K and N/ High Flow Air Filters: Test #3

High Flow Automotive Air Filters Part 3: AnthonyS tests

  ton bobistheoilguy, probably the best forum site for lubrication issues on net forums.

The results of the Spicer/TESTAND standardized 5011 tests for air filters pointed at a very clear pattern: K&N (cotton gauze) and AMSOIL (foam) filters had less flow restriction when clean, but more flow restriction when dirty than the standard AC Delco OEM air filter replacement, and there was a dramatic difference in filtration efficiency, to the advantage of the AC Delco filter. We wanted to get some corroborative evidence, to avoid the possibility of commercial collusion or test errors. We found AnthonyS' tests on bobistheoilguy, run in 2003. AnthonyS, a user of K&N filters, decided to conduct a low budget test on air filter efficiency.

AnthonyS is a forum member of the bobistheoilguy site, which archived a discussion thread on his tests. According to what he writes, he is deeply familiar with automotive technology, and has a BS in Mechanical Engineering.  AnthonyS conducted both flow and filtration tests on a set of filters, including oiled cotton filters (K&N), foam (Jackson Racing, Racing Beat, AMSOIL previous generation), and paper or similar fiber-based media (Napa Gold, Baldwin, Mazda).


Air flow

For the flow tests, AnthonyS mounted a Dwyer water manometer on the intake, to measure the pressure drop from the atmosphere across the intake and filter, and took 4 successive measurements for each filter, which he compared to intake pressure drop only.

Because there are two different sessions where car rpms are different (6,000 rpms vs. 6.500 rpms) as is atmospheric pressure, it is not possible to compare all brands to each other, but the test clear shows that:

• all paper filters are roughly equivalent in flow
• the K&N filter flows better than the other filters, although the difference is only 1.4% of atmospheric pressure, probably only noticeable in a racing environment.

Our conclusion on flow tests is that we have corroborating evidence from two tests on the relative superiority of K&N filters as far as air flow is concerned - although it is appears to us that, for regular driving use, the difference may not be noticeable.


Filtration Efficiency

In order to measure filtration efficiency,  AnthonyS inserted a secondary air filter downstream of the primary air filter, and run 500 mile tests with each filter, after which he had a third party (his wife:-) evaluate filter color.The darker secondary filters indicated the filters with worse filtration efficiency.

No flash

Flash

The color judge evaluated all paper filters as roughly the same, and definitely lighter than the foam and cotton filters. She rated AMSOIL and K&N filters roughly at the same level, with a possible advantage to K&N. The pictures tell the same tale.

We conclude from AnthonyS' filtration tests that paper filter filtration is more effective than cotton or foam, a result which corroborates the filtration findings of the previously reviewed ISO 5011 test. While we do not believe that it ranks at the level of incontrovertible evidence because of weaknesses in experience design, we feel that AnthonyS' air filter test is appropriate as corroborating evidence.

Next we discuss the conclusions to draw from the data we uncovered on high flow automotive air filters... So come back soon!

Best Air Filters: SwRI/AEM 2007 Tests

Best Air Filters Review Part 7: SwRI/AEM 2007 air filter tests

Air filters: how do you prove that your product is superior? In a field where technical performance is critical, you can ask a prestigious third party to test the competitive field for you. This is what AEM did in 2007, when they commissioned a prestigious third party research lab, the Southwest Research Institute (SwRI), to compare drop-in filter performance across all its competition. SwRI produced, as expected, an outstanding research report, whose only -minor- weakness is that the samples to test were actually provided by AEM. AEM does not provide the link any more, and it took us a lot of research to actually locate it.

Is AEM's sponsorship of these tests a credibility issue? We spent a good amount of time debating this question at ConsumerPla.net. Our conclusion is that it does not significantly impact the credibility of the report, expect, possibly, in one respect. The test procedure is standard, and was led by SwRI in isolation with no participation from AEM personnel. The samples were provided by AEM, which is the sole significant weakness of the test. From SwRI comments, it appears that competitive samples were not, in general, damaged or skewed, with one minor exception which we discount, where dust spots appeared on one competitive filter. At worst, the only skew in this regard would be that AEM would have made sure to provide a non-faulty sample of its own filter. AEM has a good reputation as a provider of quality hardware, and we do not believe that this introduces a significant restriction to the validity of the tests. As for the possibility of selective quoting form the report, it is non-existent, as SwRI requires that the totality of its reports be quoted when some of it is. Indeed, we had access (as you can too) to the totality of the SwRI report. Our conclusion, then, is that the report can be considered a valid third part comparison report.

