| Diesel, Engine Technology and Vehicle Technology. Repair, Maintenance and Diagnostics (OBD). |
Much Show, Little Value: Sports Air Filters (Articles) |
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| Much Show, Little Value: Sports Air Filters | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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dieselschrauber  23-05-2003, 14:31 |
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![[Luftfilter im Test]](/localfiles/Lufis.jpg) Why this comparison?
For an engine to deliver the highest possible power output (i.e., to burn the largest possible amount of fuel "cleanly" per power stroke), air with the highest possible density must enter the cylinders, which means -> as cool as possible and -> at the highest possible pressure. Since any pressure loss in the intake system reduces air density, the entire intake path including the filter should have the lowest possible flow resistance. Only then does the air reach the engine with the lowest (but fundamentally unavoidable) pressure losses. In reality, the pressure inside the air filter housing is already below atmospheric pressure, particularly at high airflow rates, i.e., at full throttle and high engine speeds = Pmax. The resulting air density loss is, in most cars, caused almost exclusively by the intake snorkel, whose inlet is often the narrowest point of the intake path through which all the air must flow. For example, with TDIs, vacuum of up to approx. 50 mbar behind the intake snorkel, i.e., already before the actual air filter, is a common magnitude. A 50 mbar pressure loss compared to atmospheric pressure blocks about 5% of potential power in a naturally aspirated petrol engine; in turbocharged engines (including TDIs), this loss is almost completely compensated for by the turbocharger. Factory paper filter elements cause virtually no additional pressure loss - at least as long as they are reasonably clean. If, for example, a paper air filter causes an additional 10 mbar of vacuum after several thousand kilometres and a new or different (sports) filter causes only 4 mbar of loss, replacing it would reduce the filter-related pressure loss by 6 mbar. This 6 mbar pressure gain would correspond, in naturally aspirated petrol engines, to a power gain of a mere 0.6%, while a "normal" intake snorkel still wastes about 5% of potential power. The balance should be even better with modern paper pleated filters with alternately sealed filter channels. This type of filter offers a very large surface area in a compact size. According to experiences from the forum (thanks, Julian  ), filters can also become so clogged within the normal service interval that noticeable power loss and increased fuel consumption occur.
When a paper filter should actually be replaced can be estimated by its light permeability by holding it up against the sun: with a new filter, light passes particularly clearly through the fold lines of the paper; the lamellae also let some light through. If, on the other hand, no or barely any light comes through a used filter, it should be replaced. If there is a lot of coarse dust in the filter, tapping it out (gently tapping the element flat on a smooth surface with the intake side down) can somewhat improve permeability; however, the filter is usually so clogged with fine particles that even after tapping out, no more light comes through -> replace it. As long as a paper filter is regularly checked or replaced according to operating conditions (dust accumulation, etc.), it causes neither noticeable power loss nor increased consumption. This of course does not stop sports air filter suppliers from advertising, for 1. Plate elements (also known as replacement filters) with claims such as "40% more air throughput, better acceleration, higher top speed..." to raise completely utopian hopes of power gains among interested customers - hopes that are only conceivable if, for example, a paper filter neglected for over 200,000 km is replaced with a new filter. A paper filter would, for the reasons mentioned, deliver the same power gain as a sports filter... Rainer's comparative measurements with 2 standard filters and one sports filter confirm that sports replacement filters, at least with TDIs, bring no significant improvements to the engine's air supply.
The claim of higher air throughput is to be verified here. To this end, on an Audi A3 TDI with engine code AHF, the air mass value was measured using a diagnostic device (VAG-COM, meanwhile VCDS diagnostic system, KOBD2Check also works) at 2 different defined load conditions (measuring block 3 in the engine control unit). Additionally, 3 acceleration measurements each from 1800 rpm to 4100 rpm were carried out in 3rd gear, also recording the air mass values. To rule out influences from exhaust gas recirculation, it was temporarily deactivated for the measurement. The air conditioning was likewise switched off during the measurements. The fabric air filter was lightly misted after cleaning 6,000 km ago with 15W40 oil via spray gun. Measurement conditions The test candidates Measured values at 924 rpm / 1197 rpm Since the individual measured values varied somewhat, all 20 measured values in each speed range were averaged. The result is as follows: Measured values at full load, 1800 rpm to 4100 rpm ![[Volle Beschleunigung, Papierfilter mit 12.000km]](/localfiles/Lufi_full_dreck.jpg) ![[Volle Beschleunigung, Papierfilter neu]](/localfiles/Lufi_full_neu.jpg) ![[Volle Beschleunigung, Gewebefilter mit 6.000km]](/localfiles/Lufi_full_bmc.jpg) Surprising result: the old paper air filter shows the best result, both with the engine unloaded and at full acceleration!
