I bought a K&N filter for my 1987 Toyota about 165000 miles ago and what I
have noticed is it has shrunk somewhat, it still fits good and passes smog
every time. the filter is 20 years old and is cleaned with every 3000 mile
oil change. So that's 55 air filters @ 9 dollars a piece or a savings of
about 495 dollars.

After reading all the opinions about the K&N air filters and my own
experience and reviewing, the filter designs are somewhat flawed. I think
K&N could have used better material that does not shrink over time. Advanced
Flow Engineering Air Filters claims better material than what I have in my
old, yet still good K&N air filter. I bought one for my 330CI and am hoping
for the best.

Cheers,

Brian

The problems with K&N go beyond the poor fit (and they do fit poorly..)

They also don't filter as well as the OEM paper filter - in numerous
tests it's been shown they let more dirt through than the factory filter
does. The oil on them also has a bad habit of migrating down the
air-stream through your MAF (Mass-Air-Flow sensor) - and often is
responsible for the failure of the MAF (which costs $200-500 depend on
what engine you have and how many you have..)  BMW will not warranty any
MAF on an engine using an oiled filter.  The filtering is worst when the
filter is freshly cleaned and oiled, and gets progressively better over
time as the filter starts plugging up.. until it gets so plugged up the
dirt ends up being pulled through the coarse cloth mesh.  They are not
really a good thing for your engine.

The cost savings is actually also questionable. BMW makes a filter that
tends to self-shed dirt it picks up. They typically last well beyond the
recommended replacement interval with some simple removal, and tapping
out of trapped dirt. On ones that are an updraft design - it isn't even
necessary to perform that cleaning step - the design is such the dirt
falls out of the filter into the airbox.  For the cost of one K&N you
can purchase at least 3 stock air filters from a discount vendor.

Performance gains also can't be reliably shown. BMW is awfully good at
designing engines and designing them for maximum performance. There is
no reason for them to go undersize on an airfilter since the cost per
unit increase for an adequately sized one is negligible.  They're not
going to leave HP laying around on the table for some bozos cobbling
crap together in a garage to pick up.

IMHO - K&N = marketing over performance.

I realize this is an old thread but as it happens, I was discussing
this very thing with my mechanic today. I've read about these filters
on the forums and was curious. The people backing K&N claim a 3-4 MPG
gain in normal driving, and a HP or two gain in more spirited driving.
I was also intrigued by the "clean & re-use" aspect of these filters.

My mechanic (Wittler Auto here in Albuquerque) told me that in theory,
these filters might be a good idea, but actually, they have a fairly
serious drawback. The seal is made of a silicone material, and
according to the Wittler, they've seen lots of these filters with
deformed seals. Seems that the seals deform when they get hot, i.e.
the silicone gets too soft. Thus, when the seal is deformed,
unfiltered air gets through to the engine. Thus, they aren't too fond
of these things. He did say that *if they were made better* they might
be OK. But, they would cost a lot more, too. So, for the time being,
I'm skipping them and staying with the standard OEM type filters in my
530.

Karl Winkler
http://www.sandiastrings.com

Just wondering if replacing the stock filter with the above would noticeably
improve performance or sound.

Any comments?