Lots of show, little benefit: Sport air filters (Articles)

Sportluftfilter-Test

Because many advertisements for sports air filters contain misleading statements and insinuations regarding performance gains, this aspect will be explained here based on basic characteristics of car engines.

In order for an engine to produce the highest possible power (i.e., to be able to "cleanly" burn the maximum possible amount of fuel per working stroke), air with the highest possible density must enter the cylinders, i.e.
-> as cool as possible and
-> with as much pressure as possible.

Since every pressure drop in the intake system reduces the air density, the entire intake path, including the filter, should have as low a flow resistance as possible. Only the air with the least (but essentially unavoidable) pressure loss enters the engine.

In reality, the pressure is already present in the air filter box under the external pressure, and this is particularly noticeable at high air flow rates, i.e. at full throttle and high engine speeds = Pmax.
The resulting loss of air tightness in the intake air is almost exclusively caused by the intake valve, which is often the narrowest point in the intake path, through which all the air must flow.
For example, in TDIs, the vacuum ports are typically up to around 50 mbar behind the intake valve, , so even before the actual air filter , which is a common size.
A 50 mbar pressure drop compared to the ambient pressure can block approximately 5% of the possible power output of a gasoline-aspirated engine; in charged engines (including TDIs) this loss is practically completely offset by the turbocharger.

Standard paper filter inserts cause very little additional pressure drop - as long as they are reasonably clean.
For example, if a paper air filter causes 10 mbar (additional) negative pressure after several thousand kilometers of use, and a new or different (sports) filter only causes a 4 mbar loss, then replacing the filter would reduce the pressure loss due to the filter by 6 mbar.
This 6 mbar pressure gain would, in the case of naturally aspirated engines, correspond to a power gain of approximately 0.6%, while a "standard" carburetor still results in a loss of approximately 5% of potential power.

Even better, the balance sheet should be even better for current paper-wicking filters with mutually closed filter channels. This type of filter offers a very large surface area in a small footprint.

Based on experiences from the forum (thanks, Julian ), filters can also become so clogged during normal maintenance intervals that a noticeable loss of performance and increased fuel consumption occur.
The time when a paper filter should actually be replaced can be estimated by observing its light transmission, by holding it up to the apparent sun: With a new filter, light passes through the folds of the paper particularly clearly, and the lamellae also allow some light to pass through.
However, if no or very little light passes through a used filter, it should be replaced.

If there is a lot of coarse dust in the filter, it may be possible to slightly improve its permeability by tapping (carefully placing the element with the inlet side flat on a smooth surface). However, filters are usually so clogged with fine particles that even after tapping, light no longer passes through -> replace.

However, as long as a paper filter is regularly checked and/or replaced according to the operating conditions (e.g., dust levels), it does not cause any noticeable performance loss or increased consumption.

That, however, does not prevent suppliers of sports air filters from advertising in

1. Plate elements (also known as swap filters)
...to create completely utopian expectations of performance gains for potential buyers, which seem only conceivable if, for example, an unused paper filter with over 200,000 km of mileage is replaced with a new filter. ...whereby a paper filter, due to the aforementioned reasons, would achieve the same performance gain as a sports filter.

Rainers comparative measurements using 2 standard filters and a sports filter confirm that sports replacement filters do not, at least in TDIs, provide any significant improvements in the engine's air supply.


Here, a fabric filter insert from the company *** was tested. The accessories retailer promises buyers of these fabric air filters that they will achieve a higher air flow rate than with paper filters, as well as savings on maintenance costs. The fabric insert is washable, while the paper insert must be disposed of when contaminated. To achieve reasonable filtering with the fabric filter, it must be lightly oiled after cleaning.

The claim of increased air flow needs to be verified. To this end, the air mass flow value was measured in an Audi A3 TDI, engine code AHF, using a diagnostic tool (VAG-COM, now VCDS diagnostic system, or KOBD2Check) at 2 different, defined load conditions (measurement block 3 in the engine control unit).
In addition, 3 acceleration measurements were taken between 1800min⁻¹ and 4100min⁻¹ in 3rd gear, and the air mass flow values were also recorded. To exclude the influence of the exhaust gas recirculation system, it was temporarily deactivated during the measurement. Also, the air conditioning was turned off during the measurements.
The cabin air filter was lightly sprayed with 15W40 oil after cleaning, approximately 6,000 km ago, using a spray gun.



