Snotzo said:
Everybody seems to be concerned with how much air passes through the filter, but no one as yet has addressed the subject from the actual filtering point of view, i.e how effective are the filters for preventing dirt from entering the engine.
Some few years ago, David Vizard did many tests to evaluate a large number of filters from different manufacturers, and he always placed the K&N on the top of his list for best filter. But he also made the comment that the K&N was an incredibly effective filter, and furthermore, when apparently really dirty would still allow much more clean air to pass through than any of the others.
I'm sure Jim's results are valid for the elements he tested, but they all look like new filters to me, so the tests are really only half meaningful, a set of dirty elements might tell a different story.
I see a lot of threads on other forums (tractor forums, truck forums, etc.) that deal with efficiency of dust/dirt removal of aftermarket filters vs stock paper type filters. The following study on dirt and dust loadings of different air filters for a GM Duramax diesel, is pretty good and deals with the different types of filters that we're talking about, including stock dry paper filters, and oiled media filters like Uni filters and K&N, among others. The best that an be said of aftermarket oiled filters is that they do flow more air, despite the fact that they pass more dust/dirt. It's a tradeoff. It's still much much better than running an open velocity stack.
http://www.nicoclub.com/archives/kn-vs-oem-filter.html
Some of the more interesting excerpts from the report are as follows:
- This report presents the results of an ISO 5011 test of several air filters designed for the GM Duramax Diesel. The test was independently performed under controlled conditions using a $285,000 machine at Testand Corp of Rhode Island (manufacturer of the machine).
- Every filter is initially tested at 350 CFM and the Initial Restriction or differential pressure across the filter is recorded in IN-H20 (Inches of Water). The filter is then tested by feeding test dust at a nominal rate of 9.8 grams per minute with a constant airflow of 350 CFM. The test is continued until the flow restriction exceeds the Initial Restriction + 10 IN-H
- This report presents flow restriction of a clean filter resulting from an increasing airflow. The differential pressure restriction across the filter is reported in inches of water (IN H2O) versus Air Flow in cubic feet per minute CFM. Data from these reports has been compiled and presented in the following bar graphs, plots and data tables.
- It’s interesting to note the shape of these Dust Loading Curves. The AC and Baldwin (dry paper) filters each had near linear responses until reaching maximum restriction. Restriction for these filters increased at a constant rate versus the 9.8 gms/min dust feed rate. The other filters,
most notably the oiled reusable types, had an exponential loading response before reaching maximum restriction. These filters had a lower initial restriction, but they became exponentially more restrictive under a constant flow of dirt.
This runs counter to the “myth” that oiled media filters actually “work better” as they get dirtier.
- The Flow Restriction response curves for each filter have the same basic shape. However, note how the AC Filter, which passed the smallest amount of dirt and had the highest dirt capacity and efficiency, also had the highest relative restriction to flow. The less efficient filters correspondingly had less restriction to flow. This illustrates the apparent trade-offs between optimizing a filter for dirt capturing ability and maximum airflow.
