: A word about air filters

08-28-03, 08:46 AM
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 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 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 been around the automobile all my life. 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).

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.

08-28-03, 10:03 AM
While I applaud your undertaking in this matter. I feel your findings are most likely less than accurate due to a fairly uncontrolled test procedure. The 500 mile road test is anything but a consistent control environment and I'm kind of puzzled by the second inline filter for purposes of judging filter capability of the first one.
Finally, I am very much in disagreement with your statement " 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."
It is a sweeping generalization that you could never substantiate. I would suggest a little more background info on not only the different filter media and how each type performs, but also a more controlled system for doing the tests such as a flow-chamber. I'm not necessarily saying that your conclusions are wrong, I just don't believe that your test methods will produce quality data.

08-28-03, 11:30 AM
KN air filters and cotton gauze so obviously filter poorly. No comparasin to a good paper filter... I dont understand why so many people use these awful oiled messed for maybe 2-3 hp most of the time... they let in more dirt. Look at oil analysis. Look at the filter and look at the little holes you can see.

08-28-03, 01:01 PM
This is a subject that keeps coming up in forums like this. Unfortunately, most people seem to think that just because a certain brand of filter is less restrictive than another, it's automatically an inferior filter. This is not necessarilly the truth. There are several ways to increase airflow to an engine and it does NOT have to pose a danger to the service-life of the engine. First you have to understand a little about air filters, what they do, and HOW they do it. One very simple way to increase the airflow to any engine without increasing the amount of dirt injested is to increase the filter area. This can be done by simply increasing the SIZE of the filter. Filter material is rated in a certain amount of airflow per square inch, so by increasing the square inches, you increase total flow capability of the filter. Unfortunately, there's only so much space available in the engine compartment for a filter. This is why you see those HUGE external air filters on trucks. Their filters aren't any better at filtering than the OEM one in your car but in order to satisfy the flow requirements of those large engines, you need a bigger filter (or less restrictive filter media).
So back to the issue of the aftermarket filters being better or worse than the OEM. The first thing you have to decide right off the bat is what particulate rate is acceptable for your application. Because if you want to say that less injestion is always better, then you better start hunting for a place to mount a Hepa filter the size of one of those truck filters we were talking about earlier and prepare to monitor it and shell out a few hundred dollars every few thousand miles when it needs replacement. By now, hopefully you get my point. That there has to be a compromise between several factors;
1. space available for filter housing
2. flow rate of the filter
3. filtration level of media
4. service life
5. cost
Overall, even though some aftermarket filters MAY NOT filter quite as well as an OEM paper filter, they DO attain an acceptable level of filtration. The KEY word there is ACCEPTABLE. Only you can determine what that is though. Only you know how and where you drive, how often you change your oil, or inspect and/or change your air filter. The average person doesn't change their oil or filters at a proper interval so believe me when I say that how often you change it is far more important than what level of filter you use.

08-28-03, 05:20 PM
Wow.... That was a lot of information......

I think if the OEM paper were THAT restrictive, and K/Ns were THAT much better, the OEM would have used them.....

To me, id rather spend $7/yr to replace my paper filter than to spend $50 on a filter that could potentially gum up my expensive MAF, not to mention letting in more dirt.......

Also, even if it does give 2-3 HP, you probably wont notice the POWER increase...... Maybe it will SEEM like you are going faster because of the increased intake noise......