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Today's Car Air Filters

Updated on June 26, 2012

Paper vs Oiled

Like many of us I was somewhat ignorant about today's car air filters. Ten years ago as part of my job I got involved in industrial air systems, I had visits by multiple salesmen trying to get me to buy their product and providing various literature and technical data.

Most oil soaked panel filters have smaller media areas than the original paper units, as a result, to attain a lower pressure drop they offer less resistance by virtue of passing more dirt. Once theca's air filter starts to collect dirt on its surface it increases its filtration properties but also loses the original benefit in terms of pressure drop or flow. The smaller media area means it takes a shorter time for the filter to become clogged to a point where there is a noticeable loss in performance. Also for a given volume of air the velocity of the air through a filter is determined by the area of the filter, a smaller area requires a higher velocity. Higher velocity creates more risk of the oil migrating or the dirt passing through as the tiny oil droplets can be squeezed through the filter pores. Another interesting fact about oil is that it tends to attract to other particles rather than staying in droplet form. As a result, the droplet size is unlikely to stay fine and therefore the available capture area for dirt diminishes. This is why oil type filters have such short service intervals. In fact its more than likely than the gains offered by a K & N and other manufactures are only for the first few thousand miles, particularly in dusty or dry months, when the dirt builds up quicker. Basically, the pressure drop across a filter (i.e. restriction to air flow) is based on three things... the size of the media area, the filtration rate and the volume of air its trying to filter.

As the volume of air increases through the filter with engine speed, so does the resistance or pressure drop, that is why a static test on an air filter (i.e. when fixed on a rolling road) doesn’t give a true figure in terms of horsepower, as the car isn’t moving through the air and creating additional upstream pressure into the filter. There is no doubt that an oil lubed filter will give a lower pressure drop, compared to a standard one but when the car is actually moving the gains are probably less significant as the dynamics come into play. As the car starts to travel at higher speed there is additional force applied to the filter up-stream which can cause the oil to migrate through the filter as a liquid and will naturally travel in the direction of the air. If the oil particles are then blown out of the filter they can then find their way onto the MAF (mass air flow) causing the device to read incorrectly. The inlet on an air cooled and some German designed engines already has a fine oil mist by virtue of an air oil separator unit.

Now onto the issue of dust particles, yes its correct that all filters will pass some dirt, and the standard filter even does this, but what is important is both the size and the amount. Anyone that has emptied a vacuum cleaner bag will know how fine dust can clog and be a real issue, so any filter that passes dust as a sacrifice for performance isn’t a good compromise. The dust finds its way into the throttle body and the MAF (mass air flow) building up a black tar like deposit and also into the engine on the valve stems. In itself the dust is harmless, and can be cleaned (where accessible) however, if the filter is damaged during cleaning larger particles could enter the engine and do more serious damage.

I have K & N filters on two of my cars and have not had any significant engine issues with regular cleaning of the filter and occasional cleaning of the MAF (mass air flow sensor). Unless you are a keen DIY mechanic and can fix cars, clean throttle bodies and MAF's yourself I would suggest that the gains of fitting a K & N are not worth the additional maintenance.


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