hmm, you might be on to something with the heat. Just thinking about it after reading your post. The 3bar is mounted on the firewall behind the headers.

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I wonder if the heat is affecting it. Cause just thinking about my fuel pressure also, my base FP is 45 and if I was actually running 26 psi I should see 71 psi of FP. I figured I wasn't since I was close to limit of the A1000. But just looking closely at my datalog, on the 2-3 shift my RPMs drop to ~4500 and Duty Cycle of the injs drop to 55%. So the A1000 should at the point spike up FP even higher since the 3bar shows the boost spike up. At 4800 in 3rd the AEM shows 17 psi (boost) and my Fuel Pressure shows 60 psi. 60-17 = 43 psi, pretty darn close to the 44-45 psi the Fuel pressure is showing at the start of the log.

So that really makes me think the 3bar is messing up and if it's checking ok with regulated air, something (heat maybe) has to be throwing it off. The only reason I don't think it's velocity affect is the fuel pressure regulator is T'd into the same vacuum line the 3bar is. It goes Intake 1/4" vac to T fitting and then to 3bar and Fuel Pressure regulator.

 

Sounds like your on it!

 

Bah, tried to log both sensors connected to the same regulated pressure source and heat the vacuum hose with a heat gun. But it's hard to say it made a difference or not. The PSI on the 3bar changed .2 between 21.3 and 21.5 when adding/removing the heat gun on the vacuum hose. For the next minute of log I pointed the heat gun directly at the 3bar and it didn't change at all.


Heat:
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No Heat:
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So back to the drawing board.

 

What does your signal line setup look like?

I have had this happen to me. Using a tee fitting and splicing into a line, then coming off the 90° nipple to another gauge. The guage with the signal line coming straight through the tee would see more pressure than the one that the signal had to go around the 90° angle. I switched it around so the signal coming in would have to 90 in both directions and it seemed to help.

 

or do the Y split, that might help?

 

90° fitting out of the big port on the back of the intake, to a 1/4" hose down to a tee fitting. The 3bar is on the straight through part of the tee, and the branch of the tee feeds the fuel pressure regulator. You can see the hose and 3bar in the picture above. The 1st white break in the hose is just a 1/4" straight connector, the 2nd is the Tee to the Fuel pressure regulator.

The Eboost2 is hooked to the smaller port on the back of the intake, right next to the big port the 3bar is reading off of.

The AEM sensor is connected to a 90° fitting on the turbo volute which goes to a Tee into the branch of the Tee. The straight part of the Tee goes up to the waste gate port and the eboost 2 solenoid. On the hose running up to the solenoid I put another Tee in to place the AEM, which the AEM *is* on the branch of that tee.

The only problem with thinking it's the branch showing it lower, is that the Eboost2 is reading the same as the AEM within a psi or so.


To do to the test tonight, I took a Tee and fed the compressor into the branch. So the 3bar and AEM are straight through to each other.

What I'm thinking of doing at this point, is get a new 3bar. Move it's mounting point away from the heat, get another AEM sensor and hook it up to the intake. So I can have 3 locations being logged. 1 before the intercooler and 2 after.

 

Fair enough. Maybe it is the velocity of the air rushing through the compressor housing ( or on the pipe just after) that is showing you a lower signal.

Don't know how you would feel about this but maybe tapping into the inlet tank on your ic might give you a better checking point. This plenum might act a little more like your upper intake.

You have proven that the sensors are reading correctly, now we need to figure out what the air is doing.

 

Could you have a bad earth causing a voltage shift?

 

The sensors are both using the same power and ground from the XFI. I would *assume* if one was off, the other would be off too.

 

Could the problem be that I have the Vacuum hose going from the intake and Tee'd off to the 3bar and the Fuel Pressure Regulator? Could the Fuel pressure be causing a pressure feed back into the vacuum hose and showing more pressure on the 3bar?

