Question RE: K&N finter
 

I have a K&N filter in an 04 and just cleaned the filter. The instructions suggest 50k mi. before cleaning and that as it gets dirtier that it cleans better. Does this mean that it does not filter well when clean? Why should a filter be engineered so that it works better when dirty?

The K&N filter uses a cotton media to trap dirt. I believe the cotton is not as tight a weave as the paper element thus the less restriction and better performance. As the filter traps particles they help trap more particles and work better as it gets dirty. The filter still works well when clean, just betten when a little dirty.

If I am wrong please correct me.

You have to understand the concept of an air filter. Anything you put in front of intake air to an engine will diminish its flow. Less flow, poorer breathing, less power. The K&N filter is designed for maximal airflow with "adequate" filtration. When new, they achieve good airflow and thus, can boast some impressive gains on some applications. As it gets dirtier, it filters a bit better because the holes in the filter media get filled up with dirt. This in turn, helps to filter better. However, this in turn, decreases airflow.
It's all a compromise between airflow (HP) and filtration (engine cleanliness and longevity). You must decide which you want. You can't always have both. (Your rose colored glasses have been on too long.)

ps. Personally, I would never let a K&N stay in the car for 50K MILES. That's just ludicrous. I got a K&N because I want high airflow and performance. If you're going to leave the thing for 50K miles, why get it in the first place? You will have much, much less performance gain with a "dirty", "better fitering" K&N. I clean mine annually or about every 10K to 15K miles.

xviper. What you say makes complete sense. Does this mean that it is letting in more dirt when clean than a stock paper filter? Has anyone tried the GREEN filters?

I can't quantify this for you as I don't have the numbers. I think we should get too worked up about "dirt getting in". If you kept out all dirt (smaller than "x" microns), you would have such a restrictive filter that your engine may be severely hampered in the kinds of power it can make. Remember, what dirt that does get in past an air filter will end up going out the tailpipe or getting sucked in by the oil. You then dump the oil at oil changes. That's why it gets dark. It's doing its job.
Don't worry so much about it. I've been running K&Ns for decades and as long as oil change intervals are adhered to, your engine will be fine. I think a new paper stock filter will clean the air better than a newly cleaned K&N but then, the K&N will gain you a couple HP that the OEM filter will not. It is nearly impossible to have those couple of HP and still keep that bit of dirt out. It's just the way it is.

xviper. Again good advise. That you have been using them for 10 years and still like and reccommend them is a good sign. Thanks again. I am quite good about scheduled maintenence.

Hey. what gains in hp and trq could be expected in using a K&N filter over a stock filter? '04 2.2

I had a stock filter in while I was cleaning the K&N and the cars response was "smoother" The throttle response is more immediate with the K&N back in. I can't say that it is mesurably faster or anything like that but it sounds a bit faster. i.e. it is a bit louder, but not a lot.

If you see how dirty the stock filter gets in 10K miles, it makes sense to clean the K&N filter that often too. I doubt every time you clean the filter that it is as clean as new, so there will be some residual dirt in the filter any way.

Here's an interesting article from a BMW site that talks about filtration comparisons and increased contamination of K&N filters, and rebuttal from K&N.

K&N filter article

I've seen other similar articles that show increased contamination by comparing oil analysis'. I'm not coming down on one side of the issue or the other, it's just more info you may want to consider. I do think it's worth mentioning that K&N filters developed out of the racing world. In that application any erosion that a small amount of dirt may cause is insignificant because the time between overhauls is so short. They just care about getting the flow they need. But when you apply that technology to an engine that you hope to get a couple of hundred thousand miles out of...it may become significant. As was said, it's a trade-off. You have to look at potentially increased engine wear vs. any performance or sound enhancement you expect to get and decide for yourself.

Just food for thought.