Cold air intake systems are designed to channel cooler air into the engine bay in order to improve performance and increase fuel economy. The reason this works comes from the fundamental workings of the internal combustion engine.

A fuel and air mixture is introduced into the combustion chamber before being compressed and ignited. The resulting reaction is highly exothermic, producing energy in the form of rapidly expanding gas and heat, which is used to drive the piston down hence produce useful rotational energy.

The power generated per stroke is dependant on the efficiency with which the fuel burns within the combustion chamber. The more oxygen that is present, the better the fuel will burn hence the more efficiently the engine will run. Not only will this improve the performance of the engine, but it will also help improve fuel economy.

The air used during combustion is normally taken from within the engine bay, therefore is relatively warm due to the excess heat generated by the engine itself. Unfortunately, the hotter the air, the less dense it becomes hence the lower the oxygen capacity for a given volume. The solution? Introduce cooler air from an external source, which is entirely the purpose of a cold air intake.

Cooler air carries more oxygen per unit volume, providing a much better environment for combustion to take place. The result is a more efficient burn and in turn greater performance and higher fuel economy.

Although more performance could be gained by cooling the air further, cold air intake systems provide a cost effective solution to improve performance and improve fuel economy.

The exact improvement will vary heavily from car to car, and depends on the decrease in temperature of air going into the carburettor. Several of the top manufacturers quote gains of around 8-25 horsepower depending on application, which is a very respectable increase in performance. Add this to the few extra miles per gallon and relatively low cost, and it is easy to see why cold air intake systems have become so popular.

Some also find the sound generated by the intake attractive, though this is more a side effect of having a cold air intake system than anything.

The most basic intake systems involve replacing the stock airbox with a short ram air intake, though the performance of these depends on the air flow available. Better designs attempt to shield the air filter from excess heat and draw air from cooler parts of the engine bay.

The best performing cold air intake systems involve replacing body panels to accommodate air ducts. The forward motion of the vehicle forces air through these ducts and into the intake system, providing the engine with much cooler air. These systems are much more expensive and require extra work, though remain popular, particularly on high performance road cars.