I wrote a letter to K&N and received a similar reply from Rick also. The first part is my note to K&N and the second part is his reply. See below.
Good Morning
I own a Dodge vehicle with a Cummins Turbocharged Diesel engine and I have installed one of your air filters in the stock airbox. I'm also a subscriber to the TDR (Turbo Diesel Register) magazine and frequently browse their web site. There has been a lot of discussion on this site regarding the use of your filters in these vehicles. Some have "Quoted" K&N Engineers that the K&N Air Filter should not be used in a turbocharged diesel application. Being an engineer myself this seems strange to me that if they should not be used why are they manufactured and marketed?
If you could clarify this situation it would be greatly appreciated.
Thank you for your time.
Dear customer,
That would be completely false. First, the engineers do not take phone calls, customer service does. Second, why on Earth would we make a product we do not intend to be used? They may be confused by the large canister type filters we do not make for construction equipments and
earth-movers. We do not make deep enough pleats for these and one would have to clean it every other hour! Not a real convenient situation. The
very same filter we offer is also available at Dodge dealerships through Mopar. It is Mopar part# P5249920. If they were so bad, why would Dodge and K&N offer them?
Our filters are tested by an outside, independent laboratory. They have been proven to stop at least 99% of particles on a SAE dust test. This test uses particles as low as the 0 - 5 micron range and goes up to 20 microns. For comparison, a paper filter also stops 99% on the same test and the OEM minimum standard is 96%. Foam is generally the worst media with a typical efficiency rating of 75 - 85%. To get higher ratings, the foam must be more dense and therefore way more restrictive. The "tack" characteristic
of a K&N allows for increase filtration without loss of flow as well.
The testing procedure used is SAE J-726 using ISO Test Dust. This test is the standard of the air filter industry. The test procedure consists of flowing air through the filter at a constant rate (airflow rate is determined by the application) while feeding test dust into the air stream at a rate of 1 gram per cubic meter of air.
As the filter loads with dust the pressure drop across the filter is increased to maintain the prescribed airflow rate. The test is continued
until the pressure drop increases 10" H2O above the initial restriction of the clean element (in this case . 78" to 10. 78" H2O). At this point the
test is terminated. The dirty filter element is then weighed. This weight is compared to the clean element weight to determine the total Dust Capacity. The amount of dust retained by the filter is divided by the total amount of dust fed during the test to determine the Cumulative
Efficiency.
The K&N filter achieved the following results:
Dust Capacity: 305 grams
K&N Cumulative Efficiency: 99. 05 %
Holding the filter to the light is useless, pin holes are normal. That is what makes a K&N filter. There are actually hundreds of microscopic fibers that cross these holes and when treated with oil,
capture and hold the very fine particles. On the same hand, they allow the filter to flow more air than paper or foam. The filter is 4 ply cotton gauze unlike some competitors synthetic material filters. The synthetics do not have the very small fibers that natural cotton does. Also, the oil can be pulled off of a foam filter contaminating electronic sensors. It will absorb into cotton and stay in the media. In fact, Honda and Toyota only recommend K&N filters when using aftermarket high flow filters as K&N is the only brand of filter the oil does NOT come off of. They will not cover a failed sensor if foam filters were used.
We got started over 30 years ago making filters for motorcycles and off road racers. The filters did so well that these guys wanted them for their cars and trucks. We started making filters for these applications and here we are today. If they did not work, we would not still be here and growing every year.
We now make filters for Chrysler/Mopar, Ford Motorsports, Edelbrock, Rotax Engines, and Harley Davidson. We come as original equipment on the 2000 Ford Mustang Cobra-R. We even made filters for the Apache helicopters used in Desert Storm because of maintenance problems with the original paper design. If they work in these conditions they will work for you. Links to the filtration tests are on our web site at
http://www.knfilters.com/images/factstab1.gif and
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http://www.knfilters.com/images/factstab2.gif One actually uses the 33-2056 filter made for the Cummings diesel.
> Thanks for writing, Rick
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Attention: TDR Forum Junkies 
