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Air Filter FAQ

Air Filter FAQ and Information

From: Richard Bernecker <>
Date: Fri, 24 Jul 1998 21:04:38 -0400
Subject: FWD: Air Filters – A LONG ONE

In response to a message from David R. Norton …

    • At 01:48 PM 8/19/96 +0000, David R. Norton wrote:


    • The good news – the data *was* collected. Based upon my data, I concluded that a well-maintained oiled-foam filter was superior (IMO) to just about anything else in almost any tested catagory for my motorcycle.


    I’d be interested in the methods of testing. Filters were tested by one of the magazines some time back for flow only. The method was to mount the filter on a real, live carburetor, mount the carb on a cylinder head and the head onto a flow bench, a method I can’t fault. If I recall, the Uni was the most restrictive. Because of that, I’ve avoided the Uni filter, I wonder if they’ve changed, since…

No, the filters are the same, it’s the test method the mag used which was potentially questionable. I’ll touch on that in a minute…

All filter OEM’s test their filters on a test jig, with a calibrated vacuum source (pump, downstream vacuum gauges, test vacuum gauges) and a calibrated dust feed mechanism. For my air filter eval project, I was interested in two data points; filtration efficiency and air filter airflow rates, with a weighting of conclusions in favor of filtration over airflow.

My actual datasheets are at home, so the following will be taken from memory…

1) I eval’ed publically-available test data for pleated-paper, oiled- gauze, and oiled-foam air filters; I was most interested in small and large particulate filtration efficiency AS EXPERIENCED FROM TIME OF FILTER INSTALLATION. This approach takes into account the initial dust load-up characteristics of each type of filter and provides some feedback as to the possible service life of each air filter.

The oiled foam filters seemed to show much better INITIAL filtration efficiency that either paper or oiled gauze, and filtered at a constant high rate of efficiency until the filter clogged, at which point the filters failed. Failure of the filter was marked by a significant decrease in filtration efficiency, as a result of the filter media collapsing or the filtration media simply allowing the particulate to be pulled thru the media unimpeded due to media saturation. The oiled foam filters appeared to hold significant particulate in the filtration media before vacuum levels (downstream intake restriction from filter clogging) rose to unacceptable levels and the filter failed. The vacuum level at which the filter would fail was lower than pleated-paper air filters, but higher than oiled-gauze filters. The thickness of the foam material appears to give it some self-bracing capabilities. Filtration efficiency was consistent for both small particulate (< 10 microns) as well as larger particulate.

The paper filter tests showed a relatively poor out-of-the-box dust filtration efficiency until “loaded” with a certain level of dust; then their filtration efficiency rose to quite good levels. They did clog more quickly than the oiled foam, but did not fail catastrophically upon filter failure. Filter failure was marked by a refusal to pass air; the filter’s mechanical strength allowed the filter to remain intact. (This is the property which makes them ideal for earth-moving equipment and industrial motors; the motor will run like shit when the filter is clogged but the filter will not collapse from to the high intake vacuum caused by the filter restriction. The pleats give the filter a very high mechanical strength….) Filtration efficiency was consistent for both small particulate (< 10 microns) as well as larger particulate.

The oiled gauze filters displayed roughly the same initial filtration characteristics as the paper filters, that is to say that their filtration efficiency was relatively poor until the filter became loaded with dust. Once “loaded”, the oiled gauze filters performed acceptably well in terms of filtration efficiency; their lifespan was less than oiled foam or pleated paper. Failure of the filter was much the same as oiled foam, that is to say marked by a collapse of the filtration media and a dramatic decrease in the filtration efficiency of the media. Unlike the pleated-paper and oiled-foam air filters, the oiled-gauze filters demonstrated a poor filtration ability above a certain vacuum level; the gauze is quite thin and dirt can be easily passed by the gauze under high vacuum conditions. Filtration efficiency was NOT consistent for both small particulate (< 10 microns) as well as larger particulate; the oiled gauze filters appeared to be less efficient at filtering small particulate than large, and were less efficient at small-particulate filtering than either the pleated-paper or the oiled- foam.

Based upon the above, for a motorcycle with constant maintenance intervals, I concluded that the oiled foam filters were the best solution given dust filtration as the sole criteria. They displayed the best out-of-the-box filtration rates and these rates were uniform over the lifespan of the filter. Oiled-foam filters require more frequent servicing than oiled gauze or paper, but they filter best when just serviced; a 6K-12K service interval seems ideal for the oiled foam filters. And they are reusable, unlike pleated paper filters. Oiled gauze filters were markedly less efficient in terms of initial dust filtration, but once dirtied they filtered at a level almost equal to the oiled foam. Oiled gauze filters displayed a faster failure rate than either the oiled-foam or the pleated- paper, as well, and appeared much more sensitive to vacuum levels; the filtration efficiency of the oiled-gauze filters dropped dramatically as vacuum levels rose. As well, the split between small particulate filtration efficiency and large particulate efficiency was more pronounced in the oiled-gauze filters, with small particle efficiency suffering more than the other filter types.

