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

Everything depends on the air temperature. A filter installed in the engine bay draws in air at 60 °C or more: it is less dense and gives back what it 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 aluminum.