Secondry wastegate on '94 BG5A

[Guest]

For all those die hard twin lovers out there, I am on to a wee project at the moment requiring me to know the TT system inside out and backwards. No problem, as I am 95% of the way there and the more I investigate, the more I love the idea... And the less it feels like a creation designed by aliens ;D

Most of the information available seems to surround the TT system in post EJ20G which seems to have left me with 2 or 3 burning questions and one confirmation needed To confirm. I would imagine my pair of turbos are the VF13/14 combo as I have a wastegate on the secondary.

Can anyone confirm for bog stock '94 BG5A 5MT GT SW:

VF13 - Secondry?

VF14 - Primary?

First question is: Why the secondry wastegate if the primary WG spills the same exhaust pressure zone... Is it just not capable of dumping enough because it's too small risking a boost spike? As in do all the other VF range of turbos have a large enough WG on the primary to not need the WG on secondary?

Second: The relief valve which vents secondary boost charge while spooling between stages is held closed by MAP when both chargers are in operation, we know this. The later model engines have this too, and in the Subaru training manual for these they talk about a positive and a negative pressure acting upon it by controlling two solenoids (negative pressure only under undesirable conditions for want of a better term). One is the positive manifold pressure, the other from the vacuum tank.

My ECU pin out has only one pin tagged as the relief valve (implying there is no vacuum supply solenoid), is this the one to direct manifold pressure to the valve allowing secondry boost to become usable? And does it do this when energised or relaxed? ie. When I turn it on (energise it), the solenoid opens to allow MAP pressure to act upon it?

The supposed missing solenoid (so I think) in earlier models like mine, that routes tank vacuum to the valve, opening it to relieve charge pressure really gets me. Or maybe this is an answer to the first question? Does the secondry WG exist to relieve #2 of it's operation entirely instead of this vacuum applied to the relief valve releasing boost charge? It would have the same effect on the desired result of system design safety ???

Nearly done... Sorry, smashing post I know :

If this is the VF13/14, does anyone know every day driver max boost pressures these can run? ie. Good for 10psi tops on primary, don't run anymore than 8psi regularly kind of thing?

Thanks if your still here tuffing out the long one

[Durty-Sanchez]

First question: Only subaru knows.. I suspect that as it was their first crack at a TT it was a precautionary thing more than anything (or that's just how IHI make the damn things). The secondary WG is not PWM controlled so it's a straight on/off sort of control. I can't find any mention of it in the firmware, although I haven't spent long looking.

Second question: I am fairly sure (this has been a few years now) that the solenoid that controls the secondary bypass valve switches vac or atmospheric on when it needs to be held open. It's essentially a standard BOV that triggers when the feed (provided via solenoid) has lower pressure than the input (from the secondary turbo). So.. you have a way for the secondary turbo outlet pipe to spool, and any pressure spill-over is fed back in to the intake duct rather than being allowed to build and put pressure on the turbine (because the i/c valve is still closed). The BOV's trigger feed can either be connected to Vac or even atmospheric, as both will allow pressurised air from the spooling turbo to vent back to the intake. Once the secondary has spooled sufficiently this trigger hose is fed with positive pressure to make sure it stays shut and keeps the TT system a happy place.

Of course if i've just c0cked this up and the BOV is pointed the other way then only vac on the trigger feed line will open it - provided it's at a lower pressure than the intake duct.

Anyway, might steer you in the right direction..

[Guest]

 Durty-Sanchez said:

The secondary WG is not PWM controlled so it's a straight on/off sort of control. I can't find any mention of it in the firmware

So by energising WG #2 solenoid, this will supply the actuator feed with MAP, and it's governed by a mechanical 'stop'? Should I turn this on solid when spooling and boosting on #2 100% of the time do you think? It might slow spool up. Maybe turn it on when I open the intake valve?

What you would be suggesting is the initial exhaust pressure release uses #2 WG opening, and the PWM operation of the BCS solenoid (adjacent to the vac tank) would control the boost in a "normal" fashion by trimming that spill using WG #1.

[Joker]

13 primary?

14 secondary?

other than that no more useful input from me...

[Durty-Sanchez]

Yea providing MAP to the secondary WG would open it, which would of course bleed exhaust energy from the system and slow down secondary spool times.. I think the secondary WG lies after the main exhaust actuator valve so opening this during primary operation wont do squat, it'll only factor in once the main valve is open. I'd leave it shut, it's kind of redundant in the setup - probably why it was lost in later versions.