Measurement Conditions



• Air Temperature 15°C:
  2 measurements at 924min ^-1 and 1197min ^-1 , engine idling
• Air Temperature 16°C:
  3 measurements from 1800min ^-1 to 4100min ^-1 , engine under full load
• Engine and engine oil at operating temperature
• Exhaust gas recirculation off
• Air conditioning off




The test candidates



• Used paper air filter, 12,000 kmdriven
  



• New paper air filter
  



• Woven air filter, 6,000km driven
  




Measurements at 924 min^-1Used paper air filter, 12,000km driven



• 
  Measurement 1 in mg/R at 924 min

  	Measurement 2 in mg/R 924 min-1	Measurement 1 in mg/R 1197 min-1	Measurement 2 in mg/R 1197min-1
  475	450	500	495
  485	455	500	505
  475	460	495	490
  465	475	495	490
  480	460	505	500
  485	460	490	495
  485	455	500	485
  475	460	495	500
  480	455	495	490
  475	470	495	490

  
  
  
• New Paper Air Filter
  

  Measurement 1 in mg/R at 924 minup>-1	Measurement 2 in mg/R 924 min-1	Measurement 1 in mg/R 1197 min-1	Measurement 2 in mg/R 1197min-1
  470	460	495	480
  460	465	490	480
  450	465 >	495	485
  460	445	490	490
  470	450	490	485
  465	460	505	485
  470	455	490	495
  470	450	500	480
  465	475	500	495
  470	450	490	495

  
  
  
• Fabric filter, 6,000 km driven
  <td valign="TMeasurement 1 in mg/R at 924 min^-1
  455465

  Measurement 2 in mg/R 924 min-1	Measurement 1 in mg/R 1197 min-1	Measurement 2 in mg/R 1197min-1
  465	465	490	480
  460	470	485	490
  455 </t>d>	465	480	495
  470	450	490	485
  455	450	485	495
  470	465	490	485
  		490	495
  460	460	480	495
  445	460	490	495
  460	460	495	480





Due to the somewhat scattered measurement values, all 20 measurement values within each specific engine speed range were averaged.according to Then the following picture emerges:

Measurement at 924 min
-1
• 
  Used paper air filter in mg/RNew paper air filter in mg/RFabric filter in mg/R461460

  		469

  Measurement at 1197 min

-1

• 
  Used paper air filter in mg/Rtd valign="Top">New paper air filter in mg/R
  

  	Fabric filter in mg/R
  496	490	489









Surprising Result: The old paper air filter shows the best performance, both with an unloaded engine and at full acceleration!
The reason behind this can only be speculated upon, as the two paper air filters come from different manufacturers. "Furthermore, driving in the rain may also cause a surface change to the filter, resulting in an even better air flow compared to its new condition."
Also, the measured Ulf transit times are all within a narrow range of 5.7 seconds, and any potential variation in the required drive power for the turbocharger was not detected.
Since the measured values are generally within a narrow range, it can be confidently stated that the choice of filter type does not matter.

My decision was to use paper filters because, during trips in heavy rain, the air filter housing of my A3 would get very wet, and the fabric filter would basically be washed out. A paper air filter here shows only a coffee filter similar behavior.


Measurement data for download...



• Zip file with VCDS measurement data



To address the pressure loss caused by narrow suction nozzles, the filter specialists invented...

2. Open filters (also known as mushroom filters, etc.) .
They replace the entire air filter housing, including the intake duct, which is the main cause of the pressure loss in the Pmax area.
Actually, a sensible approach, but unfortunately, open filters suck air directly from the engine compartment. Therefore, the air is practically always hotter than it would be with a standard filter – because its intake is usually located before the radiator, in an inner fender, or in another cooler area.
Based on correction calculations on dynamometer benches, a general rule can be established that gasoline engines lose approximately 1% of torque for every 6°C increase in intake air temperature. Performance loss.
Therefore, the increased airflow that promotes performance only in the high-speed range results in a permanent power loss due to hotter air.

In the middle and lower speed range and under partial load, there is practically no pressure drop behind (clean) stock filters in most vehicles. Therefore, an open filter cannot improve the situation (losses cannot be reduced further than zero), but the hot air problem is completely overcome – resulting in torque losses, poorer responsiveness, etc.

In the upper RPM range, the vehicle-specific overall balance of pressure gain and temperature increase of the sucked air determines whether the installation of an open filter increases or decreases engine performance.

