One of the easiest ways to get more power from your engine is to dump the OE air filter and air intake system and replace them with high-flow aftermarket parts. Factory systems are designed with priorities other than performance, like low intake noise and cost. As a result, most vehicles come from the factory with restrictive air boxes and intake tubes, and cheap paper air filters.
Good designed high quality after-market Cold Air Intake Systems (CAIs) fights against;
Hot Intake Air Temperature:
Not to be able to breath at a location with lowest environmental temperature and adding up the engine temperature through the air pipes result the increased air intake temperature. As a result of a simple physics, increased temperature of air reduces the ratio of the oxygen in the air. And, less oxygen reduces the combustion quality. Significantly less power, high fuel consumption, detonation/knock (killing the engine) are the main results of the hot air intake temperature. Please bear in mind that the Engine Computers (ECUs) retard the spark advance and boost (if forced induction engine) to avoid engine damages which also helps reducing the engine power.