From what I've seen the boost is controlled entirely by the BCS, which is in turn connected to the primary WG. At no point is the secondary WG ever used. During secondary spool up all exhaust gas vents through the turbine, and then once fully online boost continues to be controlled using the primary WG. During secondary operation the primary WG has less exhaust gas to pass through it, as the majority of the gas is already being expelled through both turbines, so if anything it becomes more effective

[ReubenH]

To throw a spanner in the works, some BG5A's also got VF15 and 16's.

Also, the GTB's VF18 and 19, the secondary doesn't have a wastegate, but i think you knew that.

I'm still trying to figure out your question anyway. hmmmm.

[Guest]

 ReubenH said:

To throw a spanner in the works, some BG5A's also got VF15 and 16's.

Do the 15/16's still have the secondary wastegate?

If not, then they are defo 13/14's...

Joker... Were you saying 13's the primary and 14's the secondary?

Is this a definite if I am running this pair? Confirmation greatly appreciated.

PS. It's a '95 if I said it was a '94 earlier (if that makes a difference).

[Guest]

 Durty-Sanchez said:

Yea providing MAP to the secondary WG would open it, which would of course bleed exhaust energy from the system and slow down secondary spool times.. I think the secondary WG lies after the main exhaust actuator valve so opening this during primary operation wont do squat, it'll only factor in once the main valve is open. I'd leave it shut, it's kind of redundant in the setup - probably why it was lost in later versions.

From what I've seen the boost is controlled entirely by the BCS, which is in turn connected to the primary WG. At no point is the secondary WG ever used. During secondary spool up all exhaust gas vents through the turbine, and then once fully online boost continues to be controlled using the primary WG. During secondary operation the primary WG has less exhaust gas to pass through it, as the majority of the gas is already being expelled through both turbines, so if anything it becomes more effective

So with your experience the primary WG is enough full stop, which is good in my eyes (less to control). I just want to understand their exact concept behind why it was 'thought' to be needed in the first place... It's really bugging me!

I will still operate WG#2 manually now and then I think, just so it doesn't cease up. With the way I have done the install it's completely reversible by unplugging the OEM ECU, and plugging in the Link G4 Xtreme and putting in place one fusible link to power up the supply relays. I have done this so I can pull it in and out at will and operate on one or the other easily. If the OEM uses the second WG, I can't have it ceasing on me

[telemekas]

 gadget']

[quote name='ReubenH said:

To throw a spanner in the works, some BG5A's also got VF15 and 16's.

/quote]

Do the 15/16's still have the secondary wastegate?

If not, then they are defo 13/14's...

Joker... Were you saying 13's the primary and 14's the secondary?

Is this a definite if I am running this pair? Confirmation greatly appreciated.

PS. It's a '95 if I said it was a '94 earlier (if that makes a difference).

The combination I have seen is a primary VF16 and secondary VF17 both with wastegates.

These are listed on Turbomaster.

The VF16 is essentially a compressor wheel upgrade of the VF13 (4/4) to match the VF14 (5/5) compressor wheel

[Durty-Sanchez]

I think that the primary WG is more than enough, given the bulk of the flow is through both turbos. The secondary WG has no PWM control so there's little in the way of adjustable boost control from factory. The only possible time that the secondary is used would be when the boost level in the secondary outlet pipe exceedes the secondary WG spring pressure (no idea what that is) and it opens to keep stuff from blowing up. Here's how you can test it.. disconnect the secondary WG then go give it a thrash. If your boost goes crazy then it turns out that you did need it afterall, otherwise you should be fine.

 gadget']

[quote name='Durty-Sanchez said:

Yea providing MAP to the secondary WG would open it, which would of course bleed exhaust energy from the system and slow down secondary spool times.. I think the secondary WG lies after the main exhaust actuator valve so opening this during primary operation wont do squat, it'll only factor in once the main valve is open. I'd leave it shut, it's kind of redundant in the setup - probably why it was lost in later versions.

From what I've seen the boost is controlled entirely by the BCS, which is in turn connected to the primary WG. At no point is the secondary WG ever used. During secondary spool up all exhaust gas vents through the turbine, and then once fully online boost continues to be controlled using the primary WG. During secondary operation the primary WG has less exhaust gas to pass through it, as the majority of the gas is already being expelled through both turbines, so if anything it becomes more effective

/quote]

So with your experience the primary WG is enough full stop, which is good in my eyes (less to control). I just want to understand their exact concept behind why it was 'thought' to be needed in the first place... It's really bugging me!

I will still operate WG#2 manually now and then I think, just so it doesn't cease up. With the way I have done the install it's completely reversible by unplugging the OEM ECU, and plugging in the Link G4 Xtreme and putting in place one fusible link to power up the supply relays. I have done this so I can pull it in and out at will and operate on one or the other easily. If the OEM uses the second WG, I can't have it ceasing on me