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I've been experimenting lately with using a bleed valve to get a small increase in my boost levels. No intercooler yet, so I'm only looking for 1 or 2 psi. I've tried plumbing the bleed valve into the stock plumbing every way I can think of, and the only thing I've managed to do is get it to spike.

So far I've tried: 1) Line from compressor to solenoid. 2) Line from solenoid to wastegate. 3) Solenoid to intake tract after the restrictor and 4) Solenoid to intake tract before the restrictor.

I've searched here, NASIOC and the RS Liberty club. RSLC says to plumb it in before the restrictor, but that didn't do anything for me. Plumbing it into the line from the solenoid to the wastegate nets me a spike dependant on how far I've opened the valve, but the ECU quickly compensates and lowers the boost back to 9.

So... Am I missing something? Should I open the valve more and shorten the lines as much as possible?Where should I put the valve?

I don't want to run a pure MBC, as I'm often lazy and don't want to have to worry about PTFB. Plus, I don't know how I'd make sure others that had to drive my car didn't expose the car to that either.

The expertise on NASIOC and the RSLC isn't quite applicable, it seems... Our cars use feedback from the pressure sensor in boost control, while WRXes and (at least most) RS Liberties don't.

You probably won't be successful in using a small bleed to get a little more boost while still using stock boost control.

An MBC plumbed correctly shouldn't give you PTFB, though...

Thanks Vikash. Glad to hear its not just my car

Yeah, I remember reading the discussions about MBC's and where to hook them up. Maybe I'll finally put mine in then.

Just go with a MBC (build it yourself for around $4-5). You can use a bleed valve on the pressure sensor line that comes from the #1 intake runner to the pressure sensor as a FCD later on when you have supporting mods and want more then 13 psi (sea level).

You could try to bleed a few psi from the MAP sensor like I did for a FCD. You will need a small check valve so it only bleeds positive pressure not vac. If your computer is so smart maybe it will raise the boost when it sees lower MAP pressure.

why's everyone want to bleed? Ball-and-spring style MBCs are way better...

I was thinking about trying a check valve and the bleed valve on the MAP sensor, at least to see what happens. If that doesn't work, then my MBC goes on.

Chris, for now at least, I like the idea of having the ECU trying to do something about the boost level. I'm not concerned with keeping the wastegate closed as long as possible, but rather being able to drive it however I feel and not get in trouble. Just looking for that little bit more oomph

Well don't worry about PTFB, it's a bunch of crap.

There is no way I'd use a ball&spring on the MAP sensor! As a MBC it's great but what we want here is the ECU to see a few less psi.

If it was me I'd just go with a MBC and ditch the stock boost control setup.

You can use a ball-and-spring valve as a relief valve on the MAP sensor to limit the pressure it sees.

I don't understand what you mean about PTFB being a bunch of crap.

I agree that it's probably best to just use an MBC and bag the stock control.

I didn't mean I was going to stick my MBC on the MAP sensor. I meant I would probably say to hell with the whole thing and just use my MBC to control the boost instead of the factory solenoid.

Sorry for the confusion.

All I was saying is, if you want control over your boost, a ball-and-spring MBC will do it nicely. Stock controller doesn't need to have control over the wastegate at all... why make things more complicated?

True.

I don't believe any of the crap about PTFB being bad. None of it. Not for this car.

I don't see how you could use a ball&spring to limit the pressure to the MAP sensor, or lower it. All you could do is limit all pressure till the opening of the valve and then you would get full pressure.

I still think it might work to bleed off a few psi from the MAP sensor. Just an idea I thought might work for you, throwing out some wacky stuff.

-K- wrote:I don't believe any of the crap about PTFB being bad. None of it. Not for this car.

Full boost at part throttle means overspeeding the turbo and generating lots of heat. Why isn't that bad for this car?

I don't see how you could use a ball&spring to limit the pressure to the MAP sensor, or lower it. All you could do is limit all pressure till the opening of the valve and then you would get full pressure.

You'd use a tee and maybe a restrictor, maybe like this:
That ought to prevent the sensor from seeing pressures above the setting of the valve.

If your MBC is hooked up right, off the turbo outlet not the manifold you won't be overrunning the turbo. You might be running it faster than you need to but not over what it would spin WOT. I stand behind it being a problem for the WRX because of the ECU.

I know you could use the MBC like you drew it out. But what I was looking for was an even 2psi pressure drop across the whole sensor range. That might trick the ECU into giving a few more psi with the stock boost control.

I could also see a straight ramping FCD doing the same thing. I don't know if the ECU will raise the boost but with exhaust I'd think it would go up a little.

If you hook the MBC up right -- as you describe -- you won't get PTFB.

I don't know that much about the ECU's model for variations in boost control dynamics, but you may be right that you could get a little bit more boost by tweaking the sensor signal. 2 psi seems like a lot, though.

I could get full boost without being WOT. Up a hill is the easy way. It may not be "full boost" but I can get plenty like that.

I think it would be fun to try that out and see if we can't trick the ECU into a little more boost. Opening up the exhaust would be required I think. We don't know how much range the ECU has to adjust but it has some, it adjusts for elevation right?

But if you press the pedal further, you'll get more boost, right?

Yes, it does try to adjust for elevation, shooting for an absolute manifold pressure of about 23.4 psi.

I've got my bleed valve hooked up between the pressure exchange solenoid and the MAP sensor, with a check valve between the bleed valve and the T in the line, so vacuum doesn't draw air in.... 11 psi with my rough tuning

I'm going to hook up your Scantool Vikash, and see if its consistent across the board. I also want to see what the ECU thinks its giving me. But so far, the car doesn't feel like its complaining about it. I'm gonna turn it down a bit until I get it intercooled.