Verhalten hier im Forum, Luftfilter, Liefergrad | Posts 48+

Yes, both are load-dependent quantities, although the DK value can be measured more precisely.

Hi Bertil,

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The fact that the used air filter had the highest "air mass flow rate" in Rainer's test series (but also certainly the highest flow resistance) leads me to believe that the MAF sensor is "lying" because the pressure conditions (within the MAF sensor) do not correspond to the stock condition (new air filter).

Why shouldn't Rainer's old filter, when in new condition, not allow for slightly more flow and therefore be better than other new parts even after 12,000 km?
Or is it a "natural" law that only K+N and similar companies can produce optimally streamlined filters?

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The wire in the mass airflow sensor (MAF) is cooled too much due to the higher airflow velocity, causing the reported air mass to be too high, leading to excessive diesel injection - resulting in black smoke. At K&N, it's quite the opposite.

This argument overlooks the aspect of air density: the faster-flowing, "thinner" air pushes fewer molecules per unit volume past the hot film, which can absorb thermal energy.

Since density is inversely proportional to volume (when mass remains constant), the whole process boils down to a rule like "double speed equals half the density," so that the same number of air molecules pass by the hot film per unit of time.
As if the LMM (mass flow meter) were located before the air filter (because the mass flow rate per unit of time is the same before and after the filter), and it were measuring the reduced flow rate (due to the "braking effect" of the poor filter) under normal pressure and flow rate.

Therefore, I cannot currently follow your argument that there is a measurement error in the LMM due to a "bad" LoF at this theoretical level.
And so, I remain with the assumption that the LMM (mass flow meter) generally captures the mass flow rate correctly, and Rainer's measurements are therefore correct in that regard.

But this also supports Rainer's claim that K&N filters don't provide any performance gain, because the amount of diesel being injected is being reduced!!!

IMO, the above consideration is now moot.

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Hi Ulf,

I assume that the air density behind the used air filter (despite the higher flow resistance) is the same as the air density behind the new filter.
Therefore, my assumption is that the LMM is "measuring" incorrectly.
I don't doubt Rainer's measurements, it's just surprising that the used air filter produced the highest reading and the K&N produced the lowest. That's why I suspected that the air filter, whether used or a K&N filter, was causing inaccurate readings.
The K&N filter doesn't necessarily have to be better than a paper filter (and I agree with you on that). It should definitely be true that a used paper filter is worse than a new one.

Hi Bertil,

Quote:

I assume that the air density behind the used air filter (despite the higher flow resistance) is the same as the air density behind the new filter.

Hmm... the higher the flow resistance, the more pressure "is lost across the filter" (again, the analogy to electrical resistance) - and I don't think that's something that can be easily dismissed.
A pressure drop at the oil filter means that there is less pressure behind it than in front of it.
And if air is suddenly exposed to a lower pressure (i.e., after passing through the filter), it expands, and its density decreases.

Therefore: the higher the flow resistance of an air filter, the lower the air density behind it (of course, under the same conditions, such as engine speed, etc.).

Or where am I going wrong?

Hi Ulf,


That's all okay and understandable. But which theory is correct (equal pressure or not)?
If the pressure drops at the filter, the air behind the air filter should also be colder. Or?

Hi Bertil,

Quote:

If the pressure drops at the filter, the air behind the air filter should also be colder. Or?

Yes, basically it's the reverse gear of the loader. However, this cannot result in any increase in performance, because as soon as the air is compressed again, the cooling effect disappears.

I'm going to summarize the theoretical approaches from this massive thread and post them in a new thread for further discussion.
Perhaps we can finally get some useful explanations of the theoretical foundations of this topic presented here in the forum.