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Old 06-21-2001, 09:59 AM   #1
MY99 2.5GT
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Question How does a (Sequential) Twin-Turbo work

I have a pretty good understanding of how turbos work and proper tuning. I have been a fan of turbos for a while now and am planning on fitting a turbo setup to my Legacy sometime over the next year. The other night I was thinking about the Japanese Twin Turboed Legacy B4. Is this a sequential setup?

How does this work? How do they get one turbo to kick in at lower rpms and one to kick in at higher rpms? I thought maybe the first turbo would somehow vent all pressure before it gets to the intercooler when the 2nd turbo is spooled. That didn't seem like it would work because when the first turbo doesn't give flow to the intercooler the second turbos pressure would flow into the intercooler then down the up pipe of the first turbo.

Let me know if I am thinking about this totally wrong. I really want to understand the concept of how this works.

Thanks
Brad
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Old 06-21-2001, 10:43 AM   #2
Adrian128
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I could be wrong, but I think it's got something to do with size. One turbo being smaller than the other..obviously the smaller turbo will start to spool up from down the rev range, helping low down power and torque.. and when the rpms have built up, the second turbo has spooled up and is providing more boost, at a time when the smaller turbo is running out of pufff.. so to speak. The Legacy TT set up is I think very similar to the Mazda RX7 TT set up...
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Old 06-21-2001, 11:03 AM   #3
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Question

Short answer: It is a combination of electrical and mechanical processes that allow transition from one turbo to both turbos. True sequential turbo setups are *NOT* solely limited to a simple mechanical operation or difference in turbo or turbo housing sizings... in most cases the car's ECU is heavily involved in the process.

This is the same idea but not necessarily same *implementation* used in Porsche 911 Turbos, Supra Turbos, Skyline GT-Rs, and Mazda RX-7s.

Long answer:

In my other car this is how it works (not my words, but leaving credit out merely for my other car's anonymity):

.....first off, [there] is no #2 wastegate. there is only one wastegate and it comes off #1 turbo because that turbo is always on line, therefore you always have a wastegate. there are 4 sets vsv's, actuators, and control valves for the sequential turbo system. each vsv is simply a solenoid that is either 100% open or closed, allowing manifold pressure to pressurize the different actuators that open/close the four different valves.
wastegate: when the manifold reaches 11#'s of boost, the ecu sends a signal to the wastegate vsv, this allows manifold pressure to build in the wastegate actuator which opens the wastegate.

exhaust gas bypass valve: somewhere around 3500 rpm, the ecu sends a signal to the exhaust gas bypass valve vsv, which allows manifold pressure to build in the exhaust gas bypass valve actuator which opens the bypass valve. this is a small opening inside the #2 turbine housing which allows some exhaust gas to go through the turbine of the #2 turbo which makes it start spinning, and dumps the exhaust gas out the exhaust piping coming off of #1 turbo. since it is a small amount of exhaust gas, it pre-spools the turbo and does not get it up to full operating speeds. this will smooth out the transition from 1 to 2 turbos. this valve is similar to a wastegate in design, but is located after the turbine wheel instead of in front of the turbine wheel like a wastegate would be. this is not a wastegate!

exhaust gas control valve: this valve is located in the exhaust piping downstream of the #2 turbo. when this valve is closed, all exhaust gas must go through the #1 turbine wheel to get out through the rest of the exhaust system. at around 4000 rpm, the ecu sends a signal to the exhaust gas control valve vsv, which allows manifold pressure to build in the exhaust gas control valve actuator which opens the control valve. this allows exhaust gas to go through #2 turbo and out the exhaust system which brings the #2 turbo up to full operating speed.

intake air control valve: this valve is located in the intake tract coming off of #2 turbo. it is closed below 4000 rpm so that boost pressure coming off of #1 turbo cannot backup through the #2 turbo and back out the air cleaner/suction of #1 turbo. there is also a 1 way reed valve within the same housing of the intake air control valve. as the #2 turbo starts to prespin at 3500 rpm, it will build some boost. if it builds enough boost, it will open the 1 way reed valve to allow this boost into the intake tract to join with the discharge boost pressure coming off of #1 turbo. at somewhere over 4000 rpm, the ecu sends a signal to the intake air control valve vsv, which allows manifold pressure to build in the intake air control valve actuator which opens the control valve. this allows the full boost pressure coming off #2 turbo to join in with that coming from #1 turbo and you are now fully on line. usually, the exhaust gas control valve will open first, which gets the #2 turbo spinning at full rate so that it is building good boost before the intake air control valve opens, allowing this boost to join in with that coming off #1 turbo.....
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Old 06-21-2001, 11:16 AM   #4
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Thanks for the reply. I saw Jeff Tsai had posted some B4 Dyno charts. The engine and Twin Turbo setup produced a very impressive power curve. It produces way more torque at low end than a STI ver 6 WRX. In fact take a look at the dyno charts listed at http://www.lumine.net/subaru/legacyb.../dynotest.html and you will see that the twin-turboed B4 produces at 3000rpm about 85NM higher than the STI. It doesn't end there the power keeps going up smoothly.

So I guess from looking at the Dyno charts I would have to say this is a sequential setup, a very effective sequential setup. Immagine the torgue and hp numbers on an EJ25!

Jeff or Another B4 this is your time to chime in. How does your turbo setup work??

Does anyone know where I can see a picture of the engine out of the car with the turbos installed? I would like to see how they ran the piping and turbo sizes. I would also like to know what part electronics play in get this setup to work.

Thanks
Brad
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Old 06-21-2001, 11:26 AM   #5
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Wow Plypside,
Thanks for the informative post. The last post I posted before I read your post.