Measured values for download... To counter the pressure loss through narrow intake snorkels described above, the filter specialists invented 2. Open filters (also known as mushroom filters, etc.). They replace the complete air filter housing including the intake snorkel, i.e., the main cause of pressure loss in the Pmax range. Actually a sensible approach, only unfortunately open filters draw the air directly from the engine compartment. The air is therefore practically always hotter than it would be with the factory filter - because its intake mouth is usually located in front of the radiator, in an inner fender or in other cool areas. Based on correction calculations on dynamometers, the rule of thumb is that petrol engines lose 1% of torque or power for every 6°C of increasing intake air temperature. Thus, the power-promoting pressure increase of open filters is only at high engine speeds offset by a permanent power loss due to hotter air. In the mid and lower speed range and at partial load, virtually no car has a pressure loss behind (clean) factory filters. Therefore, an open filter cannot bring any improvement here (less loss than zero is impossible), but the hot air problem hits fully - with the consequence of torque losses, poorer response, etc. In the upper speed range, the vehicle-specific overall balance of pressure gain and temperature increase of the intake air decides whether fitting an open filter raises or lowers engine power. The best power balance of open filters is thus achieved at high engine speeds and as cool as possible air in the engine compartment, i.e., with a cold engine. In other words: Open filters bring the highest power gain when you thrash the engine at full throttle up to the rev limiter immediately after a cold start 
With tuned TDIs and a strong boost increase, reducing the intake vacuum can make sense to relieve the turbocharger somewhat from extreme speeds - but the higher intake temperatures further increase the already higher thermal loads on engine and turbocharger
With untuned TDIs, the main engine-technical effect of open filters will be the hot air intake, which, as described, tends to reduce power rather than increase it. Especially in the naturally aspirated low-speed range, torque will be reduced compared to stock due to the reduced injection quantity (smoke limit). When the turbocharger then (belatedly) kicks in during full throttle and raises the smoke limit above the torque limit, the torque increase will logically be measurably stronger than in stock condition. Since a faithful sports filter driver tends to perceive only positive things, the poorer acceleration in the naturally aspirated range is simply ignored and only the more obvious turbo kick is noticed -> Wow, dude  
To solve the hot air problem, filter manufacturers usually offer 2 recipes: 1. Shielding plates between the engine block and the filter. For comparison, imagine a filter researcher who, in winter, in a well-heated room above a radiator (= the warm engine), sucks in air with a vacuum cleaner. Suddenly the man says that the intake air is actually too warm (what a miracle, since he originally sucked in cold air from outside) and presents a piece of sheet metal that he mounts between the heater and the vacuum cleaner hose as a solution to the problem. Subsequently, the filter researcher continues to suck air from the heated room, but now claims that the intake temperature has been significantly reduced and the warm air problem thus solved. Whoever believes that will be blessed... 
