To answer the question regarding airflow, one must look at other things as well when it comes to airflow.
First off there is the size of particle the filter needs to filter out. In general, 5 to 25 micron particles are of the greatest concern when it comes to air filtration, as these size particles can and do get into the smaller clearances of bearings and such other mechanisms like valve rocker arms. If the filter is to restricted to filter out smaller particles, then the airflow is reduced. However, if it is too open then the filter is not efficient, and will allow larger particles to go through the media. Air filters are generally least effective when new, and will improve efficiency as the filter media becomes loaded. However, as this occurs airflow will decrease.
Airflow is indicated and measured in (CFM) Cubic Feet per Minute, and one must know the amount of restriction at the noted airflow in order to compare filters. At an airflow rate of . 25" water (H2O) utilizing a manometer, the Amsoil oiled foam filter had 200 (CFM) Cubic Feet per Minute compared to a Fram paper filter 80 (CFM) Cubic Feet per Minute. The K&N when oiled as per the instructions for off road service measured 100 (CFM) Cubic Feet per Minute. All these filters were of equal size.
Flow Restriction is measured as the pressure differential between the inside and outside of the filter element, and is expressed in inches of H2O or mercury. This is usually tested using a manometer with one side of the unit connected to the "inflow" side of the filter (higher pressure), and the other side of the unit connected to the "outflow" side (lower pressure). The pressure difference between the "inflow" side and the "outflow" side is then measured on the manometer. Our trucks have a similar device already on the air box that measures the pressure differential, called the "filter minder". As the air cleaner media becomes clogged and the air pressure drops on the "outflow" side of the element, the yellow disc will travel down and indicate when it is time to change the air filter element.
Life expectancy and efficiency of air filters are tested using SAE test method J726C where they introduce a measured amount of "AC Test Dust" of different particle sizes in the testing apparatus. The results of the tests are measured in "percent" of contaminant removed over a given amount of time.
Wayne
amsoilman