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      03-04-2011, 11:33 PM   #1
Kodak
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Question Which air filter ?

Looking to find out if it makes any difference in a stock BMW air filter or K & N , AFE, etc, oiled or non oiled for an E90, 325 xi, complete stock. Undecided as to which one to purchase.
Thanks in advance.
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      03-06-2011, 05:26 AM   #2
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Oiled filters are tried and true, you can't go wrong with them. I would recommend K&N, so many people use them you can't have problems. And you can probably gain 2-3hp at high RPMs from it.
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      03-06-2011, 07:47 AM   #3
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I used to be a big proponent of K&N and had a K&N on every vehicle in my fleet (except the E90) since 1997. I still have a K&N in my Honda Valkyrie motorcycle and I think in my wife's Z3. The one I had for my F150 never fit perfectly like an OEM filter does, and eventually the rubber gasket fell off the filter body. My F150 suffered from serious dirt contamination that caused drivability issues and required a full intake system cleaning to resolve. I've since switched back to a paper filter for the F150.

The qualified results of HP gains and MPG gains all come from K&N testing (I'm not sure if there is true independent testing). All I can say from my experience is I didn't notice any dramatic increase in performance nor MPG gains in any of the five vehicles I've used them in (increased intake noise was a benefit).

It is nice to be able to clean them and reuse them, but the issue I have with that is re-oiling the filter element. If you over-oil it, the excess oil could possibly damage sensors in the intake tract (depending on the types used). So if you under-oil it, the effectiveness of air filtration is compromised, if you over-oil it, you may cause induction issues with the engine (especially in this day and age of strict emissions monitoring by the OBDII system). So attaining the right balance of oil on the filter is a guess game since there is no real hands-on training offered and there's no good way to tell if you've over-oiled the element; the oil is red, and more or less oil doesn't increase or decrease the intensity of the color. The other problem is the cleaning interval is 50,000 miles, so you never can get good practice at cleaning and re-oiling them (unless you have several cars that use them like I had).

I think if you swap to an open intake system that uses a cone filter then K&N is the way to go. But using one as a drop-in I'm not so sure it's worth the trouble and expense. I used a cone filter on my E30 and the K&N did work well in that application, however the E30 has an old style MAF sensor that does not use a heated element in it, so the oiling issues are not a concern.
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      03-07-2011, 08:39 PM   #4
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I have a modified intake box with a K&N in it. I noticed the bit of power right away!

If you look in the air box there is a filter on front of the replacable filter. We removed that, and cut out the whole front of the box. It sounds and performs great.
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      03-08-2011, 09:41 AM   #5
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Quote:
Originally Posted by n52330i View Post
I have a modified intake box with a K&N in it. I noticed the bit of power right away!

If you look in the air box there is a filter on front of the replacable filter. We removed that, and cut out the whole front of the box. It sounds and performs great.
Removing the charcoal filter is a pretty common thing for us to do, but I wouldn't cut my airbox like that. Now you are drawing in more hot air from the engine bay instead of it pulling from the cooler air space behind the kidney grills.
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