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

All the potential gains depend on air temperature. A filter installed in the open engine bay draws in air at 60 °C or more: it is less dense, and gives back what it had gained. Quality kits come with a heat shield and a cold-air intake; without one or the other, the gain is theoretical. Carbon transfers significantly less heat than aluminium.