Air Filter Mystery: Can It Be Solved?

Hi everyone,

In the thread about "Forum Behavior...", several interesting statements have emerged regarding the effects of air filters and the function of the mass airflow sensor.

I have collected the most important passages here and suggest that the collective intelligence gathered here attempts to develop a comprehensive explanation.
I believe that the argument "it cannot be the case because then..." is perfectly legitimate. However, I ask that we refrain from resorting to personal attacks during this discussion and instead continue it in a factual and constructive manner.
If there are any important statements missing from the following collection of quotes, please feel free to add them.

Quote:

The filling level of the engine is the ratio of the pressure in the cylinder at the intake valve to the ambient air pressure. This explains why the effects are so small in a turbocharged engine! However, in a naturally aspirated engine, even a minimal change can make a difference. (Docter)

Quote:

The sports air filter has low resistance to the air being drawn in, which reduces the intake effort. As a result, the overall efficiency increases. It's not a large increase, but it does increase. (Ingo)

Quote:

The volumetric efficiency of an engine (delivery rate) is determined by the ratio of the amount of fresh gas drawn in to the volume of the cylinder. Consequently, as the amount of air moved per cycle increases, the delivery rate inevitably rises. Therefore, if the delivery rate with K&N improves, the air mass per stroke must increase simultaneously! (Rainer)

Quote:

Because the mass of air per stroke is not dependent on the flow resistance. The amount of air! is determined by the displacement, and in turbocharged engines, also by the turbocharger. (Docter)

Quote:

TDIs have a mass airflow sensor, meaning it actually 'knows' the number of gas molecules being drawn into the engine. The air mass meter can determine a higher delivery rate (filling level) because the volume of gas drawn into the cylinder is related to the air mass, and this volume is measured under the same pressure (namely, the ambient pressure outside the engine, which is the pressure from which it draws air). A mass airflow sensor, on the other hand, only measures the volume of air being drawn in; the actual mass of air (i.e., the amount of gas molecules) still depends on air pressure and temperature! (Rainer)

Quote:

I have to correct you slightly!! The mass airflow sensor can only measure the air mass if the cross-sectional area and flow conditions (flow resistance) are known. If you reduce the inlet cross-section, the air will flow faster past the measuring wire (because the amount remains the same), the wire will cool down more, and the mass airflow sensor will claim: "there is more air" (but that is not the case)!! Unfortunately, that would also explain why you measured a higher air mass with the paper filter than with the textile filter. (Bertil)

Quote:

The mass airflow sensor also needs to determine the air mass, but it does so differently and through additional sensors (temperature). However, the mass airflow sensor cannot necessarily determine the actual pressure in the cylinder, because the pressure in the cylinder can deviate somewhat from the relatively distant air mass values that are measured. In naturally aspirated engines, it's due to resonance; in turbocharged engines, it's due to fluctuations in boost pressure, etc. (Docter)

Quote:

What does a "bad" filter do? IMO, the only possible effect is that it creates (an unnecessarily high) resistance to the airflow. This creates a pressure drop across the filter, analogous to the voltage drop across an electrical resistor carrying current. The total air mass flow rate (mass per unit of time) decreases, again analogous to electrical current. Before the filter, the average flow velocity also decreases (with a constant external pressure), because this is the only way to reduce the mass flow rate. (ulf)

Quote:

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 the speed equals half the density," which means that the same number of air molecules pass by the hot film per unit of time. As if the LMM (mass airflow sensor) 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 poorly performing filter) at normal pressure and flow rate. (ulf)

My personal conclusion:

Assuming there are no defects (normally), all the air entering the engine passes through the mass airflow sensor, and this sensor (based on my previous considerations and the "official" Bosch documentation) actually measures the air mass flow rate independently of pressure, temperature, and flow velocity. Therefore, the cylinder filling calculated by the engine computer (mg/stroke, which is the air mass flow rate divided by engine speed) is accurate.

If an air filter with higher flow resistance (and this is the only way air filters of the same size can differ) were installed, the cylinder filling (air mass!) would decrease, which Rainer also measured in his car's idle, with differences of up to approximately 2%.

What do you think about that?

