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Old 07-24-2001, 07:58 AM   #1
Obscure411
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Default WRX Overboosting BAD! Help!

OK, I installed my BPM GT turbo-back system this weekend. I love it. I tried to retune my Joe P. MBC, since it seems to be creeping boost under WOT, hitting about 17-18 psi in 3-5 gear.

I turned the set screw counterclockwise, and the darn thing popped out! I couldn't find a long enough replacement, so I used a shorter one and kept it as loose as I could in the threads. Since then, I've hit the boost cut once yesterday and once today @ WOT.

I can't seem to get this thing to stay steady anywhere near 15 psi, today I couldn't even get it to stay below 19!

Is this really terrible for my engine, and how can I fix this problem? I only want 15-16 psi max right now, until I get my VF23.

Should I just get a EBC and call it a day, or is there a way to make this darn thing work?
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Old 07-24-2001, 08:05 AM   #2
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Get an EBC and call it a day

Jorge (RiftsWRX)
www.ProjectWRX.com
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Old 07-24-2001, 08:08 AM   #3
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Sigh... thought so ... but I really wanna make this work with the MBC, or I'm gonna have to spend the money I was going to use on the VF23 on the EBC...
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Old 07-24-2001, 11:32 AM   #4
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Anyone?
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Old 07-24-2001, 11:36 AM   #5
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Default Turn, turn, turn!

Try turning it the other way until your boost settles around 15 PSI. I've noticed on very humid days I can have it set for 15 psi and see it jump to 18 PSI on the highway. At that point I pull over and turn the boost down. The next day I gotta turn it back up... but thats the nature of the beast.
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Old 07-24-2001, 11:40 AM   #6
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Problem is... I can't turn it any farther to the left than I already have! No matter what I do, I can't seem to stop it from overboosting.

I know if I take it out now, it will definitely overboost. But this morning, on the way into work @ 7:45 am before the humidity and temps rose, why was it boosting at 19?

Also, the height of the set screw wouldn't have anything to do with this, would it? The replacement set screw is about 1/3 the height of the original. Makes sense to me that it is irrelevant seeing as how it should be the area beneath the thread that matters.
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Old 07-24-2001, 11:41 AM   #7
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yeah, correct me if i am wrong but wouldn't turning the screw counter-clockwise allow for more boost? so the obvious fix would be to turn the screw clockwise as boostaholic said. this is why i went with the Blitz SBCid. if you want no hassle boost controllin, you should get one too. (i thought i saw one in the private classifieds for 525 shipped, good deal since i got mine for 500 in a GB). good luck.
pj
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Old 07-24-2001, 11:42 AM   #8
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yes, the guy selling it is rak328.
pj
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Old 07-24-2001, 11:45 AM   #9
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DOH!!!

Don't tell me that... I'll go back and check the instructions now, but I could SWEAR it said counterclockwise to reduce, clockwise to increase...

But it would be just like me to screw that up (no pun intended)
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Old 07-24-2001, 12:04 PM   #10
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righty tighty, lefty loosey. the lower the screw = less boost, the higher the screw = more boost.
pj
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Old 07-24-2001, 12:06 PM   #11
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Maybe you have a small hole or leak in the hose and the pressure is bleeding off before the boost controller. I put a hole in mine when I removed the hose from the wastegate actuator.
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Old 07-24-2001, 12:07 PM   #12
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Yeah, that's how I did it... and the first set screw popped off entirely because I tried to loosen it too much.

I dunno, maybe it can't handle a 3" exhaust? Too low of backpressure?
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Old 07-24-2001, 12:13 PM   #13
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Quote:
Originally posted by pbnj9786
yes, the guy selling it is rak328.
pj
It is still available if anybody wants it!
Brand new in box-never been installed.
More reliable than mbc.
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Old 07-24-2001, 12:21 PM   #14
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Does anyone else have experience with this while running a 3" exhaust?

Are the people experiencing success still using stock exhausts?

Forced Performance (the people I bought the MBC from) tell me this is boost creep - it's common in DSM's and that even an EBC won't fix it because the wastegate isn't large enough to vent the pressure because there's very little pressure in the exhaust, or something like that. Turbo fundamentals are still like greek to me.

Is this type of overboosting really bad for my engine?
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Old 07-24-2001, 01:15 PM   #15
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Any time you add more space behind a turbo for exhaust volume the pressure on the impeller is less and the turbo spools more quickly and with less control, thus your overboosting problem. Seems to be though that your MBC should be able to handle this. After all, thats what a boost controller is really for. Overboost in grotesque amounts is bad for your engine. Your exhaust gas temps will get high enough and you will run lean enough to risk "limp" mode on the ECU and in extreme cases detonation in the engine. Limp mode on the WRX is actually good because the ECU forces a rich fuel condition to keep you from detonation. I have not heard of anyone else boosting so high that this has been defeated and frankly you would probably need more boost than the stock turbo can put out to go past this feature.

Yeah, the statement about the wastegate acting differently when there is a larger exhaust is true too, but there are lots and lots of people out there running this type of system without problems. An EBC might help you just because it is more capable of sensing the pressure volume and capable of controlling the turbo a little better than an MBC. I would suggest one of the heavy duty units like the Blitz DSBC - it has enough balls to handle something like 30psi. Or, you could wait for one of the ECU options to come up like Cobb or TurboXS which are already tuned to handle a 3 inch exhaust in conjunction with the stock ECU.

