View Full Version : Sequential Turbos?

21-01-02, 00:27
Hi. I dont know too much about the Supra I am just here on a gerneral curiosity thing.
the sequential turbo thing, the idea is that one turbo helps spin the other?
Is that right?
Also, when you turbo upgrade, do you buy 2 huge turbos, 1 really huge turbo or can you get one large one and one smaller one. So the smaller one, being smaller has less lag, spins up faster thus spinning the larger more important turbo quicker.
I know of concept cars that had a sequential turbo and supercharger. The supercharger is basically always spinning up the turbo until the engine takes over. The result of this was a blown engine, but the theory is there.

Tell me!


Adam W
21-01-02, 01:01
Pete Betts has an excellent web page that goes into great detail about the operation of the sequential twin turbo system of the MkIV.  My basic understanding of it is as follows:

You need big turbos for big boost and big power, but they take a long time to "spin up" and start providing boost pressure.  This means that peak power is developed at fairly high revs, and you get big turbo lag (a pause between you pressing the accelerator and the engine starting to respond strongly).
The sequential system uses a small turbo at low revs (approx 1500 to 3500 I think), so it spins up very quickly to give a reasonable amount of boost pressure and minimal turbo lag.  Above 3500rpm, the engine switches over to a big turbo which can generate more boost pressure and greater peak power.

With turbo upgrades, you have many options -

1) Sequential hybrids, which uses the same system described above but with each turbo being bigger - you get more lag and more power across the whole rev range, but the same switching from one turbo to another.

2) Big twins, which are two large turbos which spin up simultaneously to develop very high boost pressure.  Very little power below 3500-4000 rpm, and big turbo lag, but enormous power once they're blowing hard.

3) Big single.  One whacking great turbo, giving huge boost (as much as two "big twins" blowing together) but because it's very large, it takes longer to spin up and needs more enbgine revs than either of the other two options.  Probably more suited to the strip than the street IMHO.

Obviously there are many different turbo choices within each of these categories, different sizes, AR ratios, etc etc . .. some big twins give MORE lag than big singles, there's a real black art to deciding which turbos are best for you.

HedgehogSandwich adi
21-01-02, 11:21
While were on the twin turbo subject, my twin turbos both spool at the same time (1JZGTE), low down the revs (around 1600 they start to slowly come in).  Presumably in this case this is purely for more boost than one turbo? Or does the twin arrangement mean that they come in lower down the rev range?  Why have two turbos producing 11 psi or whatever rather than one bigger one producing it?

Also, am i being incredibly dim, but if i bought a boost controller i would just have to buy the one as opposed to two? I know i need two seperate gauges to set it up but apart from that im stumped.

adi  (in confusion about all things tubo)

Ian C
21-01-02, 13:31
Having spent the past few weeks pulling mine apart and researching their function so I could get the 2nd turbo working, I can throw a bit of the info in to assist...

SRRAE's question:
The stock sequential setup has two turbos of the same design.  Number one is always online, and has a wastegate because of this.  Number two is switched in at 4000rpm.
#2 can't provide boost until then because two valves are closed - one in the exhaust system, one in the induction system.  Exhaust can't flow, so the turbo doesn't spin, and the air from #1 can't get into #2 and spin it backwards.

At 3500rpm the ecu opens a small valve that prespools #2.  This spins it up, but not to full speed.  This causes a slight power dropoff around this rpm.  The exhaust gas flows back out #1, as #2's valves haven't opened yet.  This smooths the transition for #2 turbo, rather than getting all the exhaust gas at full chat from standstill, it gets eased in.

At 4000rpm, #2's valves are opened, allowing exhaust gas to spin the turbo fully up to speed, and the boost produced joins the air flow from #1.

This system provides rapid low-down boost from #1 and a huge midrange to top-end kick when #2 joins it, and it's a fantastic bit of engineering when you look at it in detail.

Also, the two turbos have to be the same, otherwise one will overpower the other at the point the airflow from them meets, and it will back air through the weaker turbo - losing power briefly.  Briefly because the weaker turbo will soon go pop.

Adam - the second turbo works alongside the first, the system doesn't switch over to a bigger turbo.