2. Via hoses to supply the filter with cold air, the engine is supposed to draw in more oxygen-rich air again. The hoses are supposed to be installed so that one end lies next to the filter. Apparently, the filter suppliers suspect a kind of intelligence in their products that recognises the adjacent hose opening and from then on deliberately only sucks in that direction. Or they hope that at least their customers imagine something like that... In reality, the warm air accumulates, due to natural temperature stratification, at the top under the bonnet, around the filter. The engine will therefore continue to suck hot air instead of pulling cold air "uphill" through the hose. If the inlet opening of the hose is oriented forward, the airflow can set the desired cold air stream to the filter in motion. However, depending on displacement, speed, speed, hose diameter and engine load or boost pressure, the cold air throughput thus achievable only covers fractions of the engine's air demand, and additionally, depending on the orientation of the hose, part of the elaborately conveyed cold air will pass by the filter. But the filter strategists have a solution for that too: one simply shapes (figuratively speaking) the engine-side end of the cold air hose into a ball that encloses the sports filter and thus prevents the intake of warm air. Now only cold outside air is drawn in - and the gentle reader recognises that here merely the well-known principle of the factory filter has been reinvented once again 
So that one can nevertheless distinguish such products from factory systems (and sell them with handsome profit margins as groundbreaking developments from motorsport), they get, besides a different design, also novel names that promise performance, high-tech, dynamism, etc. We will refer to them here as 3. Sports filter boxes. Their filter housing is often considerably smaller compared to stock and is only recognisable from the outside as a thickened part of the intake path. Additionally, they usually offer generously dimensioned intake hoses or pipes and can therefore, in the upper speed range, actually achieve a power gain of several % if the factory intake paths are designed too narrowly (see above). The same effect can of course be achieved for a fraction of the cost with the factory filter by de-restricting the intake paths -> Part 1 in https://community.dieselschrauber.org/en/viewtopic.php?t=6593 What remains as a conceivable change from a sports filter box (depending on the design of the filter housing) is the extension of the continuous air column compared to the factory intake paths. This can create, strengthen or alter resonance charging effects, which will usually also change the perceived power characteristics of the engine. That means in certain speed ranges more torque is available, in other speed ranges, however, less torque. Convinced drivers of sports filter boxes will in turn only perceive the welcome changes -> cf. No. 2. With TDIs, it is fundamentally conceivable that shifted resonances improve cylinder filling in the lowest speed range. Then it is possible to increase the injection quantity via the rising smoke limit, which - with boost pressure still missing - improves torque and shifts the turbocharger spool-up somewhat towards lower speeds. In the best case, acceleration in the low-speed range is noticeably improved. Ram air charging? To protect the engine and MAF sensor from aspirated water during rainy drives, most factory intake air ducts have drain openings for aspirated water, water drainage surfaces in front of the intake openings, or the intake mouths are oriented at an angle to the airflow so that most of the water droplets fly past the intake opening. When installing sports filter boxes or during the de-restriction of factory intake paths, it seems tempting to route intake hoses so that the intake opening is directly hit by the airflow. Depending on displacement, engine speed, vehicle speed, cross-section of the intake opening and engine load or boost pressure, this creates a ram pressure charge that improves cylinder filling in naturally aspirated engines and, in favourable cases, can increase power by a few percent. In turbo engines, the effect is more limited to a slight relief of the turbocharger. All this only works, however, if the entire route between the intake opening and the engine is completely sealed, so that the laboriously generated ram pressure is not lost again. Sealed intake paths in turn mean that aspirated water (e.g., in heavy rain from the spray of the vehicle ahead) can no longer drain off before the engine and therefore enters the engine as soon as the filter, the inner sides of the intake paths and, if applicable, the intercooler are saturated with separated water. Many MAF sensors are, as is well known, allergic to water in the flowing air, which under such operating conditions lets their correspondingly earlier failure be expected. Engine damage is of course even more expensive than replacing a defective MAF sensor: If water gets into the intake opening, there is a risk of water hammer as soon as the volume fraction of liquid water in the cylinder exceeds the reciprocal of the engine compression ratio. Example: In a petrol engine with a compression ratio of 1:11, the air is compressed to 1/11 = 9% of the aspirated volume. If the volume flowing into the cylinders contains more than 9% water, the water volume is larger than the combustion chamber, i.e., the