On 11/5/2014 4:13 PM, Jim Stone wrote:
> I have been watching old episodes of Wheeler Dealers lately and the last two I
> saw were about an MX-5/Miata and BMW 3 Series. In both cases, one of the
> first things Edd did was replace the stock air cleaner assembly with a
> performance air filter. According to Edd, it is all good: increased
> horsepower, better sound, etc. However, I have to believe that the
> contraption he is replacing (filter box, filter, hoses, etc.) costs the
> manufacturers more than than performance filters would. If the KN options (I
> think that is what Edd is using) are so good and increase horsepower, why
> wouldnt the manufacturers - particularly BMW - put them in to begin with?
Well, substantially increased maintenance requirements and certain
finicky incompatibilities with some engine controls, increased noise,
potentially reduced engine life depending on the filter material, etc.
And they only offer any sort of performance improvement
The K&N and similar oiled-foam filter elements are the subject of
religious wars, some feel that they do a poor job of trapping smaller
contaminants, they certainly have less surface area than a typical
OE-type pleated paper filter and will clog up faster.
The filter itself is expensive compared to a decent paper filter, and
while some folks do just toss them every year they're intended to be
serviceable through cleaning with soap and water and re-oiling. In a
modern engine equipped with a hot-wire mass-airflow sensor, failure to
get ALL the excess oil off the filter after re-oiling can result in
contamination of the MAF, something which many automakers have gotten
quite good at detecting (and it will NOT be covered under warranty.)
The filter itself is rarely an obstruction in modern intake design; the
design of the airbox plumbing can be, though that's usually for
noise/sound control, keeping the snorkel high enough that it doesn't
suck in water from puddles, etc.
Personal opinion: they're not snake oil but they're rarely a significant
improvement. I have them on a couple cars, but they have to be
maintained. Our 195K-mile '98 540i has had a Dinan intake with such a
filter for almost all its life.
You also have to look out for the various quirks of aftermarket intake
systems - if the intake is not sealed off from hot engine compartment
air you are probably losing power from it, and in the case of our 540i
the filter is located low on the right side, and there have been cases
of cars with that setup sucking enough water to hydrolock the engine.
Certainly that's never happened to us, but I did manage to suck enough
spray from a puddle along a curb to destroy a MAF once...
Was once on a tour of the Dinan facility, eight or nine years ago? They
were doing some work on an E60 545i or 550i and Steve Dinan noted that
the air filter element was actually two separate units - the usual paper
filter, with an activated-charcoal filter underneath it to help the car
pass the EPA hot-soak evaporative-emissions test (prevent vapors from
any drops of fuel that might leak out of the injectors from making their
way back through the intake passage.)
He said just removing the charcoal filter was worth 15HP.
John.
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