MBC not controlling boost!! plz hlp

Millertime · Post by **Millertime** » Sat Aug 12, 2006 8:59 am

I installed my TurboXS MBC a little while ago and for some reason, I can't get it to actually adjust the pressure. My car wants to keep running at near 15 psi !! I could have been smoking crack when I installed it....but it seemed pretty cut and dry!

If I did something way wrong....please help! I am driving it still, but keeping the boost down.

Thanks in advance

Millertime · Post by **Millertime** » Sat Aug 12, 2006 7:05 pm

I feel pretty stupid....I accidentally disconnected the hose to my wastegate.....one of the techs at my Subaru store pointed that out to me this morning....

GotSubie · Post by **GotSubie** » Sun Aug 13, 2006 6:08 am

haha it happens to the best of us. but thats wierd because when you disconnect the hose to my waste gate it will just free boost way past 15 psi how big is your turbo?

Millertime · Post by **Millertime** » Sun Aug 13, 2006 7:27 am

it's a stock VF11 just a little guy! Does decent though. I would like to see what my car can do at full potential!

free5ty1e · Post by **free5ty1e** » Sun Aug 13, 2006 3:09 pm

Stock VF11 will overboost to like 21+ PSI if you let it.

Post by **DLC** » Sun Aug 13, 2006 6:44 pm

If you disconnect the wastegate hose it will not boost past what the spring is rated for. It's been most people's experience, and mine, that the stock turbo's spring will open at somewhere around 6.5PSI.

free5ty1e · Post by **free5ty1e** » Mon Aug 14, 2006 3:37 am

No, if you disconnect the wastegate hose the wastegate actuator will not see any pressure and therefore stay closed, allowing maximum turbo boost.

If you connect the turbo directly to the wastegate actuator, you will get wastegate spring pressure.

rallysam · Post by **rallysam** » Mon Aug 14, 2006 3:38 am

That doesn't seem right to me, Dave. If you disconnect the hose, it should boost very high. The whole wastegate system is on the exhaust side of the turbo, so it wouldn't know anything about what boost you're hitting without a hose coming in from somewhere on the compressor side or intake manifold.

To run wastegate boost, you'd have to hook the hose direct from the turbo compressor to the waste gate. That's probably what you're thinking.

or am I smoking it?

free5ty1e · Post by **free5ty1e** » Mon Aug 14, 2006 3:51 am

Ah, my ninja posting skills have landed me the top post in this admin correction

rallysam · Post by **rallysam** » Mon Aug 14, 2006 4:09 am

Damn, you beat me by a minute

GotSubie · Post by **GotSubie** » Mon Aug 14, 2006 5:09 am

ya so if you want to blow your motor by over boosting just un hook the hose to the wastegate it will be fun for about 5 min untill u blow your motor

Millertime · Post by **Millertime** » Mon Aug 14, 2006 6:51 am

It hit near 16 psi....it sounded cool, spooled fast, and sounded like it had a BOV! lol but had a fuel cut at like 4k rpms or even sooner. I gotter figured out, and I'm running something to the tune of 11. It is cutting at anything above 11.

Another question...why will it boost slightly higher sometimes. Even before the MBC it would hit near 9 psi!! So in that aspect, tuning in the exact boost pressure is becoming REALLY difficult.

(I heart boost, but she's so damn picky!)

GotSubie · Post by **GotSubie** » Mon Aug 14, 2006 7:18 am

mine will do that to but only in 3rd gear it will spike at 12 and idk y

Post by **DLC** » Mon Aug 14, 2006 7:23 pm

The wastegate has a spring in it. This is probably a failsafe, should the electronic controller fail. The spring will hold the wastegate shut till about 6.5PSI, then it'll open up from pure mechanical and pneumatic pressure. A boost controller simply applies vacuum to the wastegate actuator to keep it closed.

I've done this, others have done this, and it's not overboosted. By default, the wastegate will open at any given pressure, not stay shut, because it opens outwards.

Wastegate and boost controller info is available at wikipedia:

http://en.wikipedia.org/wiki/Wastegate
http://en.wikipedia.org/wiki/Boost_controller

free5ty1e · Post by **free5ty1e** » Tue Aug 15, 2006 12:51 am

Huh... You're saying that the pressure in the turbine / hot side of the turbo will force the wastegate spring open? That's never happened to me... with the wastegate actuator seeing no boost (a hose coming off in the boost control assembly somewheres) my stock VF11 quickly spooled up to 21 PSI in 1st gear, and when I saw that I did not try the other gears - I pulled over, found the hose, and zip-tied it in place.

