LMM (MAP vs Boost disparity)

[killerbee]

Boost2_DMA and MAP are different by 2 psi roughly. MAP reads 2 psi below Boost, and Boost is correct, at 14.0 psi.

Does anyone know the reason for the disparity? 2 sensors? The only one I know of is the one by the intake valve.

Which is referencing C0808 and C0809, "MAP" diagnostics?

[drnated]

Actual Boost is measured at the turbo. Map is measure in the boost tube after the intercooler. There is a slight pressure loss as the air is cooled in the intercooler.

[killerbee]

Actual Boost is measured at the turbo.

Care to clarify this part? Where is the sensor located?

Also, at idle airflow, which is basically nothing, there is no compression, therefore no heating, and therefore no CAC cooling. At low, and even no flow (engine Off) the disparity still exists.

[JoshH]

Actual Boost is measured at the turbo. Map is measure in the boost tube after the intercooler. There is a slight pressure loss as the air is cooled in the intercooler.

That is not correct. There is only one MAP/Boost sensor.

[killerbee]

Josh, any speculation as to why the disparity? I did notice that it appears to be a percentage, the numerical difference grows with increasing boost.

[bballer182]

Well, I'll take a stab at it and say that there might be a problem with the BOOST2_DMA, Beeing that it's a DMA pid that means that the pid is not a pid that is "standard supported" on the ECM and was created by Ross based on other pids. Very similar to the calc.pids we use but are calculated in the ECM or something

[JoshH]

Josh, any speculation as to why the disparity? I did notice that it appears to be a percentage, the numerical difference grows with increasing boost.

No idea, but I would guess maybe some sort of calculation error where EFI converts the raw data the ECM sends it into something we can understand. I know the Boost2_DMA PID follows what the mechanical boost gauge on my dad's LMM says.

[FUBAR]

This is EXACTLY the case I ran into and spent the last little bit trying to rack my brain why would there be a disparity in Data values from the SAME sensor!? Best solution after talking to some well known tuners is the stock GM MAP sensor is only capable of "x" amount measurment and is not a stable or reliable source to be accounted for and is really meaningless once one steps up to a bigger turbo and is boosting above...say 35 p.s.i. I was told a mechanical boost gauge was to be relied upon and the boost tables in EFILive are irrelevant when controlling boost with a bigger charger (or all the time (stock) for that matter.) Then more emphasis on fuel tables and Turbo Vane (if a VGT turbo is installed) tables really are the sole source for controlling boost.

I did notice that even at idle, on EFILive the MAP sensor would read 12 psi when the Boost reading would read 14.7 psi which would be correct given the low altitude I was at.

Bottom line I got, was run a mechaincal boost gauge, and throw the Desired Boost tables out the window.

[killerbee]

huh???

[FUBAR]

I was getting 1-6 psi boost difference from the MAP PID and Boost PID on my LBZ. Less difference a idle and grew greater as boost increased.

And I was explaining the possible theory's of disparity.

[FUBAR]

How would one be able to use desired boost tables if the charger is putting out 40 lbs or so and the MAP sensor is only capable of 35 (including BARO too which would only be 20 psi from the turbo)psi. How is the ECM then able to correctly interpret absolute pressure and implement boost control? i.e. 100mm3 of fuel at 3000rpm 45 psi boost when the MAP sensor's only capable of 35. Also it just seems a mechanical boost gauge would be more accurate than the stock sensor.