Filters are commonly constructed from a variety of flexible materials, such as paper, polyester, fabric, and cotton. This is the most common type of filter and is used in almost all factory production cars. The paper used is a highly compacted wood pulp that is folded into an accordion shape to increase surface area and adhere to a foam rectangle. Those who want to upgrade from the standard filter often opt for a cotton-based part.
These have layers of cotton gauze sandwiched between an aluminum mesh, which provides a less dense material for air to penetrate than paper. This should theoretically allow more air to enter and increase power, although the difference in power is likely to be small. Cotton filters can last the life of the car, but they need to be cleaned and re-oiled when they become sufficiently dirty. It is important not to lubricate the filter too much, as this can cause the mass air flow sensor to absorb oil. Foam air filters are even less restrictive, as they use several layers of foam with different densities to stop contaminants, plus a wire mesh to help maintain the shape of the filter.
This means that less dirt will be filtered out, but it is unlikely to cause engine problems. These filters can be bought as flat “panel filters” that fit inside the existing air box. Metal filters, made of layers of stainless steel mesh, usually look great and don't need to be greased. However, with fewer restrictions comes less filtration. Pleated cotton HVAC filters are also used in some cases.
They consist of the same cotton used in many other everyday products and are pleated and formed into an HVAC filter in order to filter the air. Polyester and fiberglass materials are also used for space filtering, with fiberglass being used as a coarse-grained underground filter in industrial paint booths. Spun fiberglass about an inch thick is used for most ovens and is the most effective for cleaning the air in your home, although it is not practical for most HVAC systems due to its thickness. Disposable pleated filters are made from polyester or cotton sheets of paper and are better at trapping particles than fiberglass HVAC filters. They are also more expensive and have a shorter lifespan.
Many manufacturers apply a layer of oil that is supposed to help trap dirt particles, rather than them accumulating in the filter material itself or passing to the engine. In either case, the individual strands or fibers of the material are woven close together to provide a higher level of air filtration.