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Old 07-24-2001, 01:28 PM   #16
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See, this is exactly what I don't get...

The temps are hot around here lately, but if I tune it on a hot day, and the temp doesn't vary much - it should stick give or take a pound or two.

Imprezer was even using an MBC to run less than stock boost, if I recall, before he put the TEC-II on.

That being said, why is it that this isn't working? It couldn't be related at all to the height of the set screw, could it?
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Old 07-24-2001, 01:33 PM   #17
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I have a 3" turbo back with the Hallman MBC, no spikes at all. But I put the MBC on after the exhaust though, but shouldn't make a difference.
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Old 07-24-2001, 01:33 PM   #18
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OK. There is a big problem in this thread about tightening or loosening the set screw to control boost. The first thing to find out is what KIND of MBC he has.

Basically it's two different valve concepts that can be used with MBCs.

1) You can open a valve in between the pressure source and the WG actuator and bleed off some of the signal. These are usally the cheap "fishtank" MBCs and allow for tons of boost spikes and other inconsistencies due to length of hoses. The also hurt spool because there is always some signal getting to the WG before max boost is reach.

2) The other kind is the ball and spring or check valve. You put this valve between the pressure source and the WG actuator also. However, this valve restricts signal to the WG instead of bleeding it off. All of the pressure is up against the ball and when it reaches enough to overcome the force of the spring, it pops the ball out and then the air continues on to the WG. This is the optimum MBC setup. It allows for quicker spool because the signal does not reach the WG until it dislodges the ball and can continue through the valve.

As for righty tighty, lefty loosey on a Joe P MBC, you can throw all that out the window. A tighter set screw will compress the spring inside the valve more and hence raise the amount of pressure required to dislodge the ball and open the valve.

Back the screw out == less boost. Tighten == more boost. Say it with me now.

As for EBC's being better, IMO, I disagree. Most of the EBC's I've seen in operation will start fluttering the WG before full boost is reached and hurt spool. This is just my observations and should not be regarded as gospel. I'll keep my $50 ball 'n' spring manual until I get to lazy to pop the hood for a lil' more boost.

As an alternative fix to the overboosting, you may want to consider porting the wastegate a bit to allow more air to pass through it at higher engine speeds. A bigger flapper would allow you to open that hole up even more and help to control boost ALOT better.

-jay
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Old 07-24-2001, 01:51 PM   #19
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Does anyone have a diagram of how a ball and spring MBC pieces together?

I'm wondering if I got the wrong type of set screw, or maybe something else besides just the screw popped out from the threaded hole?
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Old 07-24-2001, 02:07 PM   #20
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Even if you lost the ball or the spring out of the valve( not sure they can come flying out) it should not allow the boost to rise up out of control. It'll actually lower it. I don't have a diagram.. sorry.

Can you tell me more about what the boost does? Does it continue to rise past where you want it? Does it jump past the setting and then come back down to your set level?

Spike and creep are two different things, and it means to different routes to solve the problem.

-jay
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Old 07-24-2001, 02:10 PM   #21
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It stays at about 16 psi in 2nd. I tuned the car in 3rd yesterday at 16 psi as well, and now it is slamming into the boost cut at ~19 psi. It's hovering at about 19 in 4th & 5th yesterday and today both.

Temp didn't change at all that I can see.

I'm gonna go smoke a cigarette in the parking lot and see if I can loosen it anymore without losing it completely.
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Old 07-24-2001, 03:03 PM   #22
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Quote:
Originally posted by Obscure411
Does anyone have a diagram of how a ball and spring MBC pieces together?

I'm wondering if I got the wrong type of set screw, or maybe something else besides just the screw popped out from the threaded hole?
Check out http://www.c-speedracing.com/howto/mbc/mbc.html for how to build a Ball & Spring Type MBC. hth's
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Old 07-24-2001, 05:14 PM   #23
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Hokay, think I've learned more about MBC's than I ever wanted to at work this afternoon

At any rate, it does appear that the ball and spring are still inside the unit. It also appears that there is a certain amount of slack between the top ball/spring and where the set screw goes in.

Don't know if it's supposed to be that way, but it is nonetheless.

Today on the way home from work, I'll be removing the set screw entirely. If it still creeps hardcore, I'll assume something is wrong with the spring itself and f-- the problems, I'll get an EBC.

I am correct that with the set screw removed it should limit boost to a minimum, right?

If I can get it to work, that money will go to a new turbo instead

Wish me luck.
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Old 07-24-2001, 05:16 PM   #24
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Oh and for phast,

FYI ... it appears that it is staying pegged at the correct setting in 2nd gear, and continues to creep well past it in the rest of the gears until hitting cutoff. Never goes back down again in any gear, or at least not by a measurable amount.
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Old 07-24-2001, 05:31 PM   #25
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Sounds like creep then. I honestly don't expect your EBC to solve it. Try another MBC(borrowed or stolen) before you dump the money. Just peace of mind.

-jay
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