Twin turbos?

[FST4RD]

Hey all,

Just been reading a bit and haven't found what I am looking for.

How do the turbo's on the twin turbos work?

What I mean is does the ecu control them?

I know on some turbos all they have is a hose from the turbo to the waste gate, and they just spoolup when there is enough exhaust gas, and if you want to control the boost you tap that hose. Some have like a solinoid that's ecu controlled to control the turbo.

Can you do the same thing to both turbos on the Legacy motor, just have a hose running from the turbo to the wastegate so once enough exhaust gas is passing through it the turbo spools. Or will this cause all sorts of problems?

Hope this all makes sense...

[DRFVDR]

controlled by the ECU via the BBOD, Boost and change over all ECU controlled

you can run a tap on either or both BUT may cause issues

[telemekas]

http://www.veloce.pwp.blueyonder.co.uk/Twin%20Turbo%20Operation/ttopprt2.htm

[FST4RD]

 telemekas said:

http://www.veloce.pwp.blueyonder.co.uk/Twin%20Turbo%20Operation/ttopprt2.htm

Thanks for that... confusing looking system

[FST4RD]

 DRFVDR said:

controlled by the ECU via the BBOD, Boost and change over all ECU controlled

you can run a tap on either or both BUT may cause issues

So obviously you can't remove the ECU control? Does the ECU need to have a say in the turbo?

What would happen if you just ran a hose from the turbo to the wastegate (like a normal turbo) and ran them both on wastegate spring?

Would this mean that the turbos would then just spool up as soon as they were able to?

Sorry if this makes no sense...

[Joker]

pretty sure the BBoD controls all

ECU dont do squat except maybe boost control

[DRFVDR]

 FST4RD']

[quote name='DRFVDR said:

controlled by the ECU via the BBOD, Boost and change over all ECU controlled

you can run a tap on either or both BUT may cause issues

/quote]

So obviously you can't remove the ECU control? Does the ECU need to have a say in the turbo?

What would happen if you just ran a hose from the turbo to the wastegate (like a normal turbo) and ran them both on wastegate spring?

Would this mean that the turbos would then just spool up as soon as they were able to?

Sorry if this makes no sense...

You will get primary spool BUT there is an exhaust change over valve

if you want to have a look at a Parallel twin turbo system on a Legacy Google ( or search on here ) Prova Legacy

[FST4RD]

Exhaust change over valve... Right.

And BBOD...

So the exhaust only gets channelled to 1 turbo at a time... and that's why there is VOD, during the change over?

So theoritically if you got rid of the BBOD and the exhaust change over valve so exhaust was getting channelled to both turbos at the same time you would get rid of VOD?

What about (thinking out loud here) if you jammed the exhaust change over valve to run the big turbo only and put a stick in the small turbo to stop it spinning and ditched the BBOD? Cause the secondry turbo seems to haul the wagon along quite nicely.

[Joker]

http://sl-i.net/FORUM/viewtopic.php?t=5662

and a Kiwi's (RuebenH's) view on the system

http://www.sl-i.net/FORUM/viewtopic.php?f=59&t=10991

 

ReubenPost subject: Twin Turbo MythsPosted: Mon Sep 22, 2008 1:22 am

SLi Resident

Joined: Fri Jun 20, 2008 2:30 am

Posts: 586

Location: Queenstown, New Zealand

I thought i should spell out once and for all how the twin turbo's work, there's much misconception out there on the subject, as well as a bunch of myths...

Well, first off, as most of you know, both iterations of subaru's Twin Turbo (that is, the BD/BG Master-4's, and the BE/BH Phase-2's) are sequential setups. That is, one turbo does the first bit then the second turbo kicks in after a certain rev range. First myth: The first turbo is small, and spools early (true) and the second turbo (a bigger one) then takes over (false). What's actually happening in the crossover period is the primary turbo despools, then both of them spool up in parallel. Both turbo's are the same size! (more on that in a sec) - "secondary" is ambiguous, it will mean the second turbo, and also the phase where both turbo's are going at once i.e; "I'm boosting on secondary" Which actually means you're boosting on both turbo's, or on the secondary phase.. I guess this leads to some people thinking secondary is actually one big turbo, it's not. The difference between turbo's is the primary is reversed to sit on the left side of the engine (the passenger side in rice hand drive), and the secondary doesn't have a wastegate. And the turbo's are the same size, or at least very similar. In the VF18/19's case, the compressors are identical! If you want to get technical, there's actually a phase between primary and secondary, the exhaust valve opens slightly to start spooling the secondary turbo, before opening fully, partially despooling primary, then spooling both at once. After you drive a TT for a few months, you'll notice this as the primary running out of "puff" in the moments before VOD (valley of death) and then secondary boost.

