The engine will flow 'x' amount of air, but with the factory setup, it's not flowing that amount, its flowing less. The factory paper filter is not too restrictive but an aftermarket filter is less restrictive. Also the factory tube, narrows about halfway down. Thus restricting the volume of airflow. As does as, the air intake silencer. If they still use them on rangers. I know my 2000 stang 3.8 has an intake silencer as does my expe, but its not a ranger. Anyways, and aftermarket tube on a CAI is about 4" dia. on avg. This allows an increase in air pulled through the system. A properly tuned exhaust assist in this "pulling air through" and allows the CAI to work even more efficiently. This being while gas mileage increases with these compontents.
Everything can be done with a stock intake, but maximum gains will not be seen. Its not as efficient as aftermarket. The gains from aftermarket CAI's are proven however minimal (3-5HP at het flywheel, 1-3 at the wheels and 2-3 MPG for the avg driver). If this wasnt the case they wouldnt sell like they do. As a matter of fact, Im pretty sure if they werent all people say they are then they wouldnt be made for our vehicles.
Most if not all results from these things have been dyno tested, both by the manufacterers who obviously embellish their results and the people who use them. Its even been tested if just removing the intake silencer will increase flow volume. It does.