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

All the actual gains depend on air temperature. A filter placed directly in the engine bay draws in air at 60 °C or more: it is less dense and gives back what the kit initially gained. Proper 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.