I've spent the last 5 years designing air induction systems for Ford and Chrysler. During that time I had to do a study on K&N filters because the OEM's were wondering how K&N could get such incredible results, of course they wanted the same results out of their OEM air induction systems. To make a long story short, K&N filters do not reduce the restriction in the air induction system and therefore show no improvement in horsepower. What K&N filters do is let in more fine dust particles and contaminate your MAFS with oil. The oil on the MAFS eventually builds up, causes the sensor to read low, thus leaning out the air to fuel mixture. In extreme cases (very rare) this can cause serious engine damage. Some of the "cold" air induction systems do provide lower restriction but it is often at the cost of sucking in hot underhood air or ingesting water into the intake. Neither of which is a good thing.

__________________
1987 300SDL 167k
1992 Volvo 740 140k
1990 Volvo 740 250k
1989 Volvo 240 269k

Anyone want to trade an old Volvo for an '87 300sdl?