SwRI tested the air filters requested by AEM through the whole regular procedure outlined by ISO 5011, using ISO 12103-1, A2 fine test dust (0-80 microns) as a test medium. The choice of the dust is an interesting one. Manufacturers typically   ISO 12103-1, A4 coarse test dust (0-180 micron size), because the results are better (coarse particles get filtered more easily). Since AEM paid for the tests, they must have requested fine test dust, which indicates to us that the AEM filter probably tests better against its competitors with fine test dust than with coarse test dust. We find this a very positive attribute for AEM, as fine test dust is more likely to approximate real use patterns. Since both the Jeep Magazine study and the present one use fine dust (as opposed to the Spicer/Testand study), their results can be compared with more validity.

The selected air filters represented a majority of the high end air filters available in North America, which, unfortunately, does not include good quality throw-away paper filters in AEM's eyes: we would have liked to see some included, as they were in the Spicer/Testand study. The list:

• AEM Dryflow filters are dry element filters that are reusable and washable
• AFE Dry Pro S filters are dry element filters that are reusable and washable
• AFE ProGuard filters are oiled cotton filters with an additional synthetics barrier to enhance filtration - they are reusable and washable, and need to be re-oiled before use
• AIRAID filters are oiled cotton filters, also with an additional synthetic barrier ("Synthaflow") to enhance filtration - they are reusable and washable, and need to be re-oiled before use
• K&N filters, the most common premium filters on the market, are oiled cotton filters - they are reusable and washable, and need to be re-oiled before use.

We already saw performance analysis of K&N and AFE Proguard filters in the Spicer/Testand study, and of AEM and Airaid in the Jeep Magazine study. It is interesting to note that the results we found for AEM, K&N, AFE and Airaid filters are basically reinforced by the present one, which confirms our analysis that the present study is not damaged by AEM's sponsorship. There are two types of AFE filters in the SwRI study: while the Dry Pro S series consists of dry elements, the ProGuard product line requires oiling. AFE sales people typically recommends AFE ProGuard filters as the best filtration elements their company provides. The results of this study confirm their statements.

 An interesting aspect of this study, which was not performed by any other, is that SwRI also studied the performance of the filters it tested after multiple washes. It is particularly interesting, as these reusable filters are all sold with the expectation that they will last the lifetime of the vehicle - true for all reusable filters we know of expect for AMSOIL filters, which are sold for 100,000 miles or 4 years, whatever comes first. The present study tested clean filters, then used and washed them five times, then tested them again after the fifth wash. The results are interesting:

As already noted in the Jeep Magazine study, the present test highlights the fact that high end filters actually show less capacity than throwaway paper filters as shown in the Spicer/ Testand study. As in the Jeep Magazine study, AEM scores highest in filtration efficiency. Its capacity appears to have improved since 2006, and tops other filters as well. There is no degradation of either efficiency or performance with the number of washes. As usual, K&N scores lowest in filtration efficiency, but comparatively well in capacity. The AFE ProGuard and AIRAID filters show somewhat comparable performance, with a little edge for the ProGuard. They both lose a small amount of efficiency and capacity with reuse. The AFE Dry Pro filters scores behind all brands except K&N in filtration, and behind all others in capacity. It loses some efficiency with reuse, as does the K&N. Overall, the AEM filter does extremely well in this study so far.

How much impact does efficiency have on how much dust passes through into your engine? The average filter capacity for all filters surveyed across the present series is 234 grams.Taking this amount as the average quantity necessary to clogging your filter, this is how much pass-through dust would make it into your engine combustion chamber:

The difference between some of these filters is shocking, given how engine wear directly correlates with the amount of dust particles in the combustion chamber. So far the AEM filter has scored significantly above the others, while the K&N and the AFE Dry Pro S have trailed. For the sake of giving a complete picture, we will also cover what the tests have to say about air flow performance, although we consider it a minor criterion.