piston runs into a wall of incompressible water on the way to compression TDC. Usual consequence: one or more connecting rods and/or the crankshaft bend, and the engine only has scrap value. In a naturally aspirated diesel, due to the higher compression, a water content of around 5% in the cylinder charge is sufficient for water hammer. In engines with upstream compressors (turbocharger, supercharger...) the matter is even more critical. Let us take as an example a turbo diesel with a 1:20 compression ratio, 1 bar boost pressure and 3% water content in the intake air. The water already occupies 6% of the volume of the charge air behind the turbocharger because the water droplets cannot be compressed. Since the combustion chamber volume is only 5% of the fill volume, the water hammer is foreseeable. However, the water hammer usually does not come with the first gulp of aspirated water. A certain amount of water can initially settle in the filter, on the inner walls of the intake pipes and, if applicable, in the intercooler, which is like a grace period for the engine. Only when these areas are "saturated" with separated water will the next, sufficiently strong surge of water finish off the engine. Large-pore sports filters can let the water through faster and, in borderline cases, kill the engine, while an original paper filter would still stretch the water flow sufficiently in time and save the engine. Side effects of sports filters have been investigated, among others, by the company Hopa in connection with MAF problems; unfortunately, the document is no longer online. Here is a summary of the most important points (the water problem with MAF sensors has already been addressed above): A. Sports filters, regardless of their design, often have worse filtration performance (due to larger pores) than factory filters and therefore let more dirt into the engine. As a common remedy, the filter elements are soaked with oil, which in turn can be carried along by the airflow as mist. If filter oil deposits on the measuring membrane of a hot-film MAF sensor, its measured values drop, which sooner or later, via a reduction of the injection quantity, leads to power loss and possibly to jerking and/or other malfunctions. B. Furthermore, the installation of an open filter or a sports filter box can shift the flow profile in the MAF sensor, so that it reports too little or too much aspirated air, which in turn confuses the engine management. Possible consequences: power loss, jerking or (in diesels) increased sooting. C. Since Hopa only deals with diesel tuning, the following topic is missing from the link: Many petrol engines have an intake air temperature control (warm air aspirated above the exhaust manifold and cold air are mixed depending on the engine operating state), which is supposed to optimise smoothness and/or emission values. When installing an open filter or a sports filter box, this device must inevitably be removed, with the corresponding consequences. 4. Conclusion Open and panel sports filters offer, from a technical point of view, on balance almost only disadvantages compared to factory intake designs. If you widen the often somewhat narrow intake snorkel on factory filters, the engine (even after tuning) always gets enough air, which is additionally optimally verschandeln and cold - i.e., more than most sports air filters can offer. The real practical value of open and panel sports filters lies mainly in show effects such as the louder intake noise in petrol engines, the different look in the engine compartment - and in the profit for manufacturers and distributors. Noticeable power gains through the mere conversion to sports filters, as regularly suggested by the relevant advertising with -> hocus-pocus features such as ornate air guides in the filter (every swirl, vortex or "internal" air acceleration in the intake path costs energy, which ultimately results in a reduced air density, which in turn reduces engine power!) -> misleading descriptions -> empty buzzwords such as dynamic, powerful, optimised, etc. are practically impossible - unless the factory air filter system were a complete botched design with far too narrow flow cross-sections and/or unnecessarily convoluted air guides. Only with sports filter boxes does the balance look somewhat different: with sensible construction (i.e., without internal air accelerators or other hocus-pocus), they can, depending on the interaction with the engine, actually bring slight, localized power gains in addition to the show effect. When considering the cost-effectiveness (verschandeln one sports filter many times and keep using it instead of throwing away many paper filters), not only the prices of sports and paper filters should be considered, but also - for sports filters - the costs of the filter oil, the cleaning fluid and, if applicable, the disposal of the used cleaning fluid - and ultimately also whether one wants to take on the "dirty work" of cleaning the filter. Authors: Ulf & Rainer Translated on 03-07-2026, 15:19. |
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| Ratings - Much Show, Little Value: Sports Air Filters | |
Average rating: 5.00 - worst rating: 5 - best rating: 5 - number of ratings: 11 - View ratings |
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![[Papierluftfilter, 12.000km]](/localfiles/Lufi_dreck.jpg)
![[Papierluftfilter, neu]](/localfiles/Lufi_neu.jpg)
![[Gewebeluftfilter, 6.000km]](/localfiles/Lufi_BMC.jpg)
