jmussetter said:The MAF (Mass Air Flow) sensor is the one right after the intake filter on most cars, and uses a variety of techniques to measure the air.
There are HotWire MAF's that pass current through a wire to heat it and then measures the voltage it takes to keep the wire at a specific temp. The more air comming in, the more electricity it takes to keep the wire at temp. This is how I understand it at least.
Mitsubishi and others use a Karman MAF sensor, which has a honeycomb looking thing inserted into the intake tract and has a little diode looking unit in one of the honeycomb chambers. I beleive this one measures a portion of the air comming in and then calculates Total air based on the calibration of the honeycome unit.
I can't remember the name of the third kind of MAF sensor, but it uses a calibrated potentiometer with a plate attatched to it that swings back and forth. This plate sets in the intake tract, and the more air that comes through the tube, the more the plate swings out of the way from air pressure, which give a different voltage through the potentiometer. I havn't actually seen this type before, so I'm not sure how much they are used in newer cars.
Then there is the MAP sensor, which is the Maniford Absolute Pressure. This uses a pressure sensor inside either the Intake Manifold, or close to inside the cylinder and reads air pressure instead of volume, but uses the pressure to calculate the volume of air.
I believe that WRX actually have both a MAF and a MAP and it uses the both of them to get a more accurate reading of what is going on in relation to the turbo and intercooler and BOV.