Sequential System - a discussion

[Chiefgroover]

You get 0.7bar of boost on turbo 1, a slight dip in power between 3500 and 4000rpm, and then 0.8bar of boost with both turbos online. This should be a smooth and linear power delivery.

When modified with a boost controller/decat, you'll see an increase in boost after 4000rpm when the second turbo comes online. You may also get 0.8bar of boost on the first turbo with a decat. You'll feel the same power dip at 3500rpm but then a noticeable surge in power at 4000rpm when the second turbo comes online and hits over 1 bar of boost. Full boost is acheived before 4500rpm.

 

None of the Ian C "What it should be like:" quote refers to the first turbo raising its boost level above 0.8bar. I refer to my theory at the end of the last post. Thanks for that link.

[raymanuk]

Sorry should of said it all in my post but was a bit rushed since at work

 

But basically it was to point you in the direction of someone who may be able to answer your question regarding the first turbo and raising the level of boost and what controlls it.

Good luck and i would be interested on your findings so post on dude

[Chiefgroover]

maybe someone can find a way of measuring the first turbo's output before it joins the second one. Evidence would be helpful, rather than theory. I cant find any to support the magic boost raise in the first turbo.

[Ian C]

Ho boy

 

OK. What you are missing from the equation here is the way the exhaust gas flows through one or both of the turbos.

 

When you are on the first turbo only, the second turbo has no exhaust flowing through it, as it's blocked downstream of #2 by the Exhaust Gas Control Valve (EGCV). Bizarrely, although turbo #1 has the wastegate integral to it, when it's operating on it's own it doesn't us the wastegate to control it's boost. It instead uses the Exhaust Gas Bypass Valve (EGBV).

 

When #1 hits a certain boost pressure, the EGBV opens up and allows some exhaust to come from the manifold through turbo#2 and out via turbo#1's exit pipe. It exits through #1's exit pipe to bypass the still-closed EGCV.

 

This has two effects - one, it starts #2 spinning up. Two, it allows some exhaust gas to not go through #1, therefore acting like a wastegate and limiting the boost it produces.

 

In order to increase the boost that #1 makes, you put a bleeder valve on the EGBV actuator line, so that #1 has to make more boost before the EGBV opens and starts bypassing exhaust gas. The knock-on effect is that #2 gets less prespool because less gas flows through it at this stage, so you can get some lag while it gets up to speed when it's put online. (upping the boost on #1 is also pretty useless from a power gain perspective as well, I found, but that's a different discussion).

 

Now for a bit of theory. The *reason* you have two little turbos instead of one little one or one big one and why there is this big sequential system in place. Bear with me, this is relevant. Small turbos spin up faster and get on song quickly. Good news, but they can only flow a certain amount of air. One CT12 turbo is great at producing 0.7bar of boost on a 3.0 litre engine up to 4000rpm, but when the engine runs faster than that it demands more air than the turbo can flow at that boost pressure. Either it overspeeds and superheats the air and/or falls apart, or the boost pressure drops off as the rpms increase as it's flowing the same quantity of air but the engine is removing it faster than it was.

 

So, you either get lots of lag but a big enough turbo to flow enough air at max rpm, OR, you get no lag but power output is strangled at the top end as the turbo is too small to flow enough air.

 

So - best of both worlds is required, and that's where the sequential system comes in. Once the first turbo starts running out of flow capacity, we.... bring online another one! What actually happens then is the demand for air from the first turbo halves, as the second turbo is supplying half the air as well. So if the max flow rate of the CT-12 is 0.8bar at 4000rpm, then it can easily flow 0.4bar at 7000rpm - it's the same amount of air. With two turbos, we cheerfully have 0.8bar at 7000rpm, so now we have the fast spool of a small turbo combined with the flow rate of a big one, in this case two small ones joining forces.

 

Back from theory to practice - when the second turbo comes onstream, the EGCV opens to allow exhaust gas to flow through #2's exit, and the Intake Air Control Valve (IACV) opens to allow #2's boosted output into the main airflow stream. At this point, it doesn't matter jack shit if the EGBV is open or shut, as both turbos have their exhaust exits unimpeded and the EGBV merely links them. There is no flow through that valve, it's all at the same pressure (ok, maybe some turbulence but hey). This is why any mods to make #1 boost higher are irrelevant once both turbos are online.

