Now that you youngun's have finished talking - and making VERY good points, in my opinion, I want to throw in this one last observation:
The MAF sensor is nothing more than a heated wire whose resistance changes as its temperature changes. As air flows over it, the wire is cooled and the voltage drop across it is modified. This voltage drop is what is interpreted by the PCM as a quantity of airflow - more air molecules hitting the wire, more cooling (which is what Longshot said in a different way). Notice if the area around the MAF sensor is increased, more air gets by without changing the number of molecules hitting the wire, and, the mixture is potentially leaner.
But, we have O2 sensors in the exhaust that report the result of that leaner mixture to the PCM and it tries to compensate by adding fuel. As long as the mixture is not TOO lean, it can do this and you don't get codes, but, you're now operating near one end of the design envelope (instead of near the center).
However, as Longshot also said (not bad for a "fish guy"), fluid flow is tricky stuff indeed. As flow velocity increases, static pressure decreases, so, the air becomes less dense. The effect of having a constricted intake snorkel forces the airflow velocity to increase to fill the cylinders
at a reduced pressure! This is essentially the effect you get when driving at altitude. The MAF sensor "sees" all the flow though (as long as it fills the throat), so all is well. Opening the intake snorkel to 3" (an increase in diameter of 1" in my truck), changes the "choke point" cross sectional area from 3.142 sq inches to 7.069 sq inches! This doubling+ of the intake area greatly
reduces the intake flow velocity and increases the density of the air charge - without "fooling" the MAF sensor since it still "sees" all the air flowing around it. (Effectively, we've suddenly taken the truck to below sea level atmosphere conditions and it can make more power.)
Anyway, this is how a 70-year old sees things.
- Jack