Hello Ulf,

Although I haven't been closely following the thread, I'd like to add the following:

"If an air filter with higher airflow resistance (and this is the only way air filters of the same size can differ) were installed, the cylinder filling (air mass!) would decrease, which Rainer also measured in his car's idle, with differences of up to approximately 2%."
Sure, here is the translation of the text from German to English:

"End quote:"

The following is evident: Below the turbocharger, the pressure drop caused by the airflow resistance in the air filter significantly affects the density of the air after it passes through the filter.

Regarding the LMM (likely referring to a mass flow meter), it's claimed that it measures less air when the flow cross-section around it is reduced. However, with a constant mass flow rate per unit of time, more molecules are directed directly past the LMM (rather than being deflected a few millimeters away). Consequently, the LMM naturally measures more air because it cools down more significantly.

-------- can be skipped -------------
I don't quite understand the issue. Replacing a paper air filter with a textile air filter on a turbocharged engine makes no sense at all, especially when considering the turbocharger's performance. If someone wants to reduce the rotational inertia on the turbocharger shaft, they should use nylon stockings.

Best regards, Georg.

Hi Georg,

Quote:

I don't quite understand the issue. Replacing a paper air filter with a textile air filter on a turbocharged engine doesn't make any sense at all, especially after the turbocharger is engaged. If someone wants to reduce the backpressure on the turbocharger, they should use nylon stockings.

Well, a little bit of practical sense applies even to turbos: the pressure lost in the air filter has to be compensated for by "increased engine speed" so that the boost pressure can be maintained at the target value.
So, the loader requires more drive power under "poor" air quality conditions, which it draws from the engine – this reduces the overall efficiency, meaning that the power output decreases and/or the fuel consumption increases (in each case, minimally).


Furthermore, my goal is to clarify, using the simplest possible explanations, what different LoRaWAN features can achieve.

I can, for example, not imagine that an air filter with higher flow resistance could provide more power in a naturally aspirated engine.
Because even resonance effects in the intake manifold will not pump more air mass into the cylinders at a lower absolute pressure behind the air filter than at a higher absolute pressure.

Therefore, if "sport" air filters truly offer tangible benefits in some vacuum cleaners, it's likely because, in those specific cases, the sport filter actually has a lower airflow resistance than the standard filter.
Strictly speaking, Rainer's test result only applies to the specific model of the A3 AHF. The result could potentially be completely different for other versions.

Hello!

I have to agree with Georg, at least when it comes to turbocharging with exhaust gases. A reduced air filter flow resistance can only be beneficial in situations where there isn't enough exhaust gas energy available to effectively operate the turbocharger. So, perhaps the response at low RPMs might improve slightly. However, under normal operating conditions, the turbocharger will compensate for any air filter flow resistance, which means that only a small amount of exhaust energy is wasted going out the exhaust pipe. Any minimal increase in exhaust backpressure is probably only of theoretical importance because the excess exhaust energy is far greater than the energy required to overcome the air filter.


Regarding air intakes, I believe that only a low flow resistance can make a difference. To achieve other flow-related effects, where one might accept a higher resistance of the air filter, a more comprehensive optimization of the intake system is required.

Hi there,
Here's my input:
I've been following this topic for quite some time, especially since I'm personally affected. I've now purchased another paper filter, and I'll be installing it in the next few days to compare the results again.
'Background: Shortly before I had my car chipped, I bought a K&N air filter, but I didn't really follow up on it much. I became aware of some discussions where people claimed that after installing such a filter, the top speed (V-max) decreased, and that it would return to normal after replacing it... Hmm... Since I'm not always focused on top speed (and therefore rarely drive that fast), I decided to check it out, and indeed: when it still had 90 horsepower, it would easily reach 215 km/h on the speedometer (speedometer!!!) going downhill. Now, it simply refuses to do that (actually, it's been like that since I had it chipped), and I drive the same route regularly, so I have good comparisons.'
I'll see it in the next few days.
By the way, since I also drive a Fiat, I frequent similar forums, and the experiences are the same. The oil in the filters can even damage the mass airflow sensor (MAF) in some cases!
If you are interested, please feel free to share your experience.
Here's the translation:

'Just a quick question: Does anyone have an idea for a 10-cent fix for a 1.9 common-rail engine?'


greetings
I'm sorry, I can't translate that word because it's not a complete sentence or phrase. It seems like a single word, and I need more context to provide an accurate translation.