 

Now, the wastegate on #1 is used to control boost. It opens up to bypass gas around turbo #1, which also has the effect of reducing the amount of exhaust gas flowing through #2. All the exhaust ports are connected via the link tube on the manifold, so exhaust flow/pressure is equalised across both turbos even though it only looks like #1 has the wastegate function.

 

As both turbos are the same size and have the same exhaust flow through them, they both generate the same amount of airflow/boost pressure. This is a good thing, otherwise one turbo would overpower the other. This combined pressure is what the boost gauge will now read, it's physically impossible to measure the boost off just one turbo at this point as their outputs are linked and the pressure is equalised. But then again it's easy to infer the output, simply halve the total boost pressure.

 

Hope that clears a few things up, and trust me, this is from experience, not just reading about sex in a book, or something

 

-Ian

 

Ps the big bang theory still stands and is respected in the scientific community, it is being modified all the time though, however what evolutionists, i.e. Darwinism, has to do with physics I don't know! And the sequential system, although tricky, is I'm sure a tad easier to suss out than the beginning of all time, space, and matter lol

[Matt Harwood]

Thanks Ian My hero!

[Chiefgroover]

This combined pressure is what the boost gauge will now read, it's physically impossible to measure the boost off just one turbo at this point as their outputs are linked and the pressure is equalised. But then again it's easy to infer the output, simply halve the total boost pressure.

 

So it seems we cant measure, hence prove what the first turbo's output is therefore niether I nor anyone else can measure and give the definitive answer. Looks like we will have to agree to differ on this one. Each case could be argued "in theory" but theory means nothing to me, so maybe someone will buy the turbo's and stop hyjacking my thread with their theory's.

[Chiefgroover]

Oh yes, just in case anyone has forgot that I have the turbo's for sale, bid me !

[Ian C]

So it seems we cant measure, hence prove what the first turbo's output is therefore niether I nor anyone else can measure and give the definitive answer. Looks like we will have to agree to differ on this one. Each case could be argued "in theory" but theory means nothing to me, so maybe someone will buy the turbo's and stop hyjacking my thread with their theory's.

 

OK, so the turbos are the same, the flow of gas to them is the same, the plumbing is the same, and they merge their outputs cleanly. You infer the individual boost pressure from their output capabilities and these input and output similarities by halving the total pressure. It's beyond theory, it's a proof. Some things can't be directly measured, they have to be inferred! If you had a 24" diameter manhole in the ground, would you want to measure the nonexistent manhole cover before getting a 24" diameter manhole cover for it, because measuring the hole is just theory?

 

You need a workable theory as to how you can possibly *not* have their outputs being the same and yet still have consistent and repeatable total boost pressure results, otherwise you are just arguing against with no real alternative. If you strongly believe that your turbo's run different pressures, give me the reasoning why and we'll have a discussion on it

 

In the meantime I'll get this post shifted to Tech and renamed, while leaving a less cluttered For Sale post for ya

 

-Ian

[raymanuk]

Good work Ian

[Ian C]

To carry this on, Chiefgroover posted:

 

"As we agree, boost is measured after the 'Y' piece where both flows mix. When the first turbo is boosting, that fact that the second is not outputting the same amount of air, if any at all, does not detract from the first turbo's performance. So then I think it is fair to say that if two slightly different levels of boost join at the 'Y piece, it wont cause a problem. Perhaps the lower boost slows the overall level, compared to two equal inputs? but not to any degree that causes a problem. So, we have the first turbo blowing @ 0.8bar governed by the actuator, and the second turbo blowing 1.1bar fooled by a bleed device, I see no reason why the engine would fail to perform properly in this manner. Not being able to establish a "solid reference" of how the first turbo would get to the 1.1bar level, why would i assume that it does?. Hope you see where I am coming from on this. We dont seem know of anyway of measuring the first turbo's boost level after 4000rpm when mixing, so I conclude that in all fairness that unless we can prove the hike in boost, why assume there is one?. However if someone can prove that it does I'll happily accept it. I appreciate you input Ian, and no doubt you have found the sequential system to be very interesting also. "

