Map Sensor

If the airflow increases due to a change in the intake system, is the pressure inside the intake manifold changes or remains equal? The map is going to read the same even though more air is entering?

Your MAP sensor measure pressure only, this pressure reading is one portion of the information needed to complete the density calculation and then the fuel calculation. If you increase the air flow (supposedly both volume and mass) but keep the container the same size then the density changes (so the amount of air per specific volume). What helps me is to remember that air is a fluid and it therefore reacts and responds to stimuli in a very specific manner.

Hope this answers your question.

If you fit a larger (volume) intake with a larger cfm throttle body you will have less air speed but higher possible volume of air at any single rev range compared to a smaller intake and a smaller cfm throttle body. The change in air speed (along with other factors such as air temp and density etc) is what produces the differences in pressure which is what the map sensor is used for.

This is one of the differences between a speed density system and a MAF system - The MAF sensor directly measures airflow while speed density calculates it based on a number of parameters including manifold pressure. What this means is that within reason in a MAF system, any time you change the engine's VE by making a mechanical change, the MAF will read the increased airflow.

With a MAP sensor as your load input though if you change the engines volumetric efficiency you will need to adjust the fuel or VE table based on MAP and rpm to account for the increased airflow.