An aftermarket air intake system designed to draw cooler air from outside the engine bay, increasing engine power slightly.
Cold air intake (CAI) is an aftermarket air intake system designed to deliver cooler ambient air to the engine intake. Stock air intakes typically draw air from the engine bay, where it has been warmed by the engine. Cooler air is denser and contains more oxygen molecules per volume, allowing the engine to make slightly more power. CAI systems route the intake to draw air from the front bumper, fender, or other locations outside the heat-soaked engine bay.
The power gain from a CAI is typically modest: 5-15 horsepower on naturally aspirated cars, slightly less on turbocharged cars (where the turbocharger compresses the intake air anyway). The gain is largely from reduced intake temperature; a properly engineered CAI can drop intake air temperatures by 30-60 degrees Fahrenheit compared to stock. The gain is most visible during sustained high-load operation where the stock intake heat-soaks.
Major CAI manufacturers include K&N, Volant, AEM, Spectre, and various others. Quality varies; some CAI systems are merely longer pipes with the filter relocated, while others include heat shielding and proper airflow design. CAI is often one of the first modifications enthusiasts make to a car because it is relatively inexpensive ($300-$800), looks good in the engine bay, and produces audible engine note changes that signal modification activity.