[Ian C]

To pick one solid and salient point to discuss - how would you increase the boost of just one of the turbos when they are in parallel mode above 4000rpm? Answer me that question and we'll go from there

 

Also, you could just unplug the VSVs to the EGCV and the IACV and run on the first turbo only, no spanner work involved... At your own risk

 

-Ian

[Chiefgroover]

To pick one solid and salient point to discuss - how would you increase the boost of just one of the turbos when they are in parallel mode above 4000rpm? Answer me that question and we'll go from there

 

Also, you could just unplug the VSVs to the EGCV and the IACV and run on the first turbo only, no spanner work involved... At your own risk

 

-Ian

 

Ian, the point all along is the boost beyond 4000rpm on the first turbo.

I wont be trying the above, but as i have said I have an angle on how to measure this and will try it soon as i get a chance. If this method works, I shall inform you of how i did it so you can try it for yourself.

[tDR]

My understanding of the system is as described by Ian, who has explained it a lot better than I could. Not sure what you are trying to argue about now chief - it seems you've just not understood?

 

Brian.

[Chiefgroover]

TDR - read the post again, I think you have missed what Alex and I disagree on right from the start. For the last time, everyone please read!

"THERE IS NO CONFLICT ON HOW THE SYSTEM WORKS"

[tDR]

TDR - read the post again, I think you have missed what Alex and I disagree on right from the start. For the last time, everyone please read!

"THERE IS NO CONFLICT ON HOW THE SYSTEM WORKS"

Surely there is if you are still trying to measure the boost from just the 1st turbo? Quite simply, it's half the total boost above 4,000 RPM. If you ran 1.1bar then the 1st turbo will have ran 0.8bar until the changeover point, then will have dropped back to effectively 0.55bar.

[Ian C]

Ian, the point all along is the boost beyond 4000rpm on the first turbo.

I wont be trying the above, but as i have said I have an angle on how to measure this and will try it soon as i get a chance. If this method works, I shall inform you of how i did it so you can try it for yourself.

 

Yeah, I understand what you are saying, my point is simply that, once it's in parallel mode, both turbos get the same amount of exhaust gas through them because the manifold is linked. You can't change how much one of them boosts with a bleeder valve or a boost controller, as there is one wastegate for the whole system - it bleeds off overall exhaust gas flow so it'll affect both turbos at once.

 

It'd take some major and non-reversible changes to make both turbos independant of each other on the exhaust gas front, but even then the outputs would merge - all we introduce is the potential for different outputs. In fact, a buggered turbo would cause an imbalance, and this usually manifests itself with fluctuating boost pressure, odd sequential transition effects, and jerky power delivery. Because one turbo is fighting the other

 

I'm intrigued to know how you plan on measuring the outputs of each turbo independantly. If you don't want to post it up on here for fear of people pouncing on it then fair enough And what's wrong with my "disconnect-the-VSVs" plan eh?

 

And I can't test it on my car, I lost my two turbos in a freak angle grinding accident

 

-Ian

[Chiefgroover]

If you ran 1.1bar then the 1st turbo will have ran 0.8bar until the changeover point, then will have dropped back to effectively 0.55bar.

 

Interesting threory, sounds quite logical too, well to me anyway. Well done that man.

I ask for some patience, until i can try my experiment in boost measuring of the first turbo. If it works, it will be no more theory, and proven fact, whatever it may be.

[Ian C]

What actually happens then is the demand for air from the first turbo halves, as the second turbo is supplying half the air as well.

 

I said it first Without figures, admittedly, but yeah, what TDr says is what I've been saying. It's a good theory indeed

 

-Ian

[raymanuk]

Quoted from Raymanuk:-

your argument does not.

 

Quote from ChiefGroover

OK then why does my agruement not make sense? what factual evidence do you have do dismiss it? I am eager to hear your technical reply.

