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

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