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Discussion Starter · #1 ·
Cross-posted from wrxhackers.com

As you run your UTec at higher altitudes, I believe it becomes increasingly important to perform the 'MAP Calibration' process as described on page-28 of the owner's manual. Out of the box my UTec, (probably like yours) was showing a Gradient and Intercept of 204 and 244 respectively. At my property which resides approximately 8000ft above sea-level, this not-surprisingly resulted in a dashboard MAP reading of -4.5psi with the ignition on but the engine not running. And coincidentally I was seeing a maximum MAP reading of approximately 13.5psi on the dashboard when my boost gauges were indicating an 18psi peak.

This evening I performed the calibration process and arrived at a Gradient value of 201, and an Intercept of 196. After plugging these values in, I now have a dashboard MAP reading of -0.2psi with the engine off, and my logged MAP readings now reflect those displayed on my boost gauges. Btw, I would strongly suggest that nobody use the Gradient and Intercept values that I arrived at.

On a related topic: If you perform the Map Calibration step (4) instead of step (2), DO NOT trust the vacuum reading as reported by your Blitz SBC i-D, and instead use a good quality electronic or mechanical vacuum meter. At idle, my SBC i-D reads -12psi of vacuum, yet my GReddy electronic boost/vacuum gauge was reading ~360mmHg of vacuum (approx -7psi). The vacuum measurement as reported by the SBC i-D is bogus I believe. The SBC i-D is not terribly accurate under boost either. It consistently under-reads by about 1psi at indicated boost levels around 18psi (when compared to several other mechanical and electronic meters that I have tested/observed).

Regards,
-Pace
 

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Discussion Starter · #2 ·
That was a post I made on wrxhackers recently. It caused a little confusion as I will describe below.

The confusion arose when some members implied that the recalibration process only impacts the displayed 'relative' PSI values shown in the UTec logs, and that since the MAP sensor reads absolute pressure this recalibration has no impact upon the mapping of the load columns. Pete from TurboXS then made the assertion that the MAP sensor reads 'absolute' pressure.

While this last statement is true, the MAP sensor actually outputs a voltage indicating absolute pressure. The UTec then applies a functional transformation to this voltage using the 'Gradient' and 'Intercept' constant values. It is these constants that determine how the extrapolated MAP value is zero-referenced. With the stock values, the MAP PSI output is zero-referenced to sea-level. This causes it to show -4.5psi as the zero up here at my altitude. However, if the transformed value were really 'absolute' it would show approximately 14.7psi as the zeroth at sea-level. This is evidently not the case.

So, next time you hear someone claim that the MAP sensor shows 'absolute' pressure, just remember that it reads absolute pressure, but the meaning of the MAP value displayed in the UTec is dependant upon the MAP calibration values for the Gradient and Intercept. It is this extrapolated value that determines the meaning of the load columns in the UTec fuel and timing maps.

As Pete observed, this means that the same effect (as recalibrating the MAP) can be achieved by simply reconfiguring the MAP minimum and maximum values. i.e. In my high altitude example, I could have simply changed the MAP range from the default 0 to 18psi, to -4.5 to 13.5psi which would have given the same mapping to the load columns.

:)

-Pace
 

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Fellas,

My learning curve is coming along pretty slow, but I was wondering if you could school me on the MAF and the affect it has on tuning with the UTEC. So far, I've only been able to utilize MAP, ign, inj, and A/F.

From what I've read about the MAF, what you're describing as absolute pressure sounds like a MAF reading (or is it just for the quality of the air, ie amt of O2?).

Thanks from a UTEC dummy,

Pat
 

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Discussion Starter · #8 ·
GPatrick said:
Fellas,

My learning curve is coming along pretty slow, but I was wondering if you could school me on the MAF and the affect it has on tuning with the UTEC. So far, I've only been able to utilize MAP, ign, inj, and A/F.

From what I've read about the MAF, what you're describing as absolute pressure sounds like a MAF reading (or is it just for the quality of the air, ie amt of O2?).

Thanks from a UTEC dummy,

Pat
The MAF sensor measures mass air flow. I was referring to calibration of the MAP sensor which measures manifold absolute pressure. You do not utilize the MAF for user tuning with the UTec since the load columns are either TPS referenced (for boost) or MAP referenced (for fuel & ignition). The MAF is still utilized by the WRX ECU, although I am a little unclear as to what use the UTec makes of the MAF readings. You should direct the specific questions around the MAF utilization to somebody a little better versed on the UTec internals, such as Nathan at TXS. :)

-Pace
 
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