Home made prefilter for K&N

[2KBMW]

I'm new to this forum (1 month) and I
found it suprising about the K&N feedback.
I too bought a K&N filter 5 months ago for
my BMW 328i. I bought it not for the few
horespower I will gain, but for the price.
BMW air filter costs $35.00 and aftermarket
filters are $25 to $30 (cdn). The K&N filter
costs $70 cdn. Anyway, I checked my filter
box and everything past the filter (upper
air box and tube to engine) was clean. I came
up with this idea about using fiberglass cut to size as a pre-filter under my K&N for added
filtration. The incomming air comes from the
front grill area through a intake snorkle
and down to the bottom of the air box. I cut
a cardboard template to get the proper size
and then I cut out two sections of a fiberglass
furnace filter. I used two to fill the gap between the K&N and a chamber that is the bottom
air box from the engine area.The stock air filter
had a sponge bonded to the paper pleats that
just sat on top of that chamber. Will this work?
I have my car in storage now and won't be able
to try this out until the spring.

 

[TC]

Interesting idea. I know that many of us have thought of trying this at one time. I pulled out an unused furnace filter to get an idea of its density. I would think that such a filter addition wouldn't block flow too much, but at the same time, wouldn't capture much except "large" items such as feathers, bits of leaves, very small insects, etc. Basically, picture looking at a truly dirty filter of any type and noticing all the stuff which visually jumps out at you, like the small dead bugs, etc. This stuff probably wouldn't get past the K&N anyway, so you're simply keeping the cotton filter clean but not increasing end filtration. A HEPA type furnace filter would certainly do well, but then, it would also decrease flow to some extent.

In short, this is a cool experiment if you go through with it. But you may have answered your own question with the comment of "...everything past the filter (upper air box and tube to engine) was clean." Sounds like your particular K&N, when kept well-oiled, is doing just fine...

 

[clutchless]

Use some foam for the pre-filter. True Flow makes some foam filters that appear to be of high quality and have many applications. I have a PRM reverse cone cotton gauze air filter on my Mazda MX6. I found that a foam finger mit wheel cleaner from Griot's Garage fit perfectly inside the PRM and have been using it as a pre filter ever since. It made no difference in performance. It does catch far more stuff more quickly than the cotton gauze. I also recommend getting aerosal filter oil and re spraying it every few months as they dry out with all the air pulling the oil off the filter.