I recently realized that I haven't replaced the air filter elements in my BMW tii in almost 15,000 miles when I should have been replacing them every 8,000 miles! Since some of our members have switched to an oil soaked foam type filter rather than the paper element, I too thought I would make the switch.

Yesterday, however, I read an article in Cycle magazine that compares various air filtering methods, among them the paper element and the oil soaked foam element. That article has swayed me back in favor of the paper element, and here's why.

After thoroughly scaring me by telling what can happen to an unfiltered engine, the article proceeds to rate the various filters according to criteria such as dust capacity, overall efficiency, engine protection if cleaner service is neglected, performance under widely varying temperatures, relative cost, and some others applicable more to motorcycles than to cars, such as tolerance for rain entrance.

The paper element filter rates as high or higher than the oil soaked foam filter in all but one are an air passing ability with increasing dust accumulation. As a paper filter collects dust, its efficiency actually climbs from 99.5 % when new to virtually 100% after receiving a light coating of dust. While the dust capacity per se of the paper element and the foam element are both rated as medium, the paper element's ability to pass large volumes of air deteriorates faster than the oil wetted foam element's air passing ability. To help alleviate this problem most paper filters employ a pleated structure to increase their surface area. Also, a paper element's ability to perform well under widely varying temperatures is rated as excellent versus the oil soaked foam filter' rating of fair.

But the rating that caught my eye the most was that of overall filter efficiency, e.g., which filter actually filters the best. Here the oil wetted foam filter is rated low versus the paper filter's rating of very high.

Since it appears that the paper element is the best filter, let's consider if its one shortcoming is bad enough to outweigh its superior filtering capacity. The magazine article points out that for very dusty conditions the oiled foam's ability to keep passing air longer than the Paper filter is important and a motorcycle filter certainly encounters more dust and grit than the average car filter. But for an auto filter that is used in an urban area, I wouldn't think this factor would weigh so heavily against the paper element. My car is a living example of this since it is running fine with little or no decrease in gas mileage or performance with the same paper filters for the last 15,000 mires. So, clearly, for urban driving the paper element filter is best.

Cost, of course, is important and the oil wetted foam filter is less expensive over the long run since you don't have to replace it to renew its filtering abilities. My view is, however, that a small outlay every 8000 miles for a new paper filter(s) is worth its superior filtering qualities. The choice is yours.

Author: Rusty Barton