Several aftermarket manufacturers sell engine air filters that they claim provide better airflow and increase horsepower and acceleration as a result. Some of these companies have many loyal customers who swear that they have benefited, particularly in performance, from using these high-flow filters.
These filters are often made of cotton or nanofibers, and some can be cleaned and lubricated with a special oil and then reused, with at least one brand guaranteeing they will last a million miles. Others can be cleaned with an air hose and reused. Many are designed to directly replace the stock paper air filters provided by the vehicle manufacturers; others are included in a freer-flowing air intake system that delivers more fresh air to the engine.
On the surface, these claims make sense. Engines are like athletes in that the better they can breathe, the faster they can go.
However, independent tests have found that these aftermarket filters may provide little or no performance benefit, and some types allow more dirt into the engine. That dirt will get into the engine oil, and the abrasive effects can damage internal engine parts. In addition, some users say that the oil sprayed on some of these filter types gets into the engines' air intake systems as well.
The EPA has not tested oil-bathed filters or other free-flow air filters, but it has conducted tests that compared clogged conventional air filters against new ones. The EPA found no significant loss of fuel economy from a clogged air filter but did find in a test of gasoline-powered vehicles that acceleration improved with a clean air filter.
Though that would seem to lend some credence to claims that a freer-flowing air filter can improve acceleration, the significance of the performance gain depends on the vehicle in question. In addition, these high-performance filters cost more than conventional filters, and though they can last much longer, they require periodic maintenance instead of periodic replacement. High-flow filters and intake systems typically muffle induction sounds less than conventional paper filters do, which can give the impression of better performance — or simply a desired sound — that's worth the price of admission for some drivers.
