Air cleaner CFM question...

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rndthought - 04 Oct 2005 20:27 GMT

First, this is born out of a "mental exercise" between the parts boys
and myself...about the merits of K&N air filter...filtering efficiency
withstanding, they are being marketed on the sole concept of CFM at the
local parts store.

How would one figure the CFM required at some RPM for an "X" liter
motor?

Would it consume "X" Liters every 2 revolutions? (4 stroke,
up/down/up/down - so one time it goes down it will be due to the
combustion stroke not intake...right?)

If:
1 liter = 0.0353 ft^3 (man, can't imagine cramming 30, 2 liter
bottles in a 1 foot cube box!)

So would this be correct? (Liter) * (RPM)/2 *(0.0353) = CFM

For a 2.3 liter motor with a Red Line of 8000 RPM:
(2.3)*(8000/2)*(0.0353) = 325 CFM

Just trying to determine if anyone really needs a K&N air filter that
claims 450 CFM (in this case) - especially since most people don't
generally drive around at the Red Line!!!

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rndthought - 04 Oct 2005 22:17 GMT

If this is correct, it will only be valid for a normally aspirated
motor. Anyone know the math to come up with the equation for CFM at a
given RPM for a given boost PSI????

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jg - 04 Oct 2005 22:33 GMT

> If this is correct, it will only be valid for a normally aspirated
> motor. Anyone know the math to come up with the equation for CFM at a
> given RPM for a given boost PSI????

It's still meaningless without a comparative resistance at any particular
cfm. Generally the only way to decrease resistance is by decreasing the
capacity to catch particles, or increasing the area of the filter media
(either by making it bigger or having pleats etc). And I guess engines are
designed to operate against a certain resistance, as they are with exhaust
systems.

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James Sweet - 05 Oct 2005 03:01 GMT

> > If this is correct, it will only be valid for a normally aspirated
> > motor. Anyone know the math to come up with the equation for CFM at a
[quoted text clipped - 6 lines]
> designed to operate against a certain resistance, as they are with exhaust
> systems.

At least stock Volvo filters are generally very good, MUCH larger than those
used in many cars with similarly sized engines. I don't think there's much
to gain with an aftermarket filter.

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jg - 05 Oct 2005 06:32 GMT

> > > If this is correct, it will only be valid for a normally aspirated
> > > motor. Anyone know the math to come up with the equation for CFM at a
[quoted text clipped - 10 lines]
> used in many cars with similarly sized engines. I don't think there's much
> to gain with an aftermarket filter.

I've never noticed a difference after replacing dirty filters, makes me
think less resistance wouldn't be noticed either. Except if filtration is
worse, dust could grind the motor out faster.

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User - 06 Oct 2005 06:51 GMT

> First, this is born out of a "mental exercise" between the parts boys
> and myself...about the merits of K&N air filter...filtering efficiency
[quoted text clipped - 20 lines]
> claims 450 CFM (in this case) - especially since most people don't
> generally drive around at the Red Line!!!

Unless a filter is severely restricted the only time flow is affected by
anything other than the restriction forced on the system by the throttle
plate is at wide open throttle. If you have determined that the intake
air flow is insufficient to provide satisfactory performance then you
will have to do some serious elargement of the throttle housing diameter
coupled with intake runner improvements (both length and diameter,
interior surface preparation-streamlining and proper surface finish to
enhance turbulence for rapid fuel vaporization and so on). If you just
want eye candy K&N is the way to go--the kits are pretty.

K&N filters have captured the fancy of many enthusiasts through clever
and effective advertising. The one caveat I've found is that cleaning
and oiling the filter must be done very carefully. Any excess oil that
is drawn into the inlet piping will eventually deposit on the hot film
or hot wire of the mass airflow sensor. Even though the burn off cycle
for the sensor occurs after every shut down when the motor has exceeded
about 2000 rpm it is inadequate for vaporizing the oil that lands on it,
unlike the dust particles that it was designed to eliminate. This
ongoing accumulation of varnish on the hot film or hot wire will cause
premature, expensive failure of the part.

Bob

Signature

The goal when driving is to miss the maximum number of objects.

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Randy G. - 06 Oct 2005 18:02 GMT

>Unless a filter is severely restricted the only time flow is affected by
>anything other than the restriction forced on the system by the throttle
[quoted text clipped - 16 lines]
>ongoing accumulation of varnish on the hot film or hot wire will cause
>premature, expensive failure of the part.

The K+N's do have the ability to flow more air, but that is not always
a good thing. A LOT of years ago (over two decades, actually) I bought
one for my BMW motorcycle. This bike has dual constant velocity carbs.
The air filter flowed so mch air that the carbs would have had to be
rejetted to run correctly. I went back to the stock filter and no
problem.

Many of the K+Ns I have seen look smaller than the stock filter, but
they pass more air, more freely? At what cost? How much more dirt do
they pass as well? If you want a freer breathing motor, have it ported
and polished and have a custom exhaust system made to match. If you
are looking for simple, quick fixes that make more horsepower, I have
a fuel line magnet that I removed from my 240 you can buy from me.
Don't you think that if the manufacturers of the vehicles could
squeeze more power from their vehicles with a different air filter
that they would do it?

The problems you outlined are compelling enough reason to not use one
on a car with an AMM.

__ __
Randy & \ \/ /alerie's
\__/olvos
'90 245 Estate - '93 965 Estate
"Shelby" & "Kate"

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