Hello everyone,

After using sports air filters for about 10 years, I just can't understand all the hype surrounding them.

My first car, in which I installed a K&N replacement filter, was a first-generation Golf GTI.
Unfortunately, I also had a new paper filter, which I then used for comparison anyway.
My brother also drove the car, and he was, of course, very happy that the car was running better again (the old one was quite dirty).
He was even more surprised when, three days later, I had replaced the air filter (without him noticing).

Of course, you can tell the difference!
The engine starts up faster, and the maximum speed has always been higher.

In the Golf I GTD, it was even 5 km/h (the paper filter was only one month old and didn't need to be replaced yet).

I must add that the GTD engine is essentially mechanical, with the exception of the starter, glow plug system, and fuel shut-off.
He didn't bother to measure anything.

Dass der Motor aber viel einfacher läuft, wenn er weniger Kraft aufbringen muss um sich seine Luft zu holen, sollte doch jedem einleuchten. Or?

The SF (Sandtler) replacement filter is also very good; it's oil-free and uses static electricity to capture particles.

I've been driving my Golf III TDI with an open K&N air filter for 1 1/2 years now, and I haven't had any problems at all. (I'm aware of the pros and cons).
The only drawbacks with mine are: warm air up to approximately 80 km/h, and only one EC approval - nothing else!

Regards,
Michael.

The mass airflow sensor also needs to determine the air mass, but it does so differently and through additional sensors (temperature). However, the mass airflow sensor cannot necessarily determine the actual pressure in the cylinder, because the pressure in the cylinder can deviate somewhat from the relatively distant, previously measured air mass values.
In naturally aspirated engines, it's due to resonance; in turbocharged engines, it's due to fluctuations in boost pressure, etc. (Docter)



I need to correct myself...
The mass airflow sensor (MAF) knows the air intake level quite accurately (provided there are no contaminants affecting the actual measurement). This is related to the measurement principle. Unlike a volume airflow sensor, the MAF can measure effects like resonance in the airflow. A volume airflow sensor cannot do this, even though its measurement principle determines the air mass. However, it's not as accurate.

Hi everyone,

Has anyone ever recorded and compared the boost pressure curves of the two air filters during a 1000-4000 RPM test?
I think that should make the difference visible.

Regards,

Eike.

Michael B wrote:

After using sports air filters for about 10 years, I can't understand all the excitement surrounding them.

Hi.

Well, describing it as "excitement" might be an overstatement. We're simply trying to resolve the initial apparent contradictions between Rainer's well-founded and, in that respect, truly "objective" measurements and the conflicting anecdotal evidence.

Quote:

My first car, in which I installed a K&N replacement filter, was a first-generation Golf GTI. Unfortunately, I also had a new paper filter, which I then used for comparison anyway. My brother also drove the car, and he was, of course, very happy that the car was running better again (the old one was quite dirty). The more surprised he was when I, three days later (without him noticing), had replaced the air filter.

Hmm *frowns*... which filter was the "dirty" one, and with which filter did the car run better, or best?

Quote:

You can definitely notice the difference! The engine starts up faster, and the maximum speed has always been higher. In the Golf I GTD, it was even 5 km/h

(the paper filter was only one month old and didn't need to be replaced yet).

If the top speed actually increases by around 160 to 163 km/h, it requires almost 10% more power to overcome the increased air resistance. If I remember correctly, the GTD has a power output of 75 horsepower, so the air filter alone would have had to provide approximately 7 extra horsepower.
I consider that to be utopian... on the other hand, a tailwind of approximately 7 km/h can create the illusion of a top speed increase of about 5 km/h .

One should also not forget that "pleasant" events always take precedence in memory over undesirable things, and therefore, potentially disappointing experiences with the sports filter are simply suppressed or ignored... this is not meant as an accusation, it is simply human nature .

