[DannoXYZ]

"I still don't understand why it matters whether or not the air is above atmospheric. If the volume of air is metered, why does it matter if it is compressed?"

That's because it's the actual numbers of oxygen molecules that matter. In normal atmosphere, that's about 19-20%. The trick with measuring air going into the engine is what to measure. The old method was to measure air volume with the flapper-door AFM.

However the numbers of oxygen molecules per cubic foot of air changes with the air pressure as well. So if you measure 1-cubic foot of air at sea-level flowing past the AFM, it may have X number of oxygen molecules. But at altitude, that same cubic foot of air will have only 80% as much oxygen or 0.8X. But, the AFM still measures the SAME AIR VOLUME and injects the same amount of fuel. This would be 20% too much fuel (what was that '60s saying again?). So an altitude sensor is used along with the AFM in a two-way calculation. The AFM sends raw CF flow, the altittude-sensor sends its data and the computer figures out an appropriate amount of fuel compensated for lower air-density..

The temperature of the air also determines the number of molecules of oxygen per cubic foot as well. On a cold day, the air is denser and a CF of air will have more oxygen molecules than a super-hot day, which may only have 95% as much oxygen per CF. So the AFM has another sensor in it, a temp-sensor. This 3rd piece of data goes to the computer as well and it will re-adjust the fuel-calculations based upon air-temperature.

So at a minimum, the fuel-calculations is a function of f(CFM-flow,altitude,air-temp)

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BTW - a MAP sensor does not measure air-flow or air-volume in any way. It only measures existing air-pressure, a static value. A MAP sensor would put out the same signal with the engine at WOT@2000rpm as the engine at WOT@6000rpm. However, the CFM flow of air going into that engine is 2.5x higher at 6000rpm than at 2000rpm. So you would have to integrate MAP with respect to time in order to come up with the CFM flow rate, so CFM-flow = f(MAP,t1...t2) then plug into the previous equation above. The DME Motronic does not do this integration calculation, so you'd need an additional computer if you want to use a MAP sensor.