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Thu Nov 20 11:46:45 EST 2003

filter and is then captured on the analysis membrane we can calculate
efficiency of the TEST filter in Question.

First, the filters:

BMW Stock Filter, Eff. Area of Media: 8.4 sq ft.
K&N Replacement, Eff. Area of Media: 1.6 sq ft.

The filters are the SAME size.  They both fit in the STOCK BMW M3
The difference is that the STOCK filter has 65 pleats 1.5" deep and the
only 29 pleats each 0.75" deep.

Now, remember this ratio: " 5.25:1".  It's the ratio of the AREA of
to K&N.  It's very important and will come into play later.

The STOCK filter efficiency started at 93.4% at 0 loading and increased
99.2% efficiency as the loading increased to a max tested of 38.8 gm/sq
of dust.

The K&N filter efficiency started at 85.2% at 0 loading and increased to
98.1% at the max tested loading of 41.38 gm/sq ft.

Now, I hear you. "Jim, that's only a FEW PERCENT".  But is it?

Let's look.  If we had 100 grams of dust on a new BMW filter we would
thru a total of 6.6 grams of dust in.  If we used the new K&N filter we
14.8 grams of dust.  Thats 224% (TWO HUNDRED TWENTY FOUR PERCENT!!) more
dust ingested initially, stock vs. "free flow" and this ratio is pretty
held.  Somewhere between 200-300% more dirt gets "ingested" anywhere
loading equivalence.

The more INTERESTING thing is when you look at what happens to the DP or
Differential Pressure at a constant airflow as you dirty both filters
equally with time.

The test used a rate of 75gr of dust per 20 min.  Here's where the AREA
difference comes MAJORLY into play.  See, even though the BMW filter
a bit less at the SAME loading, it also LOADS UP 5.25 times SLOWER due
it's LARGER effective area.  So what happens is that the K&N initially
better, but as the dirt continues coming in, the K&N eventually flows
while still letting MORE dirt in.

Now, does any of this additional dirt cause problems?  I dunno. I
we could have a few people do some independent oil analyses on different
motors using both K&Ns and Stock filters.  Get enough of them, and you'd
have a
good statistical basis.  For me though, it's simple: More DIRT = BAD.

The additional short-term airflow might make sense on a track car.
it doesn't for the street.

-----Original Message-----
From: s-car-list-admin at
[mailto:s-car-list-admin at]On Behalf Of tblack
Sent: March 3, 2003 3:51 PM
To: jimk at; Randy Reimer
Cc: serge411 at; s-car-list at
Subject: Re: [s-cars] Re: MAF foul up

One of the car mags did a comparo of after market filters vs OEM and
concluded that the OEM were as good as, if not better than, the after-market
ones. In fact I recall that the filtering of dust was significantly worse
with the non - OEM replacements. I personally would not use any other than
the OEMs.
My $0.02
Tom Black.

> If there are quantifiable
> results, please post 'em.  There's no real difference in the seat of the
> pants analysis, does the dyno tell otherwise?
> - I've never seen any, and based on my experience, would probably never
> buy a non-stock type filter for an otherwise normal street car no matter
> a dyno shows.
> Jim
> _______________________________________________
> S-CAR-List mailing list
> S-CAR-List at

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