Quote:

It should be obvious to everyone that the engine runs much more smoothly when it has to exert less force to draw in its air.

Yes, indeed... and for that to be the case, the (new!) sports filter would need to have a lower flow resistance than a (new!) standard filter.
And Rainer has proven that this statement is incorrect, specifically regarding the filter for the A3 AHF.
It doesn't exclude the possibility that sports filters of other designs are actually better than standard filters, but in my opinion, the odds suggest otherwise - why would Rainer specifically have tested a faulty sports filter?

Quote:

I've been driving my Golf III TDI for 1 1/2 years with an open K&N filter, and I haven't had any problems at all. (I know the pros and cons). The only disadvantages with mine are: warm air up to approximately 80 km/h, and only one EC approval - nothing else!

Sure, this thing probably won't have any obvious or significant drawbacks, but does the TDI actually perform better than with a standard filter, according to objective criteria (measurements / which ones?)?

Oh dear, what have I gotten myself into!



If there's interest, I can ask our engineer in the fluid dynamics lab; maybe he can help me. However, I can only do that after the exams, so towards the end of next week. I can also only run the comparison test then.
Please let me know, and feel free to post any questions here! Or, you can contact me by email.




I always assume it's a naturally aspirated engine because I don't have a TDI, and naturally aspirated engines are simpler to understand (or at least, I hope they are).

The volume of air that the engine 'inhales' is always the same because the volume is determined by the stroke. Since a K&N filter has a lower flow resistance, the only thing that can change is the density of the air entering through the filter. The atmospheric pressure is constant, and the flow cross-section is not changed. Even if the K&N filter has a larger surface area, that shouldn't be relevant because the projected area is what matters – i.e., the cross-section of the intake manifold. (Or is that a misconception?)

Okay, so, based on that, the air density after the K&N filter should be higher than with a paper filter. Consequently, there's more oxygen in the combustion chamber. But does the engine control unit (ECU) or its associated components – the sensors – detect this? (Just a thought).



A finding I made with my SDI (presumably a device or system) is that it performs worse in the summer heat of midday compared to the cool night air. It seems to run best, based on my feeling, at temperatures between 4 and 8 °C. At these temperatures, the air density is also higher. Of course, it's possible I'm mistaken, but I've noticed this repeatedly without explicitly paying attention to it.
Since I almost always drive the same route, I have quite good comparisons.


I hope we can resolve this soon. How about writing a letter to VW, since the workshops probably don't have an answer?


Best regards,
Ingo

Assuming: A sports air filter has lower resistance. Otherwise, it's often really bad quality and should go straight in the trash.
This assumption is a prerequisite for me to even participate in this discussion. Otherwise, the topic would be closed. However, the discussion only becomes interesting when we consider a lower level of resistance.

Under that assumption, it's clear that an SDI (supercharger) simply delivers more power.

With a turbocharged diesel engine, things become quite complex. First, the turbocharger ensures a constant boost pressure. Especially in the area of top speed (which is supposedly increased in some cases with a K&N filter), it can generate this pressure. Therefore, the combustion process is essentially the same.
Therefore, only the analysis of the exhaust gases remains. In this case, the turbocharger presents a greater resistance with a less permeable air filter. However, it must be discussed whether this has any effect at all.

Here, we need to compare the turbocharger with other types of mechanical chargers. A mechanical charger requires energy that comes directly from the engine. How does the turbocharger work differently?
The turbocharger, while acting as a resistance in the exhaust, begins to spin even before the piston enters the power stroke. A significant portion of the hot air is evacuated from the cylinder without requiring any work to be done. This only works because the air is hot. As long as it's hot, it's fast. Further down in the exhaust, it slows down because it becomes denser. That's why the turbocharger needs to be located very close to the engine. It utilizes the hot, expanding air and is not just a resistance that the piston has to overcome. Otherwise, we would only have compressors and superchargers.

In the further considerations, it must be taken into account to what extent the extra work that the turbocharger has to perform either reduces the power available for propulsion or, conversely, largely reduces thermal losses.

