I'm a big fan of the K&N 57i. It sounds great at around 3500 revs in particular and is also, in my opinion, better for the engine. The standard air filter sits on top of the engine and gets very warm with the rising heat, so as you can imagine, that warm air is getting sucked in. The K&N doesn't have this problem as it's constantly sucking in cool air from both the cold air feeds and excess air coming in from the front grill.
I replaced my K&N 57i only yesterday after about 4 months of use. When I initially installed it I placed the cold air feeds too close to the filter element and they caused a lot of wear on the outer mesh. I bought a completely new unit which I have made sure isn't making contact with anything within the engine compartment. An expensive mistake to make, but a lesson learnt!
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God gave me feet, and Ford gave me pedals. I intend to put both to good use.