Has anyone else had the wastegate open from turbine pressure somehow?? I can't see how this would happen, as the pressure from the turbine side would tend to keep the wastegate shut -- helping the spring in the actuator, not forcing against it.

Does not compute.

BTW, the stock boost control system fails in such a way that a 0% PWM (a blown unit would put this out, basically a ground signal) or no control whatsoever keeps the pathway between the turbo compressor and the wastegate actuator completely open. It takes current to close the solenoid, not to open it. And this is only a failsafe if none of the hoses are blown off -- without boost at the wastegate actuator, the ECU has a failsafe - it's what we're bypassing with the FCD.

Post by **DLC** » Tue Aug 15, 2006 1:50 am

The wastegate opens downstream of the flow of the exhaust, not upstream. The wastegate spring holds the wastegate closed, against the flow:

If the spring fails, or there's no boost control, there should be little boost, not high boost.

With my AVC-R, if I turned off boost control, I'd get about .5BAR, not 1.5BAR.

Post by **Legacy777** » Tue Aug 15, 2006 5:28 am

Dave is correct. The spring is designed to hold a certain amount of force/boost. What the wastegate actuator solenoid does is cycle back and forth between intake vacuum & boost to regulate the pressure that the wastegate actuator sees.

rallysam · Post by **rallysam** » Tue Aug 15, 2006 1:23 pm

The portion posted by L777 is correct.

The part where Dave is wrong is that the wastegate is opened and controlled by compressor pressure coming in from a tube to fight that spring. The exhaust pressure hitting the wastegate has nothing to do with it.

free5ty1e · Post by **free5ty1e** » Tue Aug 15, 2006 2:55 pm

Exactly... its not turbine pressure it's compressor pressure and it has to be hooked up or it overboosts.

Man are you guys trying to confuse me or what?

Dave and Josh - go pull the hose from your wastegate actuator, leave it off, and go for a WOT run. Then come back here and tell me you didn't overboost.

Edit:
I mean... if turbos were failsafe with internal wastegates and no external connections to an actuator.... then what the hell did Subaru make so certain that the ECU kills fuel if the MAP sensor sees too much boost? What scenario were the engineers at SOA possibly trying to avoid, if not open wastegate actuator / overboost? Why do we need an FCD to run more than ~13 psi? Answer please if you still hold your stance on this issue by this point.

Maybe you're talking about a different turbo. I've never seen one set up to let the turbine pressure alone push the wastegate open from the inside. The boost is controlled by ONLY boost, vaccuum has nothing to do with it. With no pressure acting on the wastegate actuator, the gate stays closed tight - from the wastegate actuator spring alone. When boost presses on the actuator and overcomes the spring pressure (from the COMPRESSOR side), it opens. To prevent this from opening too early, boost controllers limit the amount of pressure allowed through to push on the actuator.

No respect, I get no respect

free5ty1e · Post by **free5ty1e** » Tue Aug 15, 2006 3:08 pm

DLC wrote:With my AVC-R, if I turned off boost control, I'd get about .5BAR, not 1.5BAR.

Okay. We're not talking about "disabling" or "turning off" boost control - that should result in the minimum boost level, which is what would happen if you removed power to the stock control solenoid.

We're talking about not having boost control at all. Uncontrolled boost. As in, you blew a hose somewhere between wastegate actuator and turbo compressor. I guarantee you will overboost in this scenario.

Post by **Legacy777** » Fri Aug 18, 2006 3:12 am

free5ty1e wrote:Dave and Josh - go pull the hose from your wastegate actuator, leave it off, and go for a WOT run. Then come back here and tell me you didn't overboost.

I'd love to....but the mother f@#$ing piece of shit isn't running....

Post by **DLC** » Mon Aug 21, 2006 8:15 pm

I'd love to...but I don't have a turbo

free5ty1e · Post by **free5ty1e** » Mon Aug 21, 2006 10:13 pm

...well then... I win!

hehehe

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MBC not controlling boost!! plz hlp

MBC not controlling boost!! plz hlp

Oops...figured it out