My feeling is: for series production vehicles, it doesn't really achieve anything.
'In tuned vehicles, we often see higher and hotter exhaust gas temperatures. There are likely areas where we can push the turbocharger a bit more without directly compromising the power output.'
So, however you look at it, I don't need a K&N filter for more power. Maybe just for a little turbo protection and a good feeling.

Ingo wrote:

The volume of air that the engine "inhales" is always the same because the volume is determined by the stroke. Since a K&N filter has a lower flow resistance, the only thing that can change is the density of the air entering through the filter. The atmospheric pressure is constant, and the flow cross-section is not changed. Even if the K&N filter has a larger surface area, that shouldn't be relevant because the projected area is what matters – i.e., the cross-section of the intake manifold. (Or am I mistaken?) Bitte gib mir den deutschen Text, den du übersetzt haben möchtest. Okay, so, according to that, the air density after the K&N filter should be higher than with a paper filter. Consequently, there would be more oxygen in the combustion chamber. But does the engine control unit (ECU) or its peripherals – the sensors – detect this? (Just a thought).

Hi Ingo,

"Since a K&N filter has a lower flow restriction..."
That's precisely the point where Rainer's measurements actually document the opposite!

The rest of your explanation is, in my opinion, largely correct - if your assumption were true.
The amount of oxygen (actually, air mass) in the cylinder is measured using the mass airflow sensor (MAF).

The filter area may be of interest (even though it is larger than the pipe's cross-section), as each unit of area - e.g., cm² - can be considered as a flow resistance element with a specific resistance value.

If the surface area were doubled, for example, while using the same filter material, the flow resistance of the filter would have to be halved, analogous to connecting two identical electrical resistors in parallel.
In my opinion, this also applies even if the filter area is larger than the cross-sectional area of the intake manifold.

Ultimately, the theoretically optimal filter would have an infinite surface area so that its flow resistance (which every filter material has) would be practically zero.

A comment from your fluid dynamics engineer would certainly be interesting.

Martin L. wrote:

Assumption: A sports air filter has lower resistance. Otherwise, it's really junk and should go straight into the trash. "This assumption is a prerequisite for me to even participate in this discussion. Otherwise, the topic would be closed. However, the discussion only becomes interesting when we consider a lower level of resistance." . . . .

Hi.

. . . hinges on a single point of aerodynamic resistance, which determines the very justification for "sports" filters.

That's already a good approach to make the whole story more understandable.

So, the question is: what are the actual flow resistances of sports filters and standard filters?

Even though it's becoming a bit embarrassing: Rainer's, in my opinion, correct measurements and conclusions show that at least , not every sports filter is better than a standard part.

To make more general statements, it would be necessary to have as many comparative measurements as possible of the drag coefficients between standard filters and their sports-oriented counterparts.

Does anyone know of a source for such documented measurements? Ideally, something as "objective" as possible, rather than from the manufacturers/suppliers of sports filters .

Well, then I'll start too .

As far as I can see, there is a consensus on this:

An air filter with a higher airflow (also known as a sports air filter) generally allows for better cylinder filling.

This also aligns with my own experience with carbureted engines in cars and motorcycles. I can confirm that effects such as increased power and improved throttle response are possible for these vehicles.

BUT:

'This effect often reverses in engines with fuel injection systems, frequently leading to the exact opposite outcome. While single-point injection systems are often unaffected (a good example being the 1.6i 75HP engines), which still perform noticeably better with open filters or modifications, multi-point injection systems often experience DRASTIC power losses. Modifications usually result in only marginal improvements, as the original air intake and air filter housing often act as limiting factors. Open filters (which are often used for the sound) frequently result in disappointing performance.'
Why? With multi-point injection, you can position the intake manifold however you want, allowing you to utilize resonance effects. And that's exactly what's done, and quite extensively. In my old Opel Ascona with a 2.0i engine, simply removing the pipe from the air filter box resulted in a significant loss of power. With an open air filter box, Fiat Pandas were able to overtake me...

Although the filter flow rate is better, the cylinder receives a much poorer fuel mixture because the resonances intended by the designer are no longer present, and the valve timing no longer matches the intake manifold.
This resonance phenomenon, incidentally, also applies to Diesels to a limited extent. Variants with a throttle body intake manifold may suffer from this (DTI).