Turbos don't have a lower rpm spinup speed, i.e. they don't 'come in' at a set number of revs.  They spin up according to the amount of exhaust gas going through them, so booting it at 1000rpm will generate (roughly) 1/5th of the gas as booting it at 5000rpm, and so it will seem to take a while to spin up.  So a smaller turbo will produce it's boost faster as it takes less gas/time to spin it up.

HedgeHog - I don't know why both your turbos would be running at the same time.  You can wire the 2jz's system to run in parallel instead of sequentially, and you get full boost earlier than 4000rpm, but don't get any low down boost until then.  Maybe yours is like that?

You get twin parallel turbos for V/Flat engines to minimise the plumbing - one turbo per bank of cylinders.  But on an inline 6 a twin parallel set seems a bit mad when (as you point out) you could minimise the plumbing difficulties by just having one big f-off turbo.  I don't know what the advantages of a set of HKS twins are over a big T-78, but I'm sure they must be more than cosmetic or willy-waving as they cost about 4000 more :)

You only need one boost controller as it controls the wastegate on #1 turbo.  #2 doesn't have a wastegate, all 'spare' exhaust gas goes out through #1 as they are linked (it's not like 3 cylinders power one and 3 cylinders power the other, which is what I thought once).


21-01-02, 13:46
Stock Parallel Turbo's will produce more power than stock sequential by all accounts....but as the others have said you get f all below 3000rpm on twin stock setup.

Steve Cargill
21-01-02, 14:05
My car appears to have had this mod done in the past, looking at cut wires on my ECU.  As they are already cut and need soldering properly I'm toying with putting a switch in and seeing what it's like.

Anyone tried this, or got any ideas - other than morbid curiosity to see what breakes first ;)


HedgehogSandwich adi
21-01-02, 16:16
Hmm, so presumably mine come in at the same time for more power then. This mush have its disadvantages though otherwise they wouldn't have altered it for the mkiv.
When i say it comes in at low revs, i mean it comes in much easier, ie with very little throttle (10-15% i guess), whereas my old 3.0t used to take a lot more on the accelerator before the boost popped its head up.

I just cant understand why use two little ones in a a parallel set up when one big one would do the job.

Adam W
21-01-02, 16:39
AFAIK for the same reason that people pay extra for HKS twins when you can get a big single cheaper that will produce the same boost.

Two small turbos accellerate quicker than one big one.  That is my understanding of it at any rate, some one let me know if I'm totally wrong.

I dunno, I thought I was just beginning to understand this turbo business . . .

Ian C
21-01-02, 19:22
I'm no expert either :)  I'm just interested, and had to research it more to diagnose why #2 wasn't coming online properly!

I really can't see why two turbos working in parallel would produce more power than the same two working sequentially.  Look at it this way - take a snapshot of a stock Supra TT's power at 5000rpm.  It's generated by both turbos being online at the same time, at full boost.  What difference does it make if the second turbo came in at 4000rpm or was spinning up at the same time as the first?  None!  Ergo, no *extra* power.

What might be noticed, though, is that both turbo's hit full boost *before* 4000rpm in parallel mode.  Therefore, the car feels faster once the initial lag has been overcome.  But will be significantly slower than the sequential setup before that.  I think that in a 1000-6000rpm run, both setups will ultimately end up neck and neck performance wise, but in a trundling about town scenario the sequential wins out every time, as it's got the low-down power.

I also can't see two smaller turbo's spinning up faster than one single (that provides the same airflow characteristics).  I say this because you would be getting something for nothing.  Exactly the same exhaust gas flow goes into either turbo setup, but one has more frictional losses (the twin setup).  Hmmm.  I would say the twins would give slightly poorer performance, but only slightly - they would probably be the same in reality.  Or I might be missing a point here.

Hedgehog - have you got a decent piccy of your engine bay, around the turbo area?  I could possibly tell you if it's supposed to be a sequential setup...


Adam W
21-01-02, 20:03
From my own research on the 1JZ-GTE I came to the conclusion that it is a parallel set up (looking at dyno graphs and from other peoples opinions).

Ian C
21-01-02, 20:13
In light of this, I would say one of two scenarios spring to mind:

1.  You get a better power curve with two smaller turbos than one big single, for reasons I don't understand yet.  (I would still maintain that you don't get any more top-end power...)

