Not a K&N fan at all...pretty widely known around here. Below is a copy/paste of some of the reasons....
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There is a lot of marketing hype surrounding certain aftermarket air filters and we wanted to know if the filters could stand up to their claims. Claims of "superior" filtering ability and dirt holding capacity are among some of these claims. Additionally, many filters are claimed to allow for "better" air flow giving you more horsepower.
This claim of better flow giving more horsepower is a debate all its own, but dyno tests run with a paper filter vs. NO FILTER AT ALL have shown NO INCREASE IN POWER OUTPUT. Therefore a filter with "better flow" will not only give you no increase in performance, it will also let in a lot of dirt while doing it.
The following data is provided by Testand Corp. in Rhode Island. Testand makes the $285,000 machines that perform the SAE J726/ISO 5011 air filter test standard. Any air filter that wants to be tested for performance and efficiency uses this test. These tests cost $1,700 per filter when done by an independent laboratory. Testand Corp. was interested in the comparison study and agreed to do the study for us.
Every filter listed was tested in an identical manner according to the SAE/ISO test standard> Here are the results:
In the order of EFFICIENCY (ability to filter dirt) the results are as follows:
FILTER % EFFICIENCY
AC Delco OE 99.93%
Baldwin paper 99.72%
No name pargain paper 99.32%
AFE Pro Guard 7 panel filter 99.23%
WIX/Napa Gold 99.03%
Purolator paper 98.73%
Amsoil, new style 98.63%
UNI 97.93%
K&N 96.80%
FLOW RESTRICTION from best to worst. Remember, 27.7 inches of water = 1 psi. So, 1 inch of water = .036 psi. The worst (AC Delco) at 6.23 in. water and the best (K&N) at 4.54 in. water is a difference of 1.69 in. of water or a "whopping" .0608 psi. Virtually negligible.
In order from least restricive to most:
FILTER RESTRICTION in inches of water
K&N 4.54
Mystery bargain 4.78
AFE Pro Guard 4.99
Purolator 5.05
WIX/Napa Gold 5.06
UNI 5.40
Baldwin 5.71
Amsoil 5.88
AC Delco 6.23
DIRT HOLDING CAPACITY. From best to worst. This is the AMOUNT of test dirt it took to create an ADDITIONAL 10 inches of restriction. At that point the test is terminated. This is an indication of HOW LONG a filter is good before it must be cleaned or replaced.
FILTER Dirt Holding Capacity
AC Delco 573.898 grams
WIX/Napa Gold 447.366 g
Purolator 388.659 g
Baldwin 388.154 g
UNI 374.638 g
Mystery bargain 350.402 g
AFE Pro Guard 7 232.516 g
K&N 211.580 g
Amsoil 196.323 g
TOTAL DIRT PASSING THE FILTER DURING THE TEST. This is how much dirt your engine will take in if you use the filter for the duration that would cause the filter to become "dirty" enough to need replacement or cleaning. The "Dirt Passing The Filter" is the dirt collected by the "POST FILTER" during the SAE/ISO test.
In order from best to worst, the filters performed as follows:
FILTER DIRT IN GRAMS PASSED
AC Delco 0.4g
Baldwin 1.1g
AFE Pro Guard 7 1.8g
Mystery bargain 2.4g
Amsoil 2.7g
WIX/Napa Gold 4.4g
Purolator 5.0g
K&N 6.0g
UNI 7.9g
NOTE: During the test the Purolator was reported to have had a seal failure which gave it higher than expected dirt passing.
