Originally Posted by Jefe
I ran this way on my P5.
This introduced oil to my AFM sensor and inaccurate reading all together, and several CELS..
I'm not sure what a P5 is, but it sounds as if you're describing a hotwire MAF sensor, not a flapper-style AFM.
The MAF would technically work in this scenario, so long as its temperature compensation was sufficiently fast and wide-ranging. With a mechanical AFM however, it absolutely will not work.
AFMs measure only the volume
of air passing through them. The ECU then references its on-board barometer and the IAT sensor to convert the AFMs output to a true mass reading. The assumption is that the air passing through the AFM is at the same pressure (roughly) as the pressure being measured directly at the ECU.
If you double the pressure of the air (without, hypothetically, raising its temperature) then the mass of air (the number of mols) per unit volume will also double. You'll now be passing twice as much air through the AFM as the ECU thinks you are, because it doesn't know about the pressure increase.
Put another way, 1 cubic foot of air at atmo (1 bar) and 1 cubic foot of air at 14.5 PSI of boost (2 bar) are not the same amount of air. But the AFM cannot tell the difference between them. As far as it's concerned, one cubic foot per second is one cubic foot per second.