A direct intake kit replaces the airbox with a short pipe and a cone filter. Two things change: airflow, because the airbox restrictions are removed, and the intake sound, which becomes clear and very pronounced under load - often the real reason for buying one.

All performance gains depend on air temperature. A filter installed directly in the engine bay draws in air at 60 °C or more: it is less dense, canceling out the gains it was meant to deliver. Serious kits come with a heat shield and a cold-air intake; without one or the other, the gain is theoretical. Carbon conducts significantly less heat than aluminum.