I believe that improved throttle response in diesel engines is a huge overstatement. A diesel engine always draws in its full volume of air. Unless the filter is extremely clogged, it makes no difference whether it has 10% more airflow or not. When I press the accelerator, more fuel is injected. So what? The air-fuel ratio is already so high; what difference does a tenth of a bar more or less make? Compared to the engine's power output, the losses in the intake system are laughable... because I'm not suddenly increasing the airflow, like in a gasoline engine!

With a turbocharger, it's different. I believe that there's a good chance you can gain advantages when installing the turbocharger. This is because the airflow is significantly altered. Less resistance here would mean a faster build-up of boost pressure. But that's about it.
In modern TDI engines, the turbocharger boost pressure is REGULATED according to a specific characteristic curve, and if necessary, forcefully, using a variable turbine geometry (VTG) system. The final power output is hardly changed by a filter, as long as the filter allows the required amount of air to pass through.

Regarding older turbocharger designs, I can imagine certain advantages. In this type, the boost pressure isn't regulated, but rather limited. This is a significant difference. Instead of following a specific characteristic curve, the turbocharger simply delivers whatever boost it receives, up to the set limit. Given that these systems often involve relatively simple technology and can exhibit turbo lag, I could potentially see benefits from allowing more free airflow. However, I don't have high expectations in this regard. I believe one might expect a better response to load changes, particularly in situations like 'full throttle, idle, full throttle' just before overtaking.

und noch eins: da diese lader den ladedruck auf eine althergebrachte art begrenzen (bypass-ventil mit federbelasteter druckdose) hab ich folgende theorie: ich würde so einen lader so konzipieren, dass er bei vollast des motors knapp über dem maximalen ladedruck erzeugen kann. auf jeden fall muss es so wenig sein, dass ich es per bypass SICHER im griff habe. da ich keine überwachung elektronischer art habe muss ich diesen weg gehen, da im fehlerfalle 'überdruck' die ESP einfach nur versucht die fördermenge dem ladedruck entsprechend zu stellen (was ja auch per druckdose geschieht). heisst im ungünstigsten fall... KNALL....

So, I need to design the system in a somewhat defensive manner and give the turbocharger a gentle pressure curve. It can't generate high pressures at low engine load. Then, at full load, it would produce too much pressure.
(Note: The TDI engine reduces the fuel injection amount if the boost pressure cannot be regulated! Therefore, the turbocharger can operate at a 'high' level.)

-> In this case, I can certainly imagine the advantages of an open air filter, because this turbocharger benefits from as little restriction as possible.
For these types of containers, regularly replacing the air filter often works wonders!

Okay, let's go...

Okay, and one more thing about the Mass Air Flow (MAF) sensor: these sensors aren't placed randomly. MAF sensors are sensitive to uncontrolled airflows. An open cone filter, placed directly in front of the MAF sensor, can completely disrupt its operation. Older MAF sensors with flaps often started to oscillate. Hot-wire MAF sensors measure the airflow in the *bypass stream*, meaning a separate part of the total airflow. If a swirl of air enters precisely at this point, the entire measurement becomes inaccurate.
'With Moto-Jetronic systems, problems are rare because they usually don't use mass airflow sensors (MAF). Instead, they typically use pressure sensors for the intake manifold pressure, throttle position sensors, and calculate the fuel requirements based on these values plus engine speed. Fine-tuning is then achieved using a lambda sensor.'

Here's what's left that Ulf mentioned: what IS a sports air filter ?

CU Gremlin.

Hmm, I'll see if I can find any technical specifications for Luffi cars tonight, and I'll also ask a friend who studies automotive engineering.


The manufacturers will likely also test and conduct experiments in that direction.



I'll look for it later, but I need to go now.


I don't doubt Rainer's measurement, but it seems strange to me.
The measurement is correct, unless proven otherwise, which probably won't happen anytime soon, or perhaps never. We'll see.




What makes me suspicious is that when you blow through a standard filter, you get the impression that all the air is...
Air passes through the filter. With the standard filter, air is recirculated.
I'll check with a parts supplier about that at some point.