2.  Toyota were edging towards the sequential setup and pioneered it's development of the small twin turbo setup in a parallel fashion first with the 1JZ-GTE.  This first phase would recover a chunk of the the primary development costs, and then the fine tuning of the sequential setup could be sorted without worrying about the 's too much.


Paul Whiffin
21-01-02, 21:41
Adam, doesn't the 2.5TT work in parrarel rather than sequential?

From what I've been told, a small single turbo like the PHR stage 1 or Sound Performance 57 will spool just as if not quicker than the stock twin set-up. Obviously really low down power will be less than with the stockers but when you get to around 3600rpm a thunderstorm is unleashed. To me the big twins/T88 large singles are a waste of time because of the lag involved, from what I've been told full boost isn't reached until around 5500rpm, bit pointless unless you increase the rev limit which is of course possible but not really needed for a road engine.  Why HKS doesn't make a small twin kit I don't know, there must be a reason for it.


HedgehogSandwich adi
21-01-02, 22:59
I haven't got a picture to hand, but from what i can tell without taking the turbo covers off, it has two alongisde the left cam cover, both the same size, and one seems to follow on to the other one. One is at the front, then it the second is exactly in line a few inches behind it.

They definately spool together though, Adam is right. Full boost is achieved very easily and quickly, which can be a bit boring as if im on full boost by 2500, theres no extra kick to put another smile on my face !
Anyway, its about to be modded soon so i'll see what happens then.

HedgehogSandwich adi
21-01-02, 23:02
Oh, an additional, stock boost is around 11psi. Doesnt really matter but i just thought id mention it.

Oh and for whoever asked for a pic, heres one, you can clearly see the tubos at the front of the pic.


(Edited by HedgehogSandwich adi at 11:36 pm on Jan. 21, 2002)

Ian C
21-01-02, 23:57
Definitely parallel, there is no mechanism for any sequential operation.  But, it's very similar to the sequential setup in layout, so that waffle about it being a prototype for the sequential design might have more weight...

I concur with Paul's observations on the big twin kit.  I don't know what market niche it's aimed at, it's bizarre.


Adam W
22-01-02, 00:13
In all this talk about rpm ranges and stuff, we appear to have forgotten something.  Even at a given rpm, you will get turbo lag as you come off and on the throttle - perhaps twin parallel turbos provide better response in this situation.

I hope the bloke who started this thread now has it all clear in his head ;)

22-01-02, 10:33
Though HKS don't make them, you can create a mid twin kit which will produce about 600hp with boost 3500rpm. Problem is you need someone to make an exhaust manifold to suit the turbo's.

It would be a case of going to an engineering company with turbo experience, buying a couple of small to medium turbo's and saying please bolt these to this engine with some really efficient piping please!

Might have dumbed that down a bit but the fact stands that it is possible you just have to be willing to pioneer a system....you can always sell the kit later and make your money back.

15-10-04, 11:11
While we're on the subject for Turbo's ...

Been contemplating a possible cause of action if my stock turbo's were to go kaput.

I was thinking of Hybrid turbo's ...
My current system is pretty much stock other than a double decat, walbro, rsr, ssq bov, vfcc

a) Is there any other compulsory mods that have to be done when upgrading to Hybrids ..

b) At the mo ... I boost to a maximum of 1.2Bar. Won't go further thanks to a CW Restrictor Ring. What would be safe max boost on Hybrids ? Obviously if > 1.2 Bar would need a new Restrictor Ring.


15-10-04, 11:22
Yes there are - try looking up fue kit upgrads and hybrids via the search button.

It has been covered before - it's not that I'm just too lazy to type!

Chris Wilson
15-10-04, 16:15
Why do 2 smaller turbos with a toatl air flow capacity equal to 1 huge one spool up faster? It's down to exhaust pulse wave tuning and in line sixes. Each turbo runs effectively on half the engine, maximisin the pulse energy. On a V8 you can get the same effect by cross over piping from one exhaust bank to the other, or by use of a flat plane crankshaft. A 4 cylinder can "sort of" get the same effect by divided turine housings. HTH. Too busy for a detailed explanation, but that's the gis of it.

Caveat: When I say busy I am really pratting about with my Skyline, so it's subtley different to "working" :D

15-10-04, 17:32
Wow, this thread is a blast from the past! Nearly 3 years between posts, that must be some kind of record!