  When looking at initial air flow performance, with a new filter and no dust in the filter, we find the K&N filter significantly ahead in performance, followed by the AFE Dry Pro and the Airaid. It is probably no coincidence that the K&N and the Dry Pro filters also account for the worst filtration performance of the lot - the Airaid performance is all the more meritorious. The AFE Proguard follows, and the AEM Dryllow brings out the rear. The situation changes somewhat when we look at the filters after 5 washes:

After 5 washes, the K&N still handsomely leads the field, followed closely by the AFE Dry Pro. The Airaid's performance has somewhat receded, while the AEM Dryflow's has improved, and they now perform equally well. The AFE ProGuard now trails the others.

The curves we looked at above assume a clean filter. Another dimension that is worthwhile looking at is airflow vs. clogging: what happens as more dust settles on the filter: what happens as the filter is actually being used? The results are, once again, interesting and surprising:

This diagram is particularly interesting, as it explains why the AEM filter has the better capacity. The K&N shows by far the better air flow as dust increases, until it reaches close to capacity, at which time it suddenly clogs quickly.  The behavior is duplicated by all other filters except for the Dryflow, with the difference that these other filters have a lower capacity than the K&N (and lower airflow as well). The AEM Dryflow starts somewhat high, but it airflow performance worsens only slowly and linearly with dust deposit, while all other filters degrade drastically in performance significantly before approaching capacity.

Let's face it: the SwRI/ AEM study is a victory for AEM, even if the test format was chosen by that company. The AEM filter showed itself excellent at filtration and with good capacity, while its airflow performance was middling, although improving with use relatively to other filters. The K&N filter, on the other hand, did poorly in filtration, had decent capacity, and showed excellent performance in airflow. The other filters did not excel in either field. There is more discussion on these and other SwRI tests commissioned by AEM on automotive.com, sporttruck.com, NASIOC, Don McBride's Air Filter Notes  and VW Vortex (although the links to the SwRI tests are not active any more, following the 2010 acquisition of AEM by K&N).

Next we discuss the ISO 5011 S&B tests published by S&B... So come back soon!

Note: accidentally published out of sequence

<<Previous                                                                                                                     Next>>

Best Air Filters: Jeep Magazine 2006 Tests

Best Air Filters Review Part 3: Jeep Magazine air filter tests

In November 2006, Jeep Magazine published a series of air filter tests comparing leading high-end drop-in air filters. Because they dealt with off-road vehicles, they decided not to test paper-based filters, which can collapse with water, and focused on reusable dry elements and oiled cotton filters. We feel that this was a mistake on their part, probably due to the fact that they underestimated the performance of standard paper-based filters.

Many auto mags spend a lot of time with dynamometer ("dyno") testing to evaluate torque and HP gains from stock modifications. Interestingly, Jeep Magazine discounts such testing for air filters: "if you've got a stock engine, basically any filter will flow enough air to keep it happy. And you aren't going to pick up any power from a filter swap alone. The restriction in the stock Jeep is the stock air-intake system, not the filter." Instead, like us, they focus on cumulated efficiency and dust capacity.

Like the Spicer/ Testand study, the Jeep Magazine team ran ISO 5011 tests. However, they used ISO 12103-1, A2 fine test dust (0-80 microns) in their tests rather than  ISO 12103-1, A4 coarse test dust(0-180 micron size), which was used in the Testand tests, because they felt that fine dust matches standard vehicle use better than coarse dust. We agree with them - but we need to note that, as a result, the Jeep Magazine tests cannot be directly compared with the Spicer/Testand tests. Manufacturers typically use coarse test dust because the are allowed by the standard and the  results are better.