 

The diaphragm moving might cause a small spike but I wouldn't think it would make sustained changes in pressure on the vacuum line.

Did you enter the data points yourself for the sensors in your engine management system?(i.e. saying that 5V is 100psi and 0V is 0psi etc.)

Is there a spec on how much current the XFI can provide? Tying a few sensors together might be overloading the power supply rail(just a random wild guess, no experience with this system). But if one sensor gets a lower voltage to start with, then the values will be biased differently and you will read lower.

 

Yes, I had to do the settings in the XFI. I used the XFI calculations and the AEM data sheet to set it up. Settings in the XFI are 25 sensor gain and -12.5 sensor offset. Which in the above posts I checked with a regulated air supply to verify the settings are correct. Set the compressor to 10 psi and AEM shows 10 psi in the XFI. Same with 20, 30, and 50 psi.

The AEM and 3bar are using the same 5v reference and ground off the XFI. So if it did change things, the 3bar would read differently then it did a month ago without the AEM being wired in. Add to that, the above post is the XFI log showing both sensors on at the same time with the same air supply and they are within .2 of each other. I don't think it's a voltage or calibration issue.

I did just try to log again with the regulated air and turn on the fuel pump to see what the sensor showed. And when I turned the FP on, the 3bar would jump from 20.2 to 20.4 psi. Not a 6 psi difference I'm seeing on the track. But it also isn't moving the regulator diaphragm up and down a lot since the injectors are not sucking fuel off the line.

 

My guess would be that it's to do with the direction of airflow at the ports that feed each sensor, as stated above. That'd explain why the static tests are fine but when there's actually air moving through the plumbing the values change so much.

The one that's reading low is in the compressor housing? Isn't that the spot in the system with the smallest cross sectional area? So the airspeed is highest? If you were to move it only a few inches downstream into the larger charge piping, the velocity should be lower due to the larger cross section, and if it's a venturi effect dropping the pressure reading it should show less effect even without changing the entry angle.

 

The one (aem) in the compressor housing, and the eboost2 connected to the upper intake are both lower then the 3bar.

 

Are you able to verify these pressures using a mechanical gauge?
That would rule out the electrical end of this.

 

I don't have a manual gauge in it, not sure if I still have one laying around. The Eboost2 uses a map sensor inside the gauge, to it's electric also.

Tonight is last T&T until December, if my wife didn't have knee surgery I would be on the way to the track tonight to test some more. Might have to wait till tomorrow and do some back road testing.

 

Isnt the backpressure supposed to measure the pressure in the up pipe VS the pressure in the down pipe?

 

Those are.both backpressure. The up pipe vs manifold pressure are usually what is compared.

 

Some say turbine vs compressor .

Some say turbine vs manifold .

 

Just tried a couple more tests, first I got on the transbrake with no changes.

@ 4600 the 3bar shows 21.8 psi, AEM still connected to the turbo housing showed 14.95 psi.

Next I moved the Fuel pressure regulator hose to a different source. And moved the AEM to the same Tee the 3bar is on.

@ 4600 the 3bar shows 20.0 psi, the AEM shows 19 psi (on the same source as 3bar). I also watched the eboost2 gauge on the dash and it showed 19 psi.

Looks like the FPR was having an affect on the 3bar reading as well, since between test 1 and 2 at the same RPM it shows less boost.

Next I turned up the 2 step to 5300

@ 5200 the 3bar shows 22.9, while the AEM shows 21 psi. eboost2 just did reach 20.2 psi.

Fuel pressure never did get higher then 57psi. Will know more when I go to the track and can hold it WOT in 3rd. Seems the 3bar reads higher then then AEM as boost goes up. But the eboost also seemed to start lagging behind reading wise. Looks like the FPR was having an affect on the 3bar reading as well, since between test 1 and 2 at the same RPM it shows less boost.


edit: also should mention that the above readings are showing more boost then the track logs because my last pass I did add 10 numbers to the boost controller, but it didn't log the last pass.