Boost gauge accuracy?

[J-MoNeY]

When ever I get on it, I happen to notice that my boost guage flutters. It won't hold a constant boost pressure and the needle shakes at 9 psi. I have a Saab 900 intercooler. Could it be a leaking IC hose?

[vrg3]

It could be a boost leak or it could be a leak in the lines to the gauge. Do you feel fluttering in the engine's acceleration?

Where is your boost gauge getting its signal and how did you tap it?

[createnew]

I had the same thing happen to me when I hooked up my MBC in line before the Controlling solenoid. You could actually feel the engine's power fluctuate. It may be because my solenoid is a little old and worn out. I tuned the MBC a little more and it went away.

If your not using a boost controller, it's probably a bad solenoid or a leak like VRG said.

[vrg3]

createnew - Why did you plumb your MBC in series with the factory boost control? Why not simply use the MBC by itself?

[J-MoNeY]

createnew - Why did you plumb your MBC in series with the factory boost control? Why not simply use the MBC by itself?

I thought I read somewhere in some post that we had to use the factory boost controller in conjunction with a MBC?

vrg3: I feel no flutter in the engines acceration. It pulls smooth, but feels like it misses at idle. Almost like there is a vacuum leak. I "t'd" into the line that comes off the 3rd intake runner I believe with a 3 way plastic T. I used vacuum hose instead of that crap they give you with it.

[vrg3]

In my opinion it doesn't make sense to try to use both systems. You'll just end up with a harder-to-control compound system.

Are you using an MBC, J-Money? If you are, I suggest connecting it directly between the compressor outlet and

About the flutter, I do know one of the reasons boost gauges typically come with 1/8" nylon tubing is to reduce flutter. The thin cross section damps oscillation, and the rigid structure avoids kinks which can cause turbulence.

You say it's almost like there is a vacuum leak -- are you positive that there isn't a vacuum leak?

[J-MoNeY]

In my opinion it doesn't make sense to try to use both systems. You'll just end up with a harder-to-control compound system.

Are you using an MBC, J-Money? If you are, I suggest connecting it directly between the compressor outlet and

About the flutter, I do know one of the reasons boost gauges typically come with 1/8" nylon tubing is to reduce flutter. The thin cross section damps oscillation, and the rigid structure avoids kinks which can cause turbulence.

You say it's almost like there is a vacuum leak -- are you positive that there isn't a vacuum leak?

I wish I knew. I concected the lines securely, but I will double check, and yes I am using a MBC. If i don't need the stock boost controller then can't i disconnect it and take it out of my car completely?

[vrg3]

You should leave the stock solenoid in there just so the ECU doesn't get upset.

Remove the two vacuum hoses going off from the side of the solenoid, and put vacuum nipple caps on them. You can keep the hose coming from its bottom going to the resonator if you like; it shouldn't matter.

Then connect your MBC directly between the compressor outlet nipple and the wastegate actuator nipple.

You can look for vacuum leaks with a can of starting fluid; do a search if you need instructions.

[ciper]

Fluttering boost gauge is sometimes normal. What type of tubing do you run?

[J-MoNeY]

Fluttering boost gauge is sometimes normal. What type of tubing do you run?

I think it's like 3/8's vacuum hose. It's the right size to fit on the T and the back of the boost gauge. I also zip tied all ends attached to something.

[Sir Yach-o]

Vrg3's right. The ECU will go limp if it sees that there is no factory solenoid, and I think you'll get a CEL too. So just leave it plugged in electrically, and just take it out of the vacuum hose loop and substitute the MBC. As far as the ECU knows, it still thinks that it's the one controlling the boost. How wrong it is!

peace,
-mike

[createnew]

VRG - "Why did you plumb your MBC in series with the factory boost control? Why not simply use the MBC by itself?"

VRG, my Turbo XS standard boost controller is simply a bleeder valve and provides no restrictian. The most boost I could get without the solenoid hooked up was about 12 psi, and it went down to 10 psi pretty quickly throughout the gear. The only way to get more than 12 psi was to hook it up with the solenoid.

I just bought a Turbo XS High Performance MBC (a restrictive type). So that should fix all my problems and I can run without that solenoid.

[TurboLegacy]

for a mbc do you really have to have a boost gauge, couldnt you just turn it down all the way and adjust it alittle, cuz even if u went to high, wouldnt the fuel cut come in at 13psi, then you could turn it down till it doesnt cut out anymore.

[eastbaysubaru]

Not a good idea. Get a boost gauge.

-Brian

[vrg3]

You could do without a boost gauge if you define your desired boost level to be "just below fuel cut."

I think that's a bad idea, though.

You should set your boost level to correspond with the other modifications you've done. A stock car should have stock boost. An intercooler or a free-flow exhaust can each justify slight increases in boost. Both together justify a larger increase. Any other approach pushes the system outside its design limits unnecessarily.

The stock fuel cut isn't even reliably at 13 psi on all cars, and varies with altitude. And, it only happens after three seconds of sustained overboost. And there's always the possibility that for one reason or another your stock fuel cut might not happen at all (it's happened).

If you can afford the modifications to back up increased boost, it only makes sense to buy $25 worth of instrumentation. Modifications need to be controlled; blindly turning up boost is asking for trouble.

[TurboLegacy]

i turned up the boost a little , without a boost gauge. i only added enough to make it a little faster. dont hate me.

[vrg3]

I don't hate you. I just disagree with how you did it . If you start doing things like installing RHD headlights on your street-driven car, though, I might start...

A way to find out how much boost you're running without buying a boost gauge could be to measure the signal from the pressure sensor with a voltmeter. You have to measure the power supply voltage to the sensor and the signal voltage at full boost, and then pop it into the transfer function of the sensor.

If you want to try, the information you need is on these pages:
http://www.graphics.cornell.edu/~v/fcd/
http://www.graphics.cornell.edu/~v/fcd/fcd_install.txt
http://www.graphics.cornell.edu/~v/ecupins/