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 intake noise, which becomes pronounced and very noticeable under load — this is often the real reason for buying one.

All the performance gain depends on air temperature. A filter placed directly in the engine bay draws in air at 60 °C or more: it is less dense, cancelling 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 significantly less heat than aluminium.