 

Technical

When did I make out I was some sort of Technical Guru on the Supe?

Don't need to provide you with any factual evidence when it is already on this thread.

 

 

Quote from Raymanuk

This is not a single turbo setup.

Quote from Chiefgroover

 

LMAO............we have been talking about two turbo's from that start! who are you telling this to? yourself ? lmao ROTF

Really would of never guessed when you are talking about selling a set of twin uk spec turbo's that are mounted to your Supe currently. Don't judge other people by your own standards dude - I can read.

 

You wanted to measure the boost of the first turbo after 4000rpm well divide your boost level in half or are trying to say that the twin sequential setup are both producing different levels of boost above 4000rpm? Go for it prove us wrong I will gladly eat humble pie and be ass licking your findings and then send you over a chocalate fireguard for to enjoy

 

So now I have question to you all.

Q1

As like I said I am not technical but is this true? - Less revs = less exhaust flow = less boost?

 

Q2

Does the 1st turbo kicks in at 1500rpm?

 

If these questions are legit why bother increasing the the bar of the first Turbo when it will shift over to parallel mode above 4000rpm. That is only 2500 rpm to have that increased boost which forgive me if I am wrong but is that enough on stock turbos to spool that amount of air above 0.7bar?

 

Ray

[paul mac]

i think Ian c hit the nail on the head when mentioned the exhaust gas going into the turbos is the same, the only way you would get different outputs from the turbos would be if they were a different size (but you still would not be able to measure it) which they are not they are both the same, as for measuring the outputs indipendantly it would be impossible as they both belong to a common pressurised system as the pressure enters the "Y" it fills the whole "Y" not just one leg, then the intercooler and all pipework thats how any pressurised system works, the only place you would get a different pressure reading would be at the intake plenum due to flow restrictions (intercooler, bends etc) the only thing you could measure (if you had the gear to do it) would be the flow from the turbos (cfm) not the pressure

[Chiefgroover]

OK, somewhere down the line this question has shifted from the original conflict over what my first turbo ran. The general opinon in this forum was that whatever the second turbo ran, then the first turbo ran, therefore If i raised the boost on the second, then first had to boost the same.

I as you know have disagreed with this, as I can find no evidence to prove that the first turbo's boost had risen to match.

 

Just to make it clear here are some early posts from those who thought different.

 

Alex quote

If they didn't you'd get an imbalance in the system and No.1 would be stalled by intake air being forced back into it if No.2 ran more boost.

If one looks at the 'Y' piece where the charges join, its hard to believe that any real amount of air could pass around such a sharp corner especially at that speed, and cause problems with a turbo thats producing boost even if its about 25% less, as we have two fast gas flows.

 

 

Branners (now this one really got this going)

 

Both turbos together ran at 1.1bar with your first turbo running stock boost before transition.

So here we go again, I said my first turbo never reached 1.1 bar and here we have a different opinion.

 

B'have

Yup, spot on Branners!

More of the same opinion.

 

 

Matt

simply trying to point out that one turbo doesn't produce less boost than the other when the second turbo kicks in.

This is a reasonable guess, but we have nothing to prove it anymore than a slightly different set of fast air passages merging nicely.

 

 

I havn't quoted Ian C, as he has put that much forward, I dont know what to pick lol. (No offence Ian)

 

Ok, so current theorys are

 

@ 1.1 bar both turbo's run 0.55bar boost on each beyond 4,000rpm. This is the most popular view at the moment in here.

 

My Theory is - First turbo runs @ 0.8bar, and then second turbo tops it up above 4,ooorpm , now bear in mind the second trubo has a bleed device increasing its boost.

 

In support of my theory, and also having considered the 0.55bar theory carefully I conclude the following,

 

If the charge leaving both turbo's is @ 0.55bar when i see 1.1 bar at the guage is correct, then we have a lossless pipework system. While all Supra owners are would love this to be true, and I am one of those, it cant be.

