A direct-inlet kit replaces the airbox with a short intake 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 - often the real reason for buying one.

All the potential gain comes down to air temperature. A filter placed directly in the engine bay draws in air at 60 °C or more: it is less dense and cancels out what the kit had gained. 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.