Ok before this gets too personal I think I need to highlight what Fernando is trying to say. He's not saying that dropping any changed air filter will result in a 6-8bhp increase on any car.. What he is saying is the particular air path on the marea is more restrictive than is normal for a car of its age/engine size.
To quote Kia.. Think about it.. If you're air intake consisted of a 20m long hose wrapped around the car twice you would be heavily restricted on your intake and your BHP would suffer terribly.
The same applies on a smaller scale with the standard marea intake. It's full of tight corners and small diameters. Whip the whole thing off and wham! the engine can breathe. Not great since its not protected so you lose about half your 'gain' by putting the best air filter you can on the car.
