CyberMunky said:
By the way, performance air filters actually increace fuel efficiency.
Not quite... Performance air filters offer less restriction to filtered air, therefore allowing more air flow to occur.
On some modern cars with AFM's (airflow meters), the ECU increases the amount of fuel injected if it detects a greater increase in air flow. The ratio however, stays the same, so really no real change occurs other than induction noise.
On cars which can't detect airflow (such as those with carburetors and older injection systems), free flow filters change the balance of the air/ fuel ratio. In effect, they make the air/ fuel mixture weaker, which
can lead to a small increase in power, BUT this is NOT a good thing for the engine long term. A weak mixture can lead to 'pinking'/ detonation, hot spots occuring in the combustion chamber, overheating, melted pistons/ burnt valves and generally it can cause damage within the engine unless the mixture is adjusted to compensate.
It is worth bearing in mind, that if manufacturers could easily get more power out of their cars by fitting a free flow filter, then don't you think they would? With all their budget they have to spend on design and testing, doesn't it seem daft that swapping the filter for a £50 K&N or whatever, can make so much difference compared to a standard airbox and filter?
Certainly designing a car involves compromises, but just swapping an air filter for a free flow item rarely makes any increase in power. See magazine tests! In conjunction with PROPER engine tuning/ modifications, they can help these mods. But not on their own.
I fitted a K&N to my car purely for A: Looks, B: Sound, and C: It makes the engine bay more tidy/ less cluttered. In terms of performance, the K&N makes the engine
feel more responsive, but there's no real change in performance. So far the top speed indicated by my Uno has been achieved with a totally standard airbox and filter...
Here's my K&N!