Jeep Magazine selected five filters for its tests (the FRAM AirHog is also listed but is not evaluated). All of them are considered premium filters, reusable, and are significantly more expensive than regular paper filters.The list:

• The AEM DryFlow is a dry element filter, made of polyester with a nylon cage. Its nominal filtration efficiency is quoted as 99.4%. It is reusable and can be washed.
• The Airaid is an oiled cotton filter than uses an additional synthetic fiber barrier to enhance filtration. Jeep Magazine quotes its nominal efficiency at 99.997% down to 10 micron. It is reusable and can be washed, although it also needs to be re-oiled prior to reuse.
• The AMSOIL EaA filter is a dry element filter, which mixes cellulose (paper fiber) with synthetic nanofibers (Donaldson technology). It is reusable and can be vacuumed/ air-blown for up to 100,000 miles or 4 years (whatever is less). This AMSOIL nanofiber filter is the present generation of AMSOIL filters, whereas the one reviewed here was from the previous AMSOIL product generation.
• The K&N was already described here. Jeep Magazine reports its efficiency as 97-99%.
• The S&B is an oiled cotton filter similar to the K&N filter. Its efficiency is claimed to be 99% for coarse dust.

Once again, the results are enlightening, although a bit less surprising to us, since we had already reviewed the Spicer/ Testand tests. The results mostly do not match manufacturers claims, but we should again bring attention to the fact that manufacturers' tests typically use coarse test dust. Coarse test dust leads to better test results, but probably does not match real life use as well as the fine test dust used in the Jeep Magazine tests.

The capacity of all these filters is significantly lower to that of many surveyed in the Spicer/Testand study. Some of it is due to the choice of filters in both studies: Jeep Magazine did not select any paper filters, which have significantly higher capacity. We suspect, however, that the other reason is the difference in dust: fine test dust probably ends up clogging filters faster than coarse test dust.

In this test again, K&N ends up last in filtration. It would probably end up first in flow - but the magazine did not conduct flow measurements. Airaid and S&B appear to have similar performance. The AMSOIL nanofiber filter appears to have a shockingly low capacity compared to the others. The AEM filter shows up superior filtration performance, along with low capacity.

The difference in filtration performance between a filter at 98.5% cumulative efficiency and one at 99.5% might appear small. But each fraction of a percent means more dust showing up in the combustion chamber of your engine and leaving more wear scar in it. We calculated the average capacity of all the air filters we surveyed in this series at 234 grams. If we run 234 grams of dust through these filters (i.e. enough to clog the average air filter in this study), this is how much dust will end up in your engine:

There is a shocking difference between some of these filters in terms of pass-through dust.

Next we review the 2007 air filter study and ISO 5011 tests by Southwest Research Institute... So come back soon!


<<Previous                                                                                                                   Next>>

K&N Air Filters: Testand Tests

Best Air Filters Review Part 6: Spicer/ Testand filtration tests

Amazingly, high flow manufacturers have not published third party tests showing the superiority their filtration technology, even for those, such as AMSOIL, who use such tests as a standard marketing strategy for their other products.

We were still able to find relevant and interesting automotive air filtration tests. In 2004, the company Testand, a manufacturer of filter test equipment, collaborated with Arlen Spicer, a Duramax enthusiast, to run a series of standardized ISO 5011 tests (equivalent to the obsoleted SAE J726) test on air filters, using one of their expensive $285,000 test machines. The results were shocking to us, and probably to many high flow air filter users. The test compared K&H (reusable cotton gauze filter),  the previous generation of AMSOIL filters (foam filter), and multiple other products made of paper or other fiber combinations, the leading OEM brand being AC Delco. The original thread for this test runs very over 40 pages (another discussion thread), and the only location where we could actually find the original data is on DieselBombers.

A primary goal of the study was to evaluate if high performance, high flow filters (in particular the K&N filter brand) were worth purchasing. The authors of the study selected a broad swath of filters. Several filters were regular OEM/ aftermarket throwaway paper filters (AC Delco, Baldwin, Wix, and a no-name model, which ended up doing pretty well in the tests). The other filters were all premium filters in different flavors:

• Purolator is a premium non-reusable paper filter
• AMSOIL foam and UNI are premium,  washable and reusable foam filters (the AMSOIL model belongs to a  previous product generation, and has been superseded by nanofiber filters)
• K&N is an oiled cotton filter, that is reusable: it can be washed and re-oiled
• AFE, another oiled cotton filter that can be washed and re-oiled, adds an additional synthetic barrier to the cotton gauze layers to improve filtration efficiency.