All of sounds pretty interesting and its funny but I understood most of it. I guess the main problem at hand if I were to try and fabricate an extremely intricate system like this would be finding compatible parts. Such as and ECU. How would I find an ECU that would work properly with all of these different bypass valves and other variables?

Jeff and AnotherB4 - does the previous explanation sound similar to how your B4 twin turbos work? Will that work on an EJ25?

Thanks
Brad
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Old 06-21-2001, 11:34 AM   #6
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Brad
I have an article in a magazine here about a GC8 WRX with an EJ25 with a twin turbo set up. It develops, according to the article, about 400 hp at the wheels. It's quite informative.. with pics and all. I can scan the whole acticle if you like.
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Old 06-21-2001, 01:02 PM   #7
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yeah but is the added weight and complexity of the system worth low end torque when

a. you can downshift
and
b. that torque might just break your tranny anyway
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Old 06-21-2001, 01:22 PM   #8
MY99 2.5GT
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To answer your question Jmott:

1. You can't down shift when your in 1st gear. I understand your point for at spead track racing.

2. Buy a new tranny (you would have to eventually even if you go single turbo low boost) By the way I have a feeling that downshifting from off boost third gear to on boost 2nd gear would be more prone to blowing a tranny than apexing a turn in third around 3000rpm and having enough power to hold the gear until max boost range. The only place where downshifting into max boost range would come handy could be coming out of apex into a straight away. With the sequential-twin setup I could power through a series of turns without having to shift, and then downshift into full on boost range when heading into a straightaway.

Adrien 128- please scan-fax-email-post whatever I want to see that write up!!! 400hp Thats amazing!!


Thanks
Brad

Last edited by MY99 2.5GT; 06-21-2001 at 01:34 PM.
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Old 06-21-2001, 01:27 PM   #9
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the legacy b4 guys have had an issue with the twin-turbo system. the transition from the first turbo to the second is not smooth. you can see in the dyno charts that there's a dip in power.

roger
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Old 06-21-2001, 04:23 PM   #10
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Default twin not always good

The grass is always greener. When I was into the 3'rd Gen RX7's, the in thing was to remove the twin turbo setup and go to 1 large turbo. Much less complex and more reliable. The twin turbo gig did work however(unless it was in the shop where it was every two to three weeks). Unlike my new WRX, the RX7 had plenty of low-end even with it being a rotary.
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Old 06-21-2001, 10:28 PM   #11
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The twin turbo system works well enough, admittedly, but the VOD (valley of death) between the turbo switchover is a pain - though Subaru is getting closer to getting rid of it with the latest models.

The biggest difference between the single and twin is that the twin makes more low rpm and high rpm power than the single - but the single makes a lot of mid rpm power. The B4 needs the low-rpm power more than the WRX because of weight differences, it's a heavy car. But with the changes to the new WRX, the weight difference is getting smaller, and the twin may be more useful for the WRX than before, for city driveability.

Single turbo system still produce more power, however.

Paul Hansen
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Old 06-22-2001, 08:55 AM   #12
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Does anybody have technical information specifically for the B4s Twin Turbo setup?
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Old 06-22-2001, 03:06 PM   #13
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I have to agree w/ Paul, I think the earlier GTB TT setup was stigmatized w/ the "valley of torque" description. That's the older Legacy series though. This latest Legacy (3rd gen) seems to be much better on the small to large turbo switchover. And yes, everything I've read suggests Subaru used this setup to improve driveability on the heavier Legacy line, and make it more appropo for a "luxury" car.
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Old 06-22-2001, 04:34 PM   #14
MY99 2.5GT
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I guess the real question is can this setup be transferred to an "American" EJ25 with favorable results. I guess the article that Adrian128 is supposed to be getting to me will probably explain this. I hope it gives email addresses or some way of contacting the people who did the adaptation.

Should be fun to be fun to see what we can do with this!!

By the way Adrien128, hows it coming with that article?

Thanks
Brad
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Old 06-22-2001, 06:15 PM   #15
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check out AMR's web page for a cool twin turbo system that uses two turbos of the same size. Almost no spool up time and still a good 10psi of boost. plus more CFM....if only I hade $7000. I bet it would be sweet. a good smooth power band no lag, and a tone of torque.
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Old 06-23-2001, 06:04 AM   #16
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Smile Missed this...

Hi Brad,

Not ignoring you, just don't visit this part of town much.

Let's cut a long story short - can it be done to a 2.5? The answer is undoubtedly yes - will it be financially logical? That is a debate for you and your wallet. You won't be able to simply transplant the standard set up because of the LHD/RHD difference, and lots of custom plumbing will have to be done (there are transfer pipes and all sorts of other stuff that need to be added = extra weight out front, too). Custom engine management, harness etc too I would imagine.

Some owners have been know to switch their cars over to parrallel so they get lots of low-mid punch and better response from 2 smallish turbines without resposne slowing transfer pipes etc in the way.

All the fuss over the VOD (a term coined by Shirokuma I might add) is a bit over done - anyone used to living with a high boost turbo set up on a small engine will be able to work around it - but fuel economy is never good when you do that - although you end up smiling a lot!! I have seen #s as low as 12mpg, although 32+ is possible too (in winter!!).

Honestly, you are running a bigger engine, and a moderate pressure single would probably do you fine - not being familiar with the 2.5's internals (esp. the phase 1 and 2 changes), that's where I'll leave it.

BTW, not taking a shot at anyone, but for accuracy's sake, the Skyline GTR's RB26DETT is a parrallel set-up. And a stonking good one at that! Not many engines can be relatively easily and safely taken to 400hp! V.

Cheers.
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