Woodie wrote:K&N filter will increase the highest output your engine can put out by a HP or two, but only at wide open throttle near the redline.
...and, allow the engine to ingest a whole lotta crap. If you travel anywhere with airborne particulate, not necessarily visible, it'll get past the K&N and be ingested. Been there, done that. Whatever meager perceived gains are maybe realized, they'll be wiped out by a guaranteed cruddy intake system. K&N needs to include, at a minimum, MAF cleaner in their filter 'service' kit.
+1. Much like Ronzuki, I've had K&N and similar oiled fabric filters (both panel and cone type) in the past and won't touch them now. They simply let too many particles through... given they were designed to improve airflow at the cost of filtration quality. The thing about K&N that many people may not realize is that they got started focused on offroad racing applications. In this context, standard paper air filters got clogged with dirt too quickly and restricted airflow. The K&N filter was developed to keep air flowing past the dirt but at the cost of filtration quality. It made a lot of sense for offroad racing
but not much for everyday driving, especially for vehicles that owners intended to last a long time. For longevity, you'll want better filtration to keep the combustion mixture cleaner. Not only that, unless you encounter a lot of dirt, the oil eventually gets sucked into the intake and gums up components like the MAF sensor as Ronzuki mentioned. Again, high dirt exposure is part of how K&N filters are supposed to work and for ordinary passenger cars, it's just not a practical reality.
Lastly, modern engines have optimized intake designs. Just look at the Kizashi's intake. It's an ideal OE intake design that combines cold air and short ram intake concepts into an OEM design. No longer are OE designs sucking in hot air from the engine compartment. It's drawing in colder, isolated air by means of a dedicated air intake behind the grill. It's another reason why I personally feel modifications to the intake and filter aren't necessary.