For more on the TT operation, the man GTB Limited's site: UK TT Register

I should also explain VoD for you too. VoD (Valley of Death) simply put, is the gap between primary and secondary. It's actually a great feeling when boosting to redline, it almost feels like switching on nos, you get a great thump in the back as you suddenly get all this extra power from nowhere. BUT, that's the only thing good about it. The non linear power curve often pisses people off - but that's not the worst part about VoD... When driving normally, VoD kicks in at about 4000 rpm... this is the speed most people like to change gears. If you change gear anywhere NEAR VoD, you get a massive great big gap where you have no boost whatsoever!!! For example, you're cruising up a hill, at, say... 10psi - Change gear at 4200 rpm or something, and - *wham* - NO power at all... and pretty much no boost until you get into secondary turbo. It's such a pain in the arse, i spent spent about $1500 (a bargain) lifting my engine, and converting the car to a single turbo! Yeah, VoD is no fun at all!

Now, er... BOV's. Myth: There is two BOV's/Bypass Valves (true), one for the primary turbo, one for the secondary (false!!!). There are two BOV's, yes. One is attached to the intercooler exactly like a single turbo subie. This BOV, if you think about it is obviously working for both turbos! Since the two turbo's meet up at the intercooler. This BOV, you can replace with an aftermarket one just like on a WRX, no fuss. The second BOV is actually for an entirely different purpose. It sits, confined, in a nest of pipes and vac lines, nicely hidden near the secondary turbo. Subaru must have done that intentionally, cause you don't want to mess with it, it's got something to do with the whole TT system, and nothing to do with actually "blowing off" air in the traditional sense . "The function of the supercharged pressure relief valve is relief of the pressurized air with the secondary turbocharger at the time of preparatory rotation. The purpose of this leakage is to provide smooth switching from single supercharging to twin supercharging"... So there ya go, the second BOV makes VoD smooth. That "preparatory rotation" is the middle phase i talked about where the exhaust valve opens a tad to start spooling the secondary turbo.

Aw heck... while i'm at it. If you want to know how it works, or what BBoD means.. well. SOLENOIDS! Lots of solenoids, and vac lines. I don't even understand it all myself. Basically, there's something like 24 different vacuum lines, about 6 solenoids, two boost control solenoids, and a differential pressure sensor, And the vacuum tank. The solenoids are all contained in the BBoD... "Black Box of DOOM" (Solenoid box). Here is a diagram.. Except for the actual wastegate solenoid, which is in the front left guard next to the vac tank of all places. And it all just magically WORKS... I think i saw someone say the exhaust valve actuator is operated electrically once.. just... NO. It's all vacuum and solenoids. So much vacuum, there's the aforementioned vacuum tank in the guard to store enough just to work the whole thing!

This is why i recommend to all in the US who are thinking about TT-ing their car, to stay the f*** away, and just go single turbo. Which works with a single wastegate actuator and a boost control solenoid. And the result is, well, a better drive! single turbo's KICK twin turbo arse.

That is all.

[FST4RD]

That makes sense joker... thanks for that, understand now. DOn't understand why Subaru did it though! ;D

So in essence to fix the problem would be when the secondary turbo is starting to spool, instead of the boost bleeding off, would be to redirect it into the pressure side along with the primary turbo so there is no change over.

[Joker]

longer wastegate hose (loke over a meter coiled up) or adjusting the actuator arm have been tried with reasonable success iirc

personally I would not bother mucking about with it at all

I have a spare motor and most bits for single conversion resting in shed for the day it might go pop

(222,000~+ and counting!

[FST4RD]

Yeah I'm not really going to go mucking around too much, want to keep it pretty standard.

TBH once your out of first gear and high enough in the rev range the wagon pulls very hard, and as long as when you change gear it stays boosting under the secondary turbo.

I thought that perhaps if the motor goes I might look at doing a single turbo conversion, or modding the twin turbo setup to work better.

[subieboy]

 JoKer said:

pretty sure the BBoD controls all

ECU dont do squat except maybe boost control

ECU controls it all with some smoke, mirrors and magic stuff, Zerosports (used to) make a controller to alter the settings...

[FST4RD]

 subieboy']

[quote name='JoKer said:

pretty sure the BBoD controls all

ECU dont do squat except maybe boost control

/quote]

ECU controls it all with some smoke, mirrors and magic stuff, Zerosports (used to) make a controller to alter the settings...

