

Edited by GTSBoy
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15 minutes ago, GTSBoy said:

7. Keep in mind that the static pressure of the manifold (once up on boost) is pretty steady. The exhaust pulses might be happening, but the whole manifold is up at 15 or 20 or 30 psi. Clearly, the exit flow from the manifold out through the turbine and the exhaust is nowhere near as pulsatory as all the above discussion makes out. There is a large base flow rate and the pulses just vary the top X% of the flow.

My comments relating to this I tried to make really clear was to do with transient/spool conditions, but even then the pressure is actually not as steady as it sounds like you'd expect - also if you want meaningful EMAP logging it is worthwhile running something like this: https://www.t1racedevelopment.com/product/gt1r-emap-kit/

Edited by Lithium

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1 minute ago, WR33KD said:

Might not be super useful but a guy in nz built a 1000bhp gtr with twins

http://www.themotorhood.com/themotorhood/2017/11/14/times-up-660kw-r33-gt-r-animal

Almost completely different kettle of fish compared to the low mounts on RBs tbh.  Gets rid of a few of the huge fails about them haha

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29 minutes ago, Lithium said:

My comments relating to this I tried to make really clear was to do with transient/spool conditions, but even then the pressure is actually not as steady as it sounds like you'd expect

Yeah, I was more spraying into the thread in general. But wrt spool.....the revs are even lower, so the whole concept of runner lengths becomes even more meaningless. And while I agree that how the whole lot works before you get boost is very important, for all the reasons that we all know, in reality there is no hope of pulse tuning to make the manifold work well in an NA sense, and the turbine is always a restriction in the exhaust, even before you get any boost, and the pressure has to be positive and rising all through the spool period, taking us further and further away from the regime where any sort of pulse tuning at the port helps.

I wasn't suggesting that the EMAP was steady - just that there is a large floor level of positive pressure with the pulses on top. Those snubbers in your link highlight the issue. Whatever "steady" pressure you get from those is the average pressure, but the lowest pressure will not be that far below that average.

I haven't logged anything myself, so I'm working from mental modelling. So here's a question for anyone who's logged EMAP without any snubbers or heavy electronic filtering.....What is the top and bottom of the pressure fluctuation range compared to the average value? +/- 5%? 10%? 25%? ie, how noisy does the signal look?

FWIW also, the blurb for those snubbers says that electronic filtering causes a delay in the measurement. Well, yes. But so does pneumatic filtering. Filtering is still filtering and putting a pulsation damper into a pneumatic signal line works by blurring the fluctuations together, which inherently means a delay, probably exactly equivalent to the electronic one.

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15 minutes ago, GTSBoy said:

spool.....the revs are even lower, so the whole concept of runner lengths becomes even more meaningless. And while I agree that how the whole lot works before you get boost is very important, for all the reasons that we all know, in reality there is no hope of pulse tuning to make the manifold work well in an NA sense, and the turbine is always a restriction in the exhaust, even before you get any boost, and the pressure has to be positive and rising all through the spool period, taking us further and further away from the regime where any sort of pulse tuning at the port helps.

I haven't logged anything myself, so I'm working from mental modelling. So here's a question for anyone who's logged EMAP without any snubbers or heavy electronic filtering.....What is the top and bottom of the pressure fluctuation range compared to the average value? +/- 5%? 10%? 25%? ie, how noisy does the signal look?

FWIW also, the blurb for those snubbers says that electronic filtering causes a delay in the measurement. Well, yes. But so does pneumatic filtering. Filtering is still filtering and putting a pulsation damper into a pneumatic signal line works by blurring the fluctuations together, which inherently means a delay, probably exactly equivalent to the electronic one.

Not sure if we are on the same page or not regarding spool, I'm not quite clear on what you are saying.... but I think runner length means a LOT in regards to spool.  Subarus should be able to spool like Evos and SR20s if runner length didn't matter.

Alas we have immediately gone to the T1 dampener so can't comment on unsmoothed signals.   I have to admit I chuckled at the "because electronic filtering causes a delay" thing but I am not certain that they would both be equivalent.   Either way, the dampener also serves a purpose in regards to temperature isolation so worth using

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44 minutes ago, Lithium said:

Not sure if we are on the same page or not regarding spool, I'm not quite clear on what you are saying.... but I think runner length means a LOT in regards to spool.

I mean runner lengths as anything that you can optimise in terms of matching the lengths or trying to make them the "right" length. I otherwise concur (and have already said so) that shorter is better.

44 minutes ago, Lithium said:

Subarus should be able to spool like Evos and SR20s if runner length didn't matter.

I think the energy lost from long pipes is part of it, but I think the bloody torturous path from under the motor to the turbine inlet has got to be at least as significant. The funky exhaust pulse pattern can't help either.

On the subject of the T1 dampener - I'm not knocking them at all. I use snubbers like that on industrial sensing applications** to knock the spikes out of pressure signals. Actually, I'm only assuming that they are like what I use, which are mostly just sintered metal inside. The T1 snubbers could be a little more sophisticated, ref their claims of temperature reduction, etc. Given where they install them (on the RB38 head) - they're not actually in a "bad" physical situation.

As to the delay - any device which is capable of providing the required smoothing will cause a similar delay. Doesn't matter if physical or electronic.

 

**Like low pressure switches on gas valve trains, where opening the safety shutoff valves to the burner can cause the gas pressure to drop below the switch point briefly. The snubber can prevent unwanted trips.

Edited by GTSBoy

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13 hours ago, GTSBoy said:

I think the energy lost from long pipes is part of it, but I think the bloody torturous path from under the motor to the turbine inlet has got to be at least as significant. The funky exhaust pulse pattern can't help either.

I don't think the path is necessarily significantly more torturous than your typical I4 or I6 single turbo exhaust manifold, it may seem exaggerated by the length of the things.   The firing order is still evenly spaced (funnily enough) so equal length/timed headers are very possible, in fact the later ones run them factory so the pulse timing is not really a factor.  

Yeah agreed on the T1 damper, as I said earlier I was already raising my eyebrows at the claim- again the temperature isolation alone makes them worthwhile still even if the smoothing doesn't change the delay versus signal processing. 

Edited by Lithium

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21 hours ago, Lithium said:

Lol truth (aside maybe for the need for twin gates), however quite a few ideas being bandied about here apply to designing any manifold.   

Well twin gates are for the "players", I might preach that but I run a single Gen V 50mm Turbosmart EWG where it's divided all the way right up to the gate seat :)

I've heard stories where people can't control boost, etc. but I have no problem running gate pressure as you've seen from my logs.

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Factory ported ones 

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47 minutes ago, Dose Pipe Sutututu said:

Well twin gates are for the "players"

I've heard stories where people can't control boost, etc. but I have no problem running gate pressure as you've seen from my logs.

Still haven't seen anything to suggest that twin gates really do anything useful, have been waiting for something to click it into place for so far all the single gate setups I've been around seem to perform comparably to twin gate ones - both in response and boost control... so I clearly wouldn't have doubted yours would be fine :)

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