Depending on valve overlap, engine vacuum is generated. The MAP sensor would be placed below the throttle plate and it compares the amount of vacuum to sea level pressure or whatever pressure you at from altitude if used with a BARO sensor. This comparison is in the fuel mapping within the ECM. Use of MAP sensors are known as a "Speed Density" system and aren't as efficient as a MAF or Mass Air Flow sensor the "counts" the incoming air and measures in grams/cm3. There are different types of MAFs like a Vane Air Flow and the hotwire that uses current to heat a resistor in the intake air charge and that charge cools the resistor. The current is then altered to keep that resistor at a specific temp. When I made the comment about the MAPs being useless in a forced induction environment, I stand by that for the simple fact that with a supercharger there isn't a vacuum. There is always pressure greater than one atmosphere within the intake plenum. In a NA environment, vacuum is greatest @ idle (normally 17-22 inHg) and then as the throttle plate opens (stepping on the accelerator) atmospheric pressure is let in equalizing the pressure above and below the throttle plate which is how that specific sensor "measures" air. It really doesn't measure air but extrapolates or makes a guess.