There are a number of independent test of various filters out there that could be quoted, most notably the SwRI test from 2007 (project # 08.12717.01.004) but let's just agree that oiled cotton gauze filters (OCG), K&N and any others, have "average" efficiency. No worse than most OE replacement filters, perhaps better than some of the cheapies, but not as good as the high efficiency media. Average is good enough in most situations, especially in low-dirt environments.
IMO,the major issues with OCG come when they improperly cleaned/oiled or cleaned too often. The instructions detail how to properly clean them but I have seen so many people who do it wrong... even my own darn self... because we get lazy. In the SwRI test above, K&N efficiency dropped 1.02 percent after five correct washings, so even if you do it right you may lose efficiency. Other filters ( AEM, AFE & AirAid) were tested the same way lost much less that that, from 0.10 to 0.50 percent. There are a few tests out there showing unoiled OCGs (about 50 percent efficiency) and improperly cleaned OCG (from 93% to about 80%) but they are old and probably no longer accurate. The most current independent test I have of a K&N shows about 96 percent using fine dust under the ISO 5011:2000 protocols.
ANY filter's efficiency will improve as it loads up, the OCG more than most. OCGs are known for their flow but they can have an eighth-inch of dirt on them and still flow well. Ironically, that's the point at which they achieve their highest efficiency.
If average is good enough for your situation, that's fine. I just don't see the point in paying a premium price for average filtration.
Some will highlight the performance aspects as icing on the cake. They can play a part, depending on the individual situation. In the stock airbox, a drop-in filter replacement doesn't offer much, in any, performance improvement. If it does, it's in the upper rpm ranges. A full system may do better but that's often as much the tuning of the new "system" (ducting, boxes, etc.) as it is the filter but it's also a product of how good the new is versus the old. Where the performance aspects really come into play is when the airflow needs of the engine have been increased via other mods and the capabilities of the stock system/filter are outstripped. You aren't so much "gaining" as "not losing" when you increase airflow capacity.
If I was running a K&N, I would use the optional foam sock if possible. That puts them into the high efficiency category, even if it costs a little flow.
IMO,the major issues with OCG come when they improperly cleaned/oiled or cleaned too often. The instructions detail how to properly clean them but I have seen so many people who do it wrong... even my own darn self... because we get lazy. In the SwRI test above, K&N efficiency dropped 1.02 percent after five correct washings, so even if you do it right you may lose efficiency. Other filters ( AEM, AFE & AirAid) were tested the same way lost much less that that, from 0.10 to 0.50 percent. There are a few tests out there showing unoiled OCGs (about 50 percent efficiency) and improperly cleaned OCG (from 93% to about 80%) but they are old and probably no longer accurate. The most current independent test I have of a K&N shows about 96 percent using fine dust under the ISO 5011:2000 protocols.
ANY filter's efficiency will improve as it loads up, the OCG more than most. OCGs are known for their flow but they can have an eighth-inch of dirt on them and still flow well. Ironically, that's the point at which they achieve their highest efficiency.
If average is good enough for your situation, that's fine. I just don't see the point in paying a premium price for average filtration.
Some will highlight the performance aspects as icing on the cake. They can play a part, depending on the individual situation. In the stock airbox, a drop-in filter replacement doesn't offer much, in any, performance improvement. If it does, it's in the upper rpm ranges. A full system may do better but that's often as much the tuning of the new "system" (ducting, boxes, etc.) as it is the filter but it's also a product of how good the new is versus the old. Where the performance aspects really come into play is when the airflow needs of the engine have been increased via other mods and the capabilities of the stock system/filter are outstripped. You aren't so much "gaining" as "not losing" when you increase airflow capacity.
If I was running a K&N, I would use the optional foam sock if possible. That puts them into the high efficiency category, even if it costs a little flow.
