Filter efficiency is measured on how much dirt the filter catches vs. how much it allows to pass through, not how well the air flows through it.* No filter element is 100% efficient (i. e., catches all the dirt). What we want is a filter that allows as much flow as possible for as long as possible while stopping just about every particle big enough to cause engine wear.
I'm not a K&N fan, so....
Has anyone done any good testing on the performance of a
new OEM air filter element vs. a
new aftermarket stock-type element like a $15 HiFloFiltro vs. a
new or newly cleaned $60 K&N element?
In my car I've seen the poor filtration of a K&N with dirt on the downstream side of the air box that was never there with an OEM filter element, and the good testing I've seen of the K&N filters has the same results. The tests I've seen show that the K&N filter flows a lot less air than OEM when it is dirty than an OEM filter with the same amount (by weight) of dirt in it. Plus, cleaning and re-oiling the K&N element can be a sloppy mess.
*Filter efficiency is the ratio of particles trapped by a filter over the total number of particles found in the air upstream of the filter. A count of the downstream particles is often used to determine the number of particles trapped by the filter. Filter efficiency can either be based on specific particle size ranges or based on the total number of particles of all sizes. There are numerous testing procedures utilized for determining filter efficiency.
http://www.filterair.info/articles/article.cfm/ArticleID/3CB961AC-F5FA-4A28-B...