Therefore, the best performance results for open filters are achieved at high engine speeds and with as cold air as possible in the engine compartment, i.e., with a cold engine.
In other words: The highest performance gains are achieved with open filters when you immediately rev the engine to the redline after cold start

While reducing the intake vacuum in heavily modified TDIs with high boost pressures can be beneficial to somewhat relieve the turbocharger from extreme RPMs - but the higher intake temperatures further exacerbate the already significant thermal stresses on both the engine and the turbocharger

For non-modified TDIs, the main engine-related effect of open air filters in the hot air intake is that they actually reduce performance, rather than increasing it, as described.
Especially in the low-speed range, the torque will be reduced due to the reduced injection quantity (soot control) compared to the standard version.
Does the charger then stop when full throttle is applied (delayed) and override the soot limit through the torque limit, will the torque increase logically be measurably stronger than in the standard state?
Since a devout sports filter enthusiast tends to only notice positive things, the poorer starting performance in the suction area is simply ignored, and only the much clearer turbo sound is perceived -&gt; Wow, man


To address the hot air problem, filter manufacturers typically offer 2 recipes:

1. Protective shields between the engine block and the filter.
To compare, imagine a filter researcher who, in winter, is in a well-heated room over a radiator (= the warm engine), using a vacuum to suck in air.
Suddenly, the man says that the air being sucked in is actually too warm (what a miracle, if he had originally sucked in cold air from the outside), and presents a piece of metal as a solution, which he mounts between the heater and the vacuum cleaner hose.
Subsequently, the filter researcher continues to draw air from the heated room, but now claims that the suction temperature has been significantly reduced, thus solving the problem of warm air.
Believe, and you will be saved.

2. About hoses for supplying the filter with cold air
should the engine suck in air that is richer in oxygen.
The hoses should be installed so that one end is located next to the filter. Apparently, the filter manufacturers assume that their products contain some kind of "intelligence" that recognizes the adjacent hose opening and subsequently only sucks in in that direction. Or they hope that at least their customers can imagine something like that.
In reality, however, the warm air stagnates due to the natural temperature stratification, above the engine compartment, around the filter. Therefore, the engine will continue to suck in hot air, instead of drawing cold air "upwards" through the hose.
If the inlet opening of the hose is oriented forward, the airflow can initiate the desired cold air stream to the filter. However, the cold air throughput that can be achieved depends on the engine size, RPM, speed, hose diameter, and engine load, respectively. The air intake only provides a fraction of the engine's air needs, and in addition, depending on the hose's orientation, a portion of the carefully drawn in cold air bypasses the filter.

But even for this, the filter strategists have a solution: You simply shape the end of the cold air intake hose that is connected to the engine into a ball that surrounds the sports filter, thus preventing the intake of warm air.
Now, only cold outside air is drawn in - and the attentive reader will recognize that here, only the well-known principle of the series filter has been re-invented

In order to distinguish these products from those in standard equipment (and to sell them as groundbreaking developments from motorsports with attractive profit margins), they also receive new, innovative names that promise performance, high-tech, dynamism, etc.

We will refer to them here as

3. Sportfilterboxes.
Their air filter housing is often significantly smaller than the series and is only visible from the outside as a thickened part of the intake passage.
Additionally, they typically offer generously sized intake hoses or pipes, and therefore can actually achieve a performance gain of several % in the upper RPM range, especially if the original intake routes are designed too narrowly (see above).

One can achieve the same effect for a fraction of the cost with the series filter, by unclogging the intake paths -> Part 1 in /viewtopic.php?t=6593

As a conceivable modification, the extension of the air column flowing in individual pieces, depending on the design of the filter housing, remains possible compared to the original intake routes. This allows for the creation, enhancement, or modification of resonance charging effects, which will typically also change the perceived performance characteristics of the engine.
D.h. in bestimmten Drehzahlbereichen, there is more torque available, while in other speed ranges, there is less torque.
Enthusiastic drivers of sports filter boxes will again only notice the welcome changes -&gt; see example in No. 2.


In TDIs, it is generally conceivable that, due to shifted resonances, the cylinder filling in the low-speed range can be improved. Then can the increasing soot limit can be used to increase the injection volume, which, in the absence of still-lacking boost pressure, improves the torque and slightly shifts the boost pressure operation towards lower speeds. In the best-case scenario, the performance at low engine speeds will be noticeably improved.


Charging speed?
To protect the engine and air mass flow meter (LMM) from water sucked into the intake during rainy conditions, most standard intake air ducts have drain openings for sucked-in water, water drainage surfaces in front of the intake openings, or the intake mouths are oriented in the direction of travel, so that the majority of the water droplets pass by the intake mouth.

When installing sports air filter boxes or during the modification of stock intake ducts, it can be tempting to route the intake hoses so that the intake opening is directly exposed to the airflow. Depending on the engine size, RPM, speed, cross-section of the intake opening, and engine load/boost pressure, this creates a ram-air induction, which improves cylinder filling in suction-type engines and, in favorable cases, can increase power by a few percent. In turbochargers, the effect is more limited to a slight reduction in the compressor's workload.
However, this only works if the entire distance between the intake opening and the engine is completely sealed, so that the painstakingly created vacuum pressure is not lost. Sealed intake passages mean that water sucked in (for example, during heavy rain from the spray of the vehicle in front) cannot drain away before reaching the engine and therefore enters the engine as soon as the filter, the inner sides of the intake passages, and possibly the air intake system (LLK) are saturated with water.

