The function of the air filter is to clean large quantities of air before the air passes into the carburetor or fuel injection system. Approximately 10,000 gallons of air pass through an air filter for every gallon of fuel used. Even the tiniest amount of dust or dirt can work its way into the engine, where over time it will cause damage.

A good air filter will effectively remove these tiny dirt particles, retaining all of the dust and dirt it filters from the air, allowing air to flow freely enough through the filter to provide an adequate supply to the engine's intake system for its combustion needs. Most cars have air filters that use a paper filtering element. This paper is pleated to provide maximum surface area and filtering capacity.

There are a few other types of air filters, mostly on older cars. These include oil bath air cleaners, oil-dampened air cleaners, and foam air cleaners.
If there is a poor seal between the air filter and the housing, unfiltered air will be allowed to flow around or past the filter and directly into the induction system. Again, to prevent this, make sure that you always use the air filter that is specified for your vehicle. A dirty or damaged air filter restricts the intake airflow, which negatively affects engine performance and life. By changing an air filter a minimum of every 6,000 miles or 6 months, the life of the engine will be extended. Also, a clean air filter keeps gas mileage and horsepower at their best levels.