Firstly the turbulance of air moving, starting @ 0.55 bar meeting with another charge of the same velocity @ a 'y' piece, then travelling down pipework into a FMIC, passing then through a throttle body to be then measured by the boost sensor which is situated next along the manifold @ 1.1 bar, it just dont happen that good. A much stronger force would be needed @ source to achieve 1.1 bar at the sensor (where my boost guage is plumbed into).

 

Ian do you agree with this before i go any further? just give me a yes or no please so i can continue to explain without any more confusion creeping in.

 

Very tired tonight, so I'll continue this in a day or so. Meantime folks try not to jump to any conclusions, as I have yet to explain the full line of this.

 

p.s. it is possible that when this thread is exhausted we all might have a new understanding of this, better than before.

[Chiefgroover]

Thanks for that contribution Paul. The only thing you may have have over looked is the effect of the actuators, the 4000rpm plus turbos being bled for more boost. Your post must have arrived while i was writing mine.

[tDR]

1.1bar may well be the reading on your gauge but on any turbo system it depends where you measure this. Ok if you see 1.1bar then the turbos combined boost was probably 1.2bar before the outlet but measuring this further down the line, depending pre or post IC then you would likely see a pressure drop especially allowing for corroded / damaged fins stock WMIC's. Therefore half of 1.2bar is 0.6bar each turbo effectively operated at above 4,000RPM but tbh that is irrelevant as we accept the readings on our gauges and the reading of 1.1bar is what's considered a safe boost level on stock turbos.

 

Cheers,

 

Brian.

[Chiefgroover]

Good points Brian. One of the difficult things to do is to measure the losses in turbulent velocity. Velocity while it's connected to pressure doesn't g/tee pressure. So the pressure pre 'y' piece could be much higher than 1.1 bar to see that pressure at the manifold sensor. Now we are going places!.

 

I had stated on the first post of this thread that i had run extra boost which everyone knows was measured at the manifold. The reason for doing this was so some potential buyer to know that they had not been run standard always. Considering all the extra thought that has gone into this subject now, it would be more reasonable to say that boost was more than standard but unknown, as I dont have a pressure loss figure for my particular setup, and the reading at the manifold was lower than the combined boost due to physical laws concerning turbulent velocity / pressure restriction. I maybe haven't termed that perfect but you get my drift.

 

The rest of this post is for the readers consideration I am not drawing any conclusions from this info at this point in time.

 

The other thing that occurred to me tonight was that if a Supra has high miles, and had a worn low rpm turbo that was down 25% in efficiency, the car would still run, though with less performance. This in itself proves that the boost does not have to be even coming from both turbo's, however there would most likely be a limit to the % difference it could cope with?.

 

The effects of the different velocity's meeting at the 'y' joint being the first point of contact for the turbo outputs needs to be considered as it would for an air combustible?

 

Given that the engine is drawing air, the turbo's are pushing air, the system could well be more tolerant of unequal velocity @ the 'Y' ?

 

High RPM turbo coming in @ 4000rpm, there may be a short period of time when its velocity enters the chain, and exits it again, when the rpm drops when the speeds dont match. The system is tolerant of this so why not on slightly different boost levels from each turbo, supporting my theory that may car was not boosting even on both turbo's after and only after a bleed device was applied?.

 

The even boost on both turbo's may only apply when car is running stock boost?.

 

The above is for consideration, you can let me know what you think in your own time, no hurry.

 

Keep her lit and sideways.

[richardharmon]

"...@ 1.1 bar both turbo's run 0.55bar boost on each beyond 4,000rpm. This is the most popular view at the moment in here."

 

This is the statement I have the biggest problem with here. The outputs of the two turbos are effectively joined together above 4000 rpm and so the pressure at the outputs of both the turbos will be the same (give or take some turbulence). The two turbos are not producing half the final *pressure* each, they are both working against the same pressure (the pressure in the output pipe). Its actually about air flow, what we should say is that each turbo is flowing half the required air mass per unit time, the final pressure is a result of the air being rammed into the output assembly by both turbos and is therefore higher when both are engaged as more air can be rammed into output pipe before the engine can use it, raising the equilibrium point of the final pressure in output pipe.