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Old 08-07-2012, 07:49 PM   #1
Wikster
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Member#: 318921
Join Date: May 2012
Chapter/Region: TXIC
Location: Addison, TX
Vehicle:
2012 Impreza WRX
Obsidian Black Pearl

Default Atmospheric/Barometric Pressure Sensor/Reading

When I monitor or datalog the barometric pressure reading, it reports 14.4 psia. It does not change at all from day to day or weather conditions and is does not match the true atmospheric pressure values reported by the airport I work at. The typical delta error I have seen has been 0.3 to 0.5 psi (14.7 to 14.9 psia actual atmospheric).

Just wondering if this normal or what? I also wonder how this affects the turbo performance if boost is calculated from manifold minus barometric. I have tried both the Cobb and Torqued Performance maps with no effect on the barometric pressure reading.

Current Setup:
2012 WRX Hatchback
COBB Access Port
TP Stage 1 91-93 Oct.

Thanks.
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Old 08-07-2012, 11:26 PM   #2
1QUIKV6
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Member#: 9427
Join Date: Aug 2001
Location: Andover, KS.
Vehicle:
2006 WRX STi
WRB

Default

I bet the values you're getting from other sources are corrected for altitude. Uncorrected baro isn't usually readily available info.

Looks like Addison's airport is right around 644' altitude which means 14.36 PSIA during standard day conditions...spot on with your logger.

http://www.digitaldutch.com/atmoscalc/

As far as the boost targets table goes it's all based on sea level/14.7 PSIA so your actual boost pressure at your altitude should be 0.3 psi lower than it would be at sea level.

Now with the guys at real altitude (IE 1000+ feet) don't need to fret because there are baro multiplier/offset tables that gains authority to influence target boost so that you stay at (roughly) X psi of boost over actual ambient regardless. I think the Subie engineers called +/- 0.3 psi good though so they didn't give it authority for the initial low altitude changes.

Last edited by 1QUIKV6; 08-07-2012 at 11:57 PM.
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Old 08-08-2012, 09:57 AM   #3
Cobb Tuning
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Member#: 4803
Join Date: Mar 2001
Location: Austin, DFW, Portland, SOCAL
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Quote:
Originally Posted by Wikster View Post
When I monitor or datalog the barometric pressure reading, it reports 14.4 psia. It does not change at all from day to day or weather conditions and is does not match the true atmospheric pressure values reported by the airport I work at. The typical delta error I have seen has been 0.3 to 0.5 psi (14.7 to 14.9 psia actual atmospheric).

Just wondering if this normal or what? I also wonder how this affects the turbo performance if boost is calculated from manifold minus barometric. I have tried both the Cobb and Torqued Performance maps with no effect on the barometric pressure reading.

Current Setup:
2012 WRX Hatchback
COBB Access Port
TP Stage 1 91-93 Oct.

Thanks.
That is a reasonable reading for your altitude. The baro pressure sensor (in the combi-valve as part of the air pump stuff) will usually give you a very off reading if there's a problem with it. You can always do a sanity check of baro reading by looking at the manifold abs. pressure reading (from MAP sensor) with key on, engine off. It should be pretty close to baro reading (won't be exactly the same always - might be off a few tenths or so, but that is normal).

Bill
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Old 08-08-2012, 07:06 PM   #4
Wikster
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Member#: 318921
Join Date: May 2012
Chapter/Region: TXIC
Location: Addison, TX
Vehicle:
2012 Impreza WRX
Obsidian Black Pearl

Default

Quote:
Originally Posted by Cobb Tuning View Post
That is a reasonable reading for your altitude. The baro pressure sensor (in the combi-valve as part of the air pump stuff) will usually give you a very off reading if there's a problem with it. You can always do a sanity check of baro reading by looking at the manifold abs. pressure reading (from MAP sensor) with key on, engine off. It should be pretty close to baro reading (won't be exactly the same always - might be off a few tenths or so, but that is normal).

Bill
Thanks Bill. I think just realized that the actual barometric sensor is calibrated in "Bar" units and not in "PSI" units so there are rounding errors in the conversion and that the reported vs. actual delta is reasonable. I guess I was expecting something like a lab sensor here at work (0.25% accuracy) but I imagine for automotive cost effectiveness Subaru would be using in this a case a sensor with lower accuracy.

Today's values
KADS METAR is 29.92 inHg = 1.013 Bar
AccessPort log is 14.36 psia = 0.990 Bar (engine running)

I haven't tried logging the value with the engine off to see if the reading is closer to actual and will try that at this point out of pure curiosity. Thanks again.
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Old 08-09-2012, 08:39 AM   #5
Cobb Tuning
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Join Date: Mar 2001
Location: Austin, DFW, Portland, SOCAL
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Quote:
Originally Posted by Wikster View Post
Thanks Bill. I think just realized that the actual barometric sensor is calibrated in "Bar" units and not in "PSI" units so there are rounding errors in the conversion and that the reported vs. actual delta is reasonable. I guess I was expecting something like a lab sensor here at work (0.25% accuracy) but I imagine for automotive cost effectiveness Subaru would be using in this a case a sensor with lower accuracy.

Today's values
KADS METAR is 29.92 inHg = 1.013 Bar
AccessPort log is 14.36 psia = 0.990 Bar (engine running)

I haven't tried logging the value with the engine off to see if the reading is closer to actual and will try that at this point out of pure curiosity. Thanks again.
Yeah, I don't think you have a problem at all. The test with engine off would be logging Manifold Abs. Pressure (from the MAP sensor) vs. Baro Pressure (from the baro sensor). With the engine off, the Manifold Abs. Pressure reading should be fairly close to the Baro Pressure reading (won't be exactly the same).

Bill
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