It's a great discussion that clarifies many interesting points -> definitely worth reading.

Best regards,
Ingo

Gremlin wrote:

An air filter with a higher airflow (also known as a sports air filter) generally allows for better cylinder filling.

Okay, that makes sense so far, but I suggest replacing the term "air flow rate" (which, as you know, also depends on environmental factors like engine displacement and RPM, etc.) with "flow resistance," because that's a property specific to the filter itself.

Quote:

While single-point injection systems are not affected by this (a good example are the 1.6i 75HP engines) and often run noticeably better with open filters or inserts, multi-point injection systems often experience DRASTIC performance losses. Inserts usually only result in marginal changes, as the original air intake and air filter housing often act as limiting factors. OPEN filters (which are often used for the sound) often result in long faces. Why? With multi-point injection, you can position the intake manifold however you want, allowing you to utilize resonance effects. And that's exactly what's done, and quite extensively. In my old Opel Ascona with a 2.0i engine, simply removing the pipe from the air filter box resulted in a significant loss of power. With an open air filter box, Fiat Pandas were able to overtake me... Although the filter flow rate is better, the cylinder receives a much poorer fuel-air mixture because the resonances designed by the manufacturer are no longer present, and the valve timing no longer matches the intake manifold. This resonance phenomenon, incidentally, also applies to Diesels to a limited extent. Variants with a throttle body intake manifold may suffer from this (DTI)

.

Therefore, this also addresses the "secrets" of resonance charging (or whatever it may be called).
It is clear that this effect depends on the design of the intake manifold. Consequently, open filters, which inevitably have a shorter overall intake pipe length, can completely disrupt the overall tuning, which can sometimes significantly increase performance.

However, in my opinion, this aspect of the filter question should not be discussed further here, as the overall effect is based on the sum of two completely different phenomena (namely resonance and flow resistance), and one could argue endlessly about which effect contributes which specific part of the overall result.

Quote:

I believe that improved responsiveness in diesel engines is a huge overstatement. A diesel engine ALWAYS sucks in its full volume of air. Unless the filter is extremely clogged, it doesn't make a difference whether it has 10% more flow rate or not. When I press the accelerator, more fuel is injected. So what? The air-fuel ratio is so high, what difference does a tenth of a bar more or less make? Compared to the engine power, the losses in the intake manifold are ridiculous... because I'm not suddenly increasing the airflow, like in a gasoline engine! When it comes to turbochargers, it's different.

In that case, I believe it's quite possible that you can gain advantages when installing the turbocharger.

Quote:

This is because the airflow is significantly altered.

Less resistance would mean a faster build-up of boost pressure.

Quote:

But that's all there is to it.

Basically, I agree.

Quote:

In modern TDI engines, the turbocharger boost pressure is REGULATED according to a characteristic curve, and if necessary, forcefully, using a variable turbine geometry (VTG). The final power output is hardly changed by a filter (as long as it allows the required amount of air to pass through).

What Rainer's measurements confirmed...


With the old pre-chamber turbocharger technology, I could imagine certain advantages. Here, the boost pressure is not regulated, but limited. This is a significant difference. No characteristic curve is developed; instead, the turbocharger takes whatever it gets, up to the set pressure limit. Since this often involves "stone age" technology with turbo lag, I could definitely see advantages with unrestricted airflow. However, I don't have high expectations. I think we might expect better responsiveness to load changes, especially in the "full throttle, idle, full throttle" situation just before overtaking...

"Free airflow" would likely again refer to an open filter with a completely redesigned intake geometry.

Quote:

-> In this case, I can certainly imagine the advantages of an open air filter, because this turbocharger is happy with as little obstruction as possible. For these boxes, a regular air filter replacement often works wonders!

Ah, yes, that's it... I don't want to contradict you factually, but I'd rather steer the discussion back to a

Quote:

more manageable aspect: drag. :-)so....fire away...

Hihi... I don't see any reason to fire }.


}there's still something Ulf mentioned: what IS a sports air filter }}

for me: any part that is advertised as offering improved performance compared to the original filters, or where such an impression is created, e.g., "more airflow for your engine..."