I hesitate to make this post because I don't want to dig up an old argument, but I just completed tests of a drop-in K&N air filter in a 2005 Frontier 4.0. Let me state here that I have no axe to grind in this discussion, but I know that many owners buy a K&N filter looking for horsepower and/or economy and I wanted to see an objective test on my truck to see if it delivered on either. Well-controlled dyno tests showed almost identical charts on the stock air filter and the K&N drop-in; it's pretty clear that the K&N offers no performance gains ON THIS ENGINE, IN STOCK TRIM. I don't find that surprising since the 4.0 uses the same size airbox/filter as does the V-8 Titan and probably flows all the air it needs.
I have used K&N filters on performance cars and bikes for many years and have found that they can increase performance in some applications; it just doesn't seem to be the case here. To see the information and dyno charts:
http://www.reverendbiker.com/frontier/knfilter.htm
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