Do you know would happen if you removed the BBoD and removed the exhaust change valve?

Would the ECU have a wee cry?

[telemekas]

 FST4RD said:

That makes sense joker... thanks for that, understand now. DOn't understand why Subaru did it though! ;D

So in essence to fix the problem would be when the secondary turbo is starting to spool, instead of the boost bleeding off, would be to redirect it into the pressure side along with the primary turbo so there is no change over.

That in essence is how the Toyota Supra Sequential Twin Turbo IACV change over operated. (a small passive reed valve in conjunction with the butterfly valve)

The Subaru TT can be improved by replacing the IACV with a passive one way check valve. This allows you to remove the Supercharger pressure relief valve.

However for optimization the ECV needs to be ported or removed and a new type installed low down in the 2nd turbo dump pipe.

[GorGasmSpecB]

A common misconception is that the system "switches" between turbos at the changeover. It actually runs both at the same time after the changeover.

As mentioned in that article above.

[Jord]

 telemekas said:

That in essence is how the Toyota Supra Sequential Twin Turbo IACV change over operated. (a small passive reed valve in conjunction with the butterfly valve)

The Subaru TT can be improved by replacing the IACV with a passive one way check valve. This allows you to remove the Supercharger pressure relief valve.

However for optimization the ECV needs to be ported or removed and a new type installed low down in the 2nd turbo dump pipe.

That would cut down the VOD significantly wouldnt it? As the SRV is dumping all the boost the secondary makes until it equals primary boost. Not to mention removing two solenoids from the equation!

[telemekas]

Yes, boost drop off is cut by more than 50% (ie 7psi-->2-3psi)

[Jord]

Cant really see why FHI didnt go down that route in the first place. How much did it roughly cost? The ECV flapper would be the other big restriction as it still blocks near half of the port when fully open. Would a butterfly valve work as a replacement ?

[telemekas]

 Jord said:

Cant really see why FHI didnt go down that route in the first place. How much did it roughly cost? The ECV flapper would be the other big restriction as it still blocks near half of the port when fully open. Would a butterfly valve work as a replacement ?

Removed the ECV and replaced it with vacuum operated butterfly valve (low down in the 2nd turbo dump pipe).

[subieboy]

 telemekas']

[quote name='Jord said:

Cant really see why FHI didnt go down that route in the first place. How much did it roughly cost? The ECV flapper would be the other big restriction as it still blocks near half of the port when fully open. Would a butterfly valve work as a replacement ?

/quote]

Removed the ECV and replaced it with vacuum operated butterfly valve (low down in the 2nd turbo dump pipe).

Pics and details please

How much did the butterfly valve cost. what butterfly valve did you use, etc

[Joker]

sounds like one out of the OEM GTB exhaust (lol motor operates it in jack cubby hole)

[telemekas]

 subieboy']

[quote name='Jord said:

Cant really see why FHI didnt go down that route in the first place. How much did it roughly cost? The ECV flapper would be the other big restriction as it still blocks near half of the port when fully open. Would a butterfly valve work as a replacement ?

/quote]

Removed the ECV and replaced it with vacuum operated butterfly valve (low down in the 2nd turbo dump pipe).

Pics and details please

How much did the butterfly valve cost. what butterfly valve did you use, etc

Jun B.L ECV $250 operates via port No:6 solenoid box

Similar to this one:

http://www.ebay.com.au/itm/400062383069?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1438.l2649

Replaced the ICV with one these:

http://www.aid-inc.com/product_50/check_valve.html

[subieboy]

 telemekas']

[quote name='Jord said:

Cant really see why FHI didnt go down that route in the first place. How much did it roughly cost? The ECV flapper would be the other big restriction as it still blocks near half of the port when fully open. Would a butterfly valve work as a replacement ?

/quote]

Removed the ECV and replaced it with vacuum operated butterfly valve (low down in the 2nd turbo dump pipe).

Pics and details please

How much did the butterfly valve cost. what butterfly valve did you use, etc

Jun B.L ECV $250 operates via port No:6 solenoid box

Similar to this one:

http://www.ebay.com.au/itm/Ferrari-F355-355-F50-Exhaust-Bypass-Valve-155006-/220653327792?pt=Motors_Car_Truck_Parts_Accessories&hash=item335ff695b0

Replaced the ICV with one these:

http://www.aid-inc.com/product_50/check_valve.html

Sweet