Air Filtration and Flow Test

As automotive enthusiasts, we are always looking for ways to increase the power output of our motors. Many aftermarket companies now manufacture and offer high performance air filters. Most all claim a power gain through increased airflow and some claim better filtration as well. In this test, many air filters will be compared. I personally have owned various makes of performance automobiles. This particular test is being conducted using a 1992 Mazda Miata with a bone stock engine (can't wait to modify it personally). I have many different turbocharger parts and components collected waiting to modify the Miata. The air filters tested include a Napa Gold, Amsoil two stage foam, Jackson Racing two stage foam, a Baldwin fiberous, a K&N, and a Mazda factory replacement.

The air filters will be mounted in the stock air box and filtration tested by placing a 2nd filter (after filter if you will) just before the airflow meter. A picture, as you will soon see, is worth a thousand words and shows this setup. A water manometer will be used to test pressure drop across each filter. Obviously the filters with lower pressure drop flow better, and have the potential to yield more power given the engine actually needs the airflow increase. The pressure drop across each filter will be measured in inches of H2O (water). The pressure drop will be from atmosphere using the stock Miata induction system to the air box just after the filter. 1 psi of pressure = 27.7 inches of water for reference.

I have a degree in mechanical engineering and am currently pursuing a master's in engineering management. I have been around the automobile all my life, and will be obtaining certification as an ASE certified mechanic this fall (for fun). My father worked in the automotive test industry my entire adolescent life. I've also spent much of my life at the race track, either racing as an amateur or with my father who was a crew chief for a stock car racer many years. He also was a crew chief at the 24 hours of Daytona and LeMans twice. I regret not being able to go to LeMans to this day. I'm a certified gage calibration technician, certified quality assurance inspector, certified as a refrigerant worker by the EPA, and work in the Navy as a mechanic in nuclear propulsion. I like to think I know a few things about machines including automobiles, but there is always something to learn.

I choose to conduct this test because of the conflicting information I see in advertisements and have read on the internet. Everyone claims that their filter flows the best, and removes the most dirt. If you think about it, flow and filtration ability are actually linked. A solid piece of metal would prevent any particles from entering the engine, but it wouldn't flow any air at all. On the other hand, the screen used on a window screen will flow well, but won't filter well at all. So if you think about it, the best flowing and best filtering is really contradictory in claim. I wanted to find out which filter really does filter the best, and which one really does flow the best. I have used almost every brand and type of air filter over the years including K&N. I had not used a foam filter until conducting this test. I have used every manner of off the shelf fiberous or paper filter.

There are basically four types of filtration materials currently in use for automobiles: paper or other fiberous (some appear much like fiberglass), foam, cotton gauze and stainless steel mesh. This test has used the first three, but not stainless steel mess. Steel mesh bathed in oil is some of the very first filtration materials ever used in an automotive application. There is a reason they were abandoned for paper in the '60s. I find it surprising that some aftermarket manufacturers are touting them now. In the links at the end, there is an interesting test of a stainless mesh filter.

The filters in this test were tested for both flow and filtration. The pressure drop across a filter is a good indication of its ability to rob the engine of needed airflow and hence power. Obviously the air filter with the least pressure drop is the highest performing. For the filtration test, I used a secondary filter after the filter being tested to catch any particles that passed through the first filter. The secondary filters were made by cutting apart an off the shelf Fram carburetor filter. The particles passing through the filter being tested leave a deposit on the secondary filter. The lightness or darkness on the secondary filter is an indication of how much dirt is getting through the filter being tested.

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.

The filtration test has been the cause of much argument and debate in some circles. Many contend that a color comparison (comparing shades of gray) is not scientific or appropriate. Again, this is a very low budget test, and there are other scientific analyses where color comparison is valid. In water chemistry a color comparison is often used to determine concentrations as low as parts per billion. Search for information on titrations (of which some are by color) or color comparators. In chemistry the color is compared to a known standard of specific ion concentration by color. If you have ever played with a fish tank chemistry sampling kit, then you have done color comparison yourself. In this air filter test, no attempt is made at determining actual concentration. A color comparison is used to determine real world filtration ability. Each test filter was used in the same car, on the same roads for 500 miles. The darker deposits indicate poorer filtration, and lighter ones better filtration. That said, both the cotton gauze type (K&N) and foam filters (Amsoil & Jackson Racing) showed the same levels of filtration. Both performed poorly compared to the fiberous or paper filters (Napa, Baldwin, and Mazda).

If you are interested in the filtration pictures or pressure test data, follow the links provided to my other pages. You will also find links to other tests, which I found interesting on the web. 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.