The differential pressure test was performed using a water manometer where one psi of pressure is equivalent to 27.7" of water. The differential pressure was measured between atmospheric and the pressure drop after the air filter in the stock airbox. The max pressure drop in this test was seen at only 7.0 inches of water or 0.25 psi. The factory airbox and piping with no filter yielded a pressure drop of 5.0 inches of water or 0.18 psi. That means that the worst filter in this test only caused a pressure drop of 0.07 psi. In my opinion, this means that if you are picking an air filter based on performance, you probably aren't doing your car any favors. For the record, the K&N was the best flowing filter. Of the 3 types of media tested, the cotton gauze type filters flow best. There are other brands besides K&N for sale, of which most are probably made by K&N for resellers. Foam air filters flowed marginally better than paper.

In the end, paper or fiberous filters do remove more particles from the air before they enter your engine. The cotton gauze filters indeed offer better airflow. You have to decide for yourself whether you value ever last ounce of power or filtration. I cannot, nor will I make this decision for you. I do know that on a relatively stock car with a properly sized air filter, you indeed have very little if any performance to gain by swapping filtration material.