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Old 01-10-2008, 04:16 PM   #1
Scooby Guru
Member#: 71092
Join Date: Sep 2004
Chapter/Region: MWSOC
Location: Delaware County, Ohio
2005 2.5RS Wagon
Regal Blue Pearl

Default Effect of Temperature on MAFv, MAFg/sec, and Engine Load

OK, so being Ohio in the winter, the temperature is fluctuating up and down by 40F every week. This means that I'm getting data with IATs that swing that much. It makes comparing one run to another pretty crazy.

1) Due to the way the MAF sensor works, it reads higher voltage at lower temps if flow velocity/volume is held constant, right?

2) Is the car actually getting more molecules of O2 at colder temps since the cold air is denser? (aka, do the actual mass of O2 and the MAF sensor trend in the same direction with temperature)

3) I'm looking at the MAF Compensation A/B (Intake Temp) table. When is this compensation applied? Obviously MAFv is pre-compensation, but if I log MAF g/sec, is the compensation already applied? What about Engine Load (g/rev)?

Basically, I'm trying to figure out how to compare two runs made at different temps to each other to figure out if one flowed more air than the other.
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Old 01-10-2008, 07:16 PM   #2
Scooby Guru
Member#: 10108
Join Date: Sep 2001
Chapter/Region: TXIC
Location: Lake Travis, Austin, TX
07 STI
60Trim spun rod bearing


Log MAF G/S to see that. Tea cups will have to chime in on the specifics on when the compensations are applied, but all in all, colder air = more air mass. This is why you need injector headroom (for when it gets cold out).

You think temp fluctuations are bad there? Try 34 in the morning and 85 in the evening.

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Old 01-10-2008, 07:18 PM   #3
Scooby Newbie
Member#: 75643
Join Date: Nov 2004
Location: NV
2004 WRX
San Remo Red


The ECU has a compensation table for pressure and temperature for MAF. g/sec is g/sec no matter what temperature. That is the beauty of using g/sec and not CFM. Now your loads will be higher when it is colder out given the same throttle-RPM-gear.
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Old 01-10-2008, 09:18 PM   #4
Tea cups
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Member#: 103136
Join Date: Dec 2005

MAFv would increase if temperature drops given the same volume of air (greater density). Therefore, so would measured mass airflow (g/s) and load (g/rev) which is calculated based on airflow and rpm.

When you log mass airflow and load it is after the compensations (IAT and manifold pressure) are applied.
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