The air filter’s primary function is to remove dirt and debris from the induction charge before entering the intake tract, and therefore, the engine. Keeping dirt out of the engine is a primary means of helping it last longer. If unfiltered air can enter the engine oil, dirt can cause premature wear of internal parts. Replacing or cleaning the air filter is an important part of regular vehicle maintenance, outlined in the vehicle owner’s manual.
This article outlines some of the many different kinds of filters. Cotton oiled, dry cotton, and foam filters are the most popular aftermarket performance filter media. Be aware of the different types, and that some that are universal, non-OE style filters that may need adapters to install properly (cone filters).
Panel versus Cone
Most factory air filter elements are flat panels that are sandwiched into a box in the intake tract (airbox). The box fasteners are removed, the box opened, and the filter panel lifted out. The factory filter is a specific shape with a gasket around the outer edge to seal the airbox lid. Aftermarket panel replacements are designed similarly to the factory panel in shape and size to fit the OE application but are made from different materials to change the air-flow characteristics.
Some original equipment and most aftermarket applications use a cone filter. Cone filters go over the end of an intake tube and attach to it with either a clamp or a flange. These open-element cone filters are typically used in place of a factory airbox, doing away with the potentially restrictive plumbing associated with them. Higher performance applications will almost always use an open-element cone filter. Please note that custom mounting may be required if not using these universal cone filters with a complete air intake system upgrade.
Most factory filters are made of paper material. The factory air filters are not bad for performance and do last reasonably long. These filters cannot generally be cleaned and are replacement only. Some enthusiasts will claim a factory filter is the best for use on road cars, where there is a lot of debris and a long time in between service intervals. Many enthusiasts will replace the factory filter with aftermarket cotton or foam filters to increase the air-flow into the engine. The power gains are reported inconsistently, but it is considered common knowledge that replacing a factory filter with a higher flowing aftermarket one will help increase the power potential of the engine.
For more airflow, a popular upgrade is using cotton drop-in replacement filters. The cotton element is a more open matrix than the factory paper and lets more air through. With oiled cotton filters, a light-weight oil is applied to the element which provides a “sticky” place for dirt to attach to before it goes into the engine. The oil is effective in trapping dirt and must be cleaned periodically. These are permanent filters that can be cleaned and re-oiled with a kit from the manufacturer. Some owners have condemned the use of oiled filters because excess oil can be drawn through the filter into sensitive mass-air flow reading components, causing the engine to run poorly. Our stance is, be careful not to over-oil when servicing your oiled filterâ€¦it is easy to do this and then blame the filter for the problem, when in reality these filters are often very viable performance parts.
Because of the growing stigma against oiled filters, and to reduce maintenance effort and cost, there is a newer style of cotton elements called dry filters. A dry filter replacement which flows more than stock like an oiled does, but without the oil. The matrix is tighter which allows the trapping of dirt, but the cotton still fundamentally flows more than paper. The cleaning is easier without needing to apply oil. Just wash with water, allow to dry, and reinstall.
Used in many racing applications, foam filters are known to flow a lot of air. We’ve heard reports of foam filters being the highest flowing available. These filters can be made with a foam of various porosity, or layers of foam with different density, with the more open elements using oil to help trap dirt. They are softer and more pliable making them useful in a wide variety of applications, such as wrapped over intake velocity stacks. Foam filters can often be cleaned, however foam filters are less durable than the factory paper or cotton filters, and may need replacement sooner if used and cleaned frequently. Many enthusiasts feel the power gains are a good trade-off for less durability compared to cotton filters.