Many LMM are, as is well known, allergic to water in moving air, which is expected to lead to their premature failure under such operating conditions.

A motor failure, of course, is still more expensive than replacing a faulty O2 sensor:
If water enters the intake port, there is a risk of a water hammer, once the proportion of liquid water in the cylinder exceeds the reciprocal of the engine compression ratio.
In a gasoline engine with a compression ratio of 1:11, the air is compressed to 1/11 = 9% of the intake volume. If the volume of air entering the cylinder contains more than 9% water, the volume of water will be greater than the combustion chamber, i.e., the piston will move forward against the non-compressible water during the compression stroke. Common occurrence: One or more connecting rods and/or the crankshaft bend, rendering the engine useless.
Even with diesel, due to the higher compression, a water content of 5% in the cylinder charge is sufficient to cause a water hammer.

In engines with upstream compressors (turbochargers, compressors . . .) the matter is even more critical.
Let's consider a turbocharged diesel engine with a compression ratio of 1:20, a 1 bar boost pressure, and a 3% water content in the intake air. The water occupies 6% of the volume of the intake air behind the turbocharger, because the water droplets cannot be compressed. However, since the volume of the combustion chamber only represents 5% of the total volume, the water hammer effect is predictable.

However, the water hammer usually doesn't occur with the first gulp of water that is sucked in. In the filter, on the inner walls of the suction tubes and possibly in the LLK, a certain amount of water can initially settle, which is equivalent to a grace period for the motor.
Only when these areas are "saturated" with isolated water will the next, sufficiently strong water surge destroy the engine.
Here, large-pore sports filters can allow water to pass through more quickly and, in extreme cases, can cause the engine to stall, while an original paper filter can still adequately regulate the water flow and save the engine.



Side effects
Examples of sports filters include those from the company: Hopa, in the context of LMM problems, has been investigated. Unfortunately, the document is no longer available online.
Here is a summary of the key points (the water issue in LMM has already been mentioned above):

A.
Sport filters often have a poorer filtering effect (due to larger pores) than standard filters, regardless of their design, and therefore allow more dirt to enter the engine.
A common solution is to saturate the filter elements with oil, which can then be carried away as a mist by the airflow.
If filter oil accumulates on the measuring membrane of a hot-film type LMM, its measuring values will decrease, which will eventually lead to a reduction in fuel injection volume, resulting in power loss and possibly, jerky operation and/or other malfunctions.

B.
Furthermore, the installation of an open filter or a sports filter box can alter the airflow profile in the catalytic converter, causing it to either under- or over-report the amount of air being sucked in, which in turn disrupts the engine management system. Possible consequences: Reduced power, rough running, or (in diesel engines) increased soot emissions.

C.
Since Hopa only deals with diesel tuning, the following topic is missing in the link:
Many gasoline engines have an intake air temperature control system (where warm air and cold air, drawn in above the exhaust manifold, are mixed depending on the engine's operating condition), which is intended to optimize the engine's running characteristics and/or emissions values.
When installing an open filter or a sports filter box, this device must necessarily be removed, with the corresponding consequences.


4. Conclusion
Open and plate-type sports filters offer almost only disadvantages compared to standard intake designs from a technical point of view.

If you extend the often somewhat narrow intake hose when using a series filter, the engine (even after tuning) will always get enough air, which is also optimally verschandeln and cold – meaning more than most sports air filters can offer.

The real value of open and flat sports filters lies mainly in show effects such as the louder intake noise in gasoline engines, the second image in the engine compartment, and the profit for manufacturers and distributors.

Significant performance gains through simply replacing with sports filters, as regularly advertised by relevant advertising with

-&gt; Hokuspokus features, such as intricate air passages within the filter (each curve, swirl, or "internal" air acceleration in the intake path consumes energy, which ultimately results in a reduced air density, which in turn reduces engine performance!)
-&gt; misleading descriptions
-&gt; empty buzzwords like dynamic, powerful, optimized, etc.


are practically impossible - unless the factory-installed air filter system is a complete design flaw with excessively narrow flow passages and/or unnecessarily convoluted air ducts.

Only with sport filters, the balance looks a bit different: They can, with proper design (i.e., without internal air accelerators or other gimmicks), actually achieve small, localized power gains depending on their interaction with the engine, in addition to the show effect.

When considering the cost-effectiveness (e.g., using a sports filter multiple times instead of discarding many paper filters), it is important to consider not only the prices of sports and paper filters, but also, for sports filters, the costs of filter oil, cleaning fluid, and potentially the disposal of used cleaning fluid – and ultimately, whether you are willing to take on the "dirty work" of filter cleaning.

Authors: Ulf & Rainer