The engine management is performed by a ML4.1 Motronic ECU.
The air flow is measured by a flap-controlled Air Flow Meter (AFM). Strictly speaking this is a volumetric flow meter, which means its measure depends on the air temperature and the ambient pressure. The ambient pressure has a wider relative range of values and also a higher influence on the volumetric flow then the air temperature (in theory and in practice too).
The following picture shows how the output voltage depends on the volumetric flow (the graph os inversed). On X we have the whole output signal range as measured by Motronic. On Y are shown values corresponding to the air flow, the absolute values being meaningless. This dependence was extracted from the Motronic maps which translate the measured value into LOAD. The dependence has the same shape on another Motronics used in Porsches.
The next picture is something more sophisticated. It shows the volumetric
air flow depending on the RPM and the intake pressure while VVT was turned ON
(and timing shift actually finished).
It has been captured on a car featuring a number of additional sensors and
a computer that collects all this measures while the engine is running.
The pressure was measured with a GM 0-1000Hpa (0-100Kpa) standart sensor.
The value range 0-1000 corresponds very closely to the 0-1000 Hpa pressure range.
The absolute Z value is meaningless.
What is important to note is that the dependences of the AFM on both RPM and pressure are very linear.
Yes, I'm an old-school boy, I'm still using gnuplot :) .