Air Filter Flow Testing

As previously mentioned, the pressure drop testing was done using a water manometer.  The manometer was procured from Dwyer Instruments for a very reasonable price.  

Dwyer Manometer

The green dye in the manometer is specific gravity 1.00, and the remainder of the fill was with deionized water.  I also put a test port on the airbox housing after the air filter.  A test tube was attached to the airbox above the filter being tested and routed into the car via the passenger window for testing.  The test port is threaded into the airbox and capped off when testing is not being conducted.  

No one is paid me to perform this test.  I am conducting it out of curiosity, interest, and as a service to all automotive enthusiasts.  I want to especially thank my wife for putting up with me, Scott H. (K&N), Chris O. (Baldwin) Kevin B. (Amsoil & Jackson Racing), and Cliff K. (Mazda filter) for donating air filters.  I bought the Napa filter, which is made by Wix.  Thanks to the support of the Miata community this test is costing me very little.  If it weren't for encouragement from fellow enthusiasts and their donations, this test would have taken much longer and probably would not have happened at all.  The price of gas and lack of time I have thanks to my job and school sure didn't help though.

Pressure Drop Test #1

Pressure drop testing is now complete for each filter.  The test measured the differential pressure from the atmosphere including the stock Miata air inlet pipe, airbox and across the filter.  The pressure port (actually a vacuum port due to pressure loss) was attached to airbox just above the air filter.  The air flow in the stock Miata flows through and inlet pipe to the bottom of the airbox and up through the filter and then through the air flow meter etc.  A tygon hose (clear plastic tube) was attached the pressure port and routed to the manometer inside the car.  I had my friend and coworker, Matt, ride in the passenger seat and record all data.  Each filter was tested a minimum of 4 times.  All of the filters were tested on the same day during a 3 hour span limiting any effects of atmospheric conditions.  The 3 hour period included a much needed stop for a late lunch and beverage.  Any other filter pressure tests will be conducted using one of the filters from this test as a control filter (most likely the paper filter).  All of the measurements in inches of water were taken at 6500 rpm at WOT in 2nd gear while climbing hills locally in western WA.  In all 22 WOT runs were made and 22 data points taken.  4 data points were taken for 5 of the filters.  Due to an ambiguous result on run #4 for the Amsoil filter, two more data points were obtained.  All of the data will be presented in the chart below.  All comparisons will be made using an average of all data points taken (4 data points for 4 filters and 6 data points for one).    

So what do these results mean?  For one, there is very little pressure drop across any air filter, and the difference between the best (K&N) and worst (paper) is very small.  Yes as total power output increases, air flow increases, and differential pressure would also increase.  So a K&N probably does yield some power on higher output race motors where every last ounce of power must be squeezed out.  On lower powered street cars, it is probably not much of an improvement over paper.  Basically, an air filter is first and foremost a filter, and should be chosen for it's filtration ability.  I know after this enlightening experiment, that is exactly how I will select my air filters.  Also remember, that this total differential pressure is measured from the atmosphere via the stock Miata air intake tube, airbox, accross the filter, though the test port, about 6 feet of tygon tubing and the manometer and back to the atmosphere.  This is not the pressure drop across the filter itself.  I may do a quick test with no filter ( I really hate to do it, but might anyway) to get a good estimate of how much is the system itself sans the filter.  

Also bear in mind the K&N is cotton gauze, the Racing Beat is a single stage foam type filter, the Jackson Racing and Amsoil are dual stage foam, and the Napa is a paper filter.  So cotton gauze does flow more air than foam as claimed.  Amsoil is the best foam filter for flow.  The foam filters are a real pain in the rump to service (this just my opinion and not fact).  For that reason, I will never ever own a foam filter myself.  I've owned several K&Ns and they are easier to service.  If you are buying a high performance filter for airflow, K&N is tops in this test.

Pressure Drop Test #2

Test #2 was conducted to find the pressure drop for the Mazda and Baldwin filters.  The Napa filter was also compared to them so the test results could be extrapolated to the first test.  During this test, no filter was also tested to determine the amount of pressure loss due to the factory airbox and intake plumbing.  This test was done in 2nd gear also, but data was taken at 6,000 rpm instead of 6,500 due to a mistake by yours truly.  The conclusions are still obvious, but it would've been nice to go to 6,500 like in the first test.  Here are the results.

For this test differential pressure for the Napa filter is lower due to different atmospheric conditions (different day) and 500 less rpm at the data collection point.  That said, it is still noteworthy that the Baldwin and Mazda filter do not flow as well as the Napa, but are still very, very close.  In all honestly, all 3 flow about the same or very nearly the same.  So it is safe to say from the first test, that the K&N is still the best flowing filter.  What is interesting to note is that there was a 5.1 inch of H2O differential pressure in the stock air box and intake piping.  If you subtract this different it says worst case one can see a 2 inch of H2O differential pressure loss due to the air filter.  This is equivalent to 0.072 psi or very nearly nothing.  The air filter posses very little restriction at all in this application.  As long as an air filter is properly sized for an application, the lost airflow will be very minimal.  This means that there is very little if any power to be found from removing the air filter, much less changing the filter type.  The K&N did flow better than the Napa Gold in the first test to the tune of 0.02 psi.  That is less than 1/3rd the loss in the factory piping, and it is a whopping 0.14% of atmospheric pressure (14.7 psi).  If you need the 0.14% better airflow, than the K&N is hands down the best filter.  

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