A direct-fit 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 is direct and very noticeable under load - often the real reason for buying one.

All the performance benefit comes down to air temperature. A filter sitting in the engine bay can draw in air at 60 °C or more: it is less dense and cancels out what the kit was meant to improve. 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.