"OK fellas, here's the deal. I spoke directly with Jordan Flagstad - I was the one he gave those flow numbers to. He is an engineer at Donaldson. Donaldson makes the big filter (B085011) that Dale Isley puts on his Tymar setup. Donaldson makes the AIS kit and the Power Core filter that goes into it.

Donaldson did the comparison flow testing. They tested the flow through their entire AIS kit as they sell it (or as Ford dealers sell it), and they tested their B085011 filter alone hooked up to the flow test equipment. And the flow through the Power Core filter installed in the AIS housing, plastic housing and all, showed 8.3" of restriction at 500 CFM flow. The flow through the B085011 filter alone, hooked directly up to the test equipment (ie. - open to airflow all the way around the filter) showed 10" of restriction at the same 500 CFM flow. This is how Jordan explained it to me, and this is why I am replacing my Tymar with the AIS. "

It seems to me that the ability of an air filter to protect your engine should be the top criteria for selecting an air filter and the "Dirt Filtering Efficiency" of an air filter is given by...

"Dirt Filtering Efficiency"={"Dirt Holding Capacity"}/{"Dirt Holding Capacity"+"Dirt Passed"}

...and if you multiply this result by 100 it gives the percentage of the dirt in the incoming air stream that's stopped by the air filter so that it doesn't enter your engine.

For an ISO-5011 test the measurement of "Dirt Holding Capacity" and "Dirt Passed" is done by establishing a 628 CFM airflow through an initially clean air filter and then introducing into the airflow stream a controlled amount of "standardized dust" at a controlled dust feed rate of 17.58 grams/min. The "standardized dust" meets an ISO-12103 standard which has a specified distribution of particle diameters ranging from less than 0.5 microns to 150 microns and the peak of the particle distribution curve is at a 60 micron particle diameter.

As the filter does its job of filtering out the dust that's been introduced into the airflow stream the filter becomes progressively more restricted and the suction is increased so as to maintain a constant 628 CFM airflow through an increasingly dirtier filter. When the inches H2O restriction increases to 10 inches H2O above the initial restriction for a clean filter at a 628 CFM airflow the test is terminated and the results are tabulated. The "Dirt Passed" is collected with a downstream sub-micron HEPA filter and then weighed and the "Dirt Holding Capacity" is determined by weighing the clean and dirty filter under identical conditions of ambient pressure, temperature, and relative humidity.

For the stock FA-1750 filter the "Dirt Filtering Efficiency"={542.0}/{1.31+542.0}=0.9976=99.76% but this means that 0.24% of the ingested dirt still passes through the filter and as the miles accumulate this adds up to a lot of dirt going into your engine especially if you drive in dusty conditions!

The Donaldson Power Core Ultra Web technology blue media used in the AIS FA-1757 element has a "Dirt Filtering Efficiency" of 99.97% which means that only 0.03% of the ingested dirt passes through that filter and this means the stock FA-1750 filter passes a factor of x8 more dirt than the AIS FA-1757 does!

So what's the "Dirt Filtering Efficiency" of a WIX 46637 or any of it's crossed-referenced "so-called" equivalents such as the Baldwin PA2818 or the Donaldson B085011? Well the only information I've been able to find is from the Donaldson website which states that "Donaldson’s DuraLite Air Cleaners" which includes the Donaldson B085011 are rated for... "Gas and diesel engines in light to medium dust conditions".

So the Donaldson B085011 filter doesn't seem to be a good choice for the dusty conditions that many trucks encounter! Also based on the price difference the WIX/NAPA 6637 versions of the Donaldson B085011 might not be of equal quality or perhaps by now they're just like most other commodities and all made at the same factory in China?

A dry paper element filters on a "go or no go" basis where dirt particles that are larger than the "openings" in the filter media are trapped while particles that are smaller than the openings can pass right through. For a treated or impregnated paper element with the same size "openings" in the filter media many of the smaller diameter particles stick to the surface and don't pass through the element resulting in improved "Dirt Filtering Efficiency" of "invisible" particulate matter with very small micron diameters. Since the surface of the stock FA-1750 element has a kind of "waxy" feel and the 6637 seems to employ a plain dry paper element I doubt the 6637 filters any better than the stock element does and perhaps not even as well!

In case anyone's curious as to why a "628 CFM airflow" is used it's because the ISO-5011 test standard includes a formula for the required testing CFM that depends on engine displacement and on whether or not it has a turbo and for any 7.3L engine with a turbo a "628 CFM airflow" is required for the dirt filtration testing.

Some filter vendors have been known to test at a CFM that's lower than the ISO-5011 test standard CFM and since a larger percentage of dirt gets filtered out at a lower CFM this allows claims of higher "Dirt Filtering Efficiency"! Also some filter vendors test with "coarse dust" which only includes particles ranging in size from 5.5 microns to 176 microns as opposed to the ISO-5011 test standard which includes particle diameters ranging from less than 0.5 microns to 150 microns.