2) Now, on to airflow… Each filter is tested (usually by an “independent lab”) for each OEM; the OEM will specify the vacuum levels to be used during the test and so forth. This makes a direct correlation of airflow data quite difficult. However, one fact was made quite clear by all OEM’s who participated in this little exercise; airflow is directly proportional to filtration media surface area. IOW, a large paper air filter will out- flow a small oiled-gauze filter, all other things being equal. As well, the presence of an intake restriction upstream or downstream of the air filter itself in many instances makes the air filter’s air flow data reasonably irrelevant. The air filter is often not the primary intake tract restriction. However, all other things being equal, an oiled-gauze filter appears to flow approx. 10-25% more CFM than an oiled-foam filter of identical surface area proportions, given a moderate intake vacuum level. The higher the vacuum level used, the smaller that gap becomes until the oiled-foam filter appears to be equivalent to the oiled-gauze in terms of absolute airflow at high vacuum. As well, filter media “loading” characteristics also have a dramatic effect on airflow. Pleated-paper filters drop in airflow rates much more than oiled-gauze or oiled-foam as the filters become dirty. Oiled-foam filters appear to be more stable across their lifespan than oiled-gauze, in terms of airflow rates. A dirty oiled-foam filter will begin to approach the airflow rates of a dirty oiled-gauze, all other things being equal. (A dirty pleated-paper filter will have a high filtration efficiency rating but will flow like a dog.)

To speak specifically to the airflow test Dave Norton referenced above; an airflow test of new filters will be testing the pleated-paper and oiled- gauze filters when they are least restrictive as well as least-efficient in terms of actual filtration. An airflow test of used filters (6-10K mileage) would be much more realistic of a test, in that you would be testing each filter when it is running at approximately 98-99% filtration efficiency. As well, the vacuum used for the test will have a fundamental impact on the results. Certain engine designs have an inherently low intake velocity (the Honda CBX leaps to mind), while others (such as big twins and high-rpm large-displacement 4-cylinder mills) tend to have higher intake stream velocities. It would be reasonable to expect that the oiled- gauze filters would work better, from an airflow standpoint, on the CBX than an Airhead. And finally, if an individual “pod-type” air filter was used for the test, as opposed to a stock airbox and filter, the results will ALWAYS favor an oiled-gauze filter since it flows best per square inch of surface material and the surface area is limited due the small size of the air filter itself…

My conclusions regarding airflow; who knows? This is going to have to be determined on an application-by-application basis, as the size of the air- box and the presence of upstream or downstream air flow restrictions are variables from bike to bike. The only real test I am aware of, other than to flow-bench an actual intake system (airbox and *all* intake components for *all* cylinders) for your particular machine, is to run the motorcycle briefly without an air filter to determine the power level increase without any air filter restriction. A series of timed accelerations (off-the-line and roll- on) in various gears and loads is optimal, within a specific time period of the same day and with jetting adjusted as needed during the test. If the power level increases dramatically, then the filter is a restriction and the machine may benefit from a less restrictive air filter. If the power level remains fairly constant (which most likely means less than a 5% increase in power), then the machine has intake restrictions other than the air filter and moving to a less-restrictive air filter without also addressing the rest of the intake system is pointless.

My overall conclusions? I value filtration efficiency over maximum airflow numbers, so my first inclination is to favor the oiled-foam filter, then the pleated-paper, and last the oiled-gauze. I ran my R1100GS (closed-loop engine management system which ensured appropriate jetting) without a filter, and then with the stock pleated-paper as well as the oiled-gauze filter. Any power gains netted from running without an air filter were less than the margin of error in my timed testing; I’ll call it less than 5%. Overall engine smoothness was increased above 4.5K RPM without an air filter. I concluded that the filter was a restriction in airflow above 4500RPM, but not significantly enough to provide a demonstrable decrease in power. The oiled-gauze filter displayed the same characteristics as did running with no air filter installed. I have yet to wring out the oiled-foam filter from the standpoint of engine manners and performance. Given that the “no filter” test resulted in minimum acceleration-timed power gains, I expect to be fairly comfortable with whatever the oiled-foam filter provides in terms of airflow…. Bottom line; a sheet of (thin) oiled gauze will flow more and filter less than a sheet of (thicker) oiled foam, and a paper filter is a good compromise between sturdiness and filtration requirements. Kinda a common-sense expectation and the test data does appear to support that conclusion.

Whew! This turned out to be most of the data, minus the actual numbers and graphs…. Anyway, I hope this helps answer some folks questions regarding air filters…. As an aside, I encountered lots of OEM “fudgies” in this project. For example, K&N uses test data in their marketing literature showing a high filtration efficiency; what they don’t tell you is that this data was garnered after the filter was pre-dirtied, to overcome the natural inefficiency of the oiled-gauze media to filter effectively when new. UNI provides airflow test data measured at much higher vacuum rates than the K&N test data, to overcome the fact that the oiled-gauze filter will almost always outflow an oiled-foam filter under lesser vacuum conditions. And so on… If anyone desires to pursue this issue further, I strongly recommend that the data be compared back-to-back from each filter OEM, to help weed out each filtration media’s signature characteristics.

Now it’s time for me to get back to work… 🙂
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