AutoXdriver

My apologies if someone has mentioned this already since I haven't read the ENTIRE thread <img src="graemlins/icon501.gif" border="0" alt="[icon501]" /> but even though it may appear a box type K&N has less surface than the stock one due to less folds, it actually has more surface filering area. The reason is that it is foam.

Once dirt makes it through the thin surface of the paper filter its home free and on its way to your cylinder walls. A foam filter has larger pores but it has 100 times (or more - just a guess) thickness and the pores (or holes) are offset all the way through the material so the dirt (theoretically) is constantly bumping into the walls as the airflow direction is changed as it makes its way through the thickness of the filter. This is where the oil comes in. As the dirt bumps into the walls of the pores as it winds its way through it sticks due to the oil coating.

So foam/oil filters are supposedly superior to paper filters because of:

1) Ability to hold more dirt (increased time between changes), due to the increased surface area, meaning the surface area the air comes in contact with all the way through the filter which is many times greater than the surface of the material that you can see - paper filters don't have this quality.

2) Increased flow, due to the larger pores which is still supposed to trap as much or more dirt than paper due to the dirt collection capability of the oil.

Does it really work? Haven't a clue ;-) I do believe however that some tests have shown them to be superior in both of those categories over paper filters. After reading the 928 thread I'm not sure I would want to put one on a MAF metered car, but it also is possible those people were over oiling the filter. I think the biggest problem with foam/oil filters is that most people don't maintain them correctly.

Badue

More folds is just more places for a filter to trap dirt and get stopped up. When you look at the paper filters, the dirt is always in the crease of the fold. No other part of the filter really does any work because the angle of incidence is too low, it just bounces off. That's why the K&N has less folds because it allows more of the filter to be at a higher angle relative to flow.
I can say this. I have a Cummins diesel which has a vacuum guage in between the Filter and Turbo. With a new paper filter the peak and hold guage will have a reading on it after the first hard launch. ( Yes , It's modified and makes 40psi boost) That's a LOT of air being ingested. With the K&N clean and oiled the vac guage doesnt budge untill a few thousand miles. I wont say it makes as much difference on a NA car, but I think for turbos (I have 5 turbo vehicles) I can tell a difference both in seat of pants and EGTs.

Stefan

No big study or anything, but my porsche mechanic (who's been doing it for many years and trained at porsche in germany) said that K&N is fine for track, but in real world driving with dust and debris paper is safer for the engine.

I just did a rebuild due to scoring on my cylinder walls and I was running K&N for the last 40,000 km or so before that had to be done. I didn't feel a seat of pants difference using the K&N and I've got a funny feeling that it let the particles in which scored the cylinders (1 & 2).

Since my track time is minimal I'm sticking with the factory paper filter. Less maintenance is also a plus.

sasilverbullet

Thank you everyone for responding! I think the preponderance of evidence is 'use paper for street'. So that's what I will keep doing, I've got a paper filter in now, and I'll keep changing it every time I change the oil (it's cheap).

Thanks again! This is a great group!

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