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

All the potential gains come down to air temperature. A filter installed in the engine bay draws in air at 60 °C or more: it is less dense and cancels out what it was supposed 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.