Replacing an air filter is one of the cheapest and easiest ways of improving a car's performance. The basic principle is that increasing the airflow through the engine results in an increase in power, and letting the air enter more freely is a good starting point. It can be achieved by removing the filter altogether, but this is not a good idea since small particles of dirt or grit can do a remarkable amount of damage to the inside of an engine.

Manufacturers of performance filters aim to increase the flow of air without seriously reducing the filtration properties of their products. The simplest performance filters are relatively cheap, very easy to fit (open air box, take out standard filter, put in new version) and have a moderate effect on performance - perhaps a couple of bhp on top of the standard maximum power figure, depending on the individual car.

Air filters work well in conjunction with other inexpensive tuning items such as exhaust back boxes (see exhausts). They are, however, less effective than air induction kits, which replace most of the standard air induction system.

It is also worth noting that a clogged filter will have a considerable effect on an engine's performance. It may be that a car will perform much better if an old filter is simply replaced by a new standard one, allowing the engine to develop the power the manufacturer intended it to in the first place.

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