The first part of the test measured flow restriction in each filter, while clean, and progressively increased the volume of air being pushed through: which filter introduced the least resistance to air being brought into the engine? While the differences were not enormous, the winner was the premium-priced K&N filter, as shown in the graph below:

This result augured well of the success of the K&N filter. The next question was to find out how the filter behaves as it gets progressively clogged by dust, as happens during actual use. The test used a constant 9.8grams/ minute dust flow, using standardized ISO 12103-1, A4 coarse test dust, 0-180 micron size. The outcome of this test was the first real shocker -there are several to come. The test below shows that, as dust progressively clogs the filter, many filters, including the K&N and the AMSOIL filters, see the flow restriction (i.e. the resistance to air flow) growing exponentially, to very largely exceed the restriction seen on the standard OEM paper filter made by AC Delco:

The result of this test was a big surprise: the flow restriction advantage for the K&N (or other alternative filters) only existed when the filter was largely clean! Hopefully, this meant that the alternative filters were particularly efficient at removing dust, and, as a result, might have clogged faster. The next step was then to evaluate filtration efficiency, which is when we encountered the next shocker: the most efficient filtration was provided by the AC Delco filter, why the high-end K&N provided the worst filtration of all filters, and the AMSOIL filter mediocre filtration at best, as demonstrated by the test below, using the same standardized dust:

Given the proof provided by SAE, more than 15 years before this test, of how critical dust was to engine wear (we discussed it here), we could not understand how it was possible for alternative filter manufacturers to actually market a premium product when its efficiency was significantly below that of the standard, regular price product. We felt that, possibly, other filtration tests would show superiority for the premium priced, alternative filters. The ultimate test would compare the different filters, according to the amount of dust that was being passed up to a specific restriction of air flow, which, in the ISO 5011 test, was set at an increase of 10" of H20 from the original flow restriction as measured with a clean filter. The result was another shocker. The best performing filter, both in terms of time to clog and amount of dust passed, was the standard paper-made AC Delco filter, while the worst was, again, the K&N filter, the AMSOIL filter not being far behind:

In the test, the standard OEM replacement made by AC Delco took 60 minutes to increase flow restriction by 10" H2O, and passed 0.4 grams of dust to the engine in the process, while the premium K&N filter took 20 minutes to get to the same stage, passing 7.0 grams of dust (!!!) in the process, the AMSOIL filter clogging as fast but passing less dust through in the process. The amount of pass-through dust can be illustrated by the pictures in this article.

Finally, we could not help wondering how much dust overall could accumulate into the filter before causing a clog (measured in ISO 5011 tests as 10" H2O restriction, not quite equivalent to clog):

Again, AC Delco was the runaway winner, AMSOIL running last in the tests, and K&N second to last.

To say that we were shocked by the results of this test would be a strong understatement. We were also surprised by the poor availability of this test, which, while somewhat known on the forums, was not broadly available, as the original archives for the test, along with several replacements, had disappeared off the web. Could there be a problem with the credibility of the test? We did not think so, but felt that corroborating test results would be needed to truly feel comfortable with the outcome.

Next we review the results of  Jeep Magazine's 2006 ISO 5011 air filter study and tests... So come back soon!

Note: accidentally published out of sequence

<<Previous                                                                                                                   Next>>

Dirty Filters May Impact Performance

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 10: Dirty air filters and performance

In our last post on air filters, we reviewed dirty filter air flow and concluded that there was possibly a range of air flow restriction which could impact engine performance. The air flow curves were characteristically shaped, most of them being exponential with dust accumulation, with the exception of a small number being linear. Can we corroborate this data? 

The Southwest Research Institute published a research report commissioned by AEM on similar tests:

Regrettably, the SwRI restriction tests occurred at a low flow level of 240 CFM, while the Oak Ridge National Lab tests occurred on the vehicle itself at wide open throttle, and the Spicer/Testand ISO 5011 tests occurred at 350 CFM, a better level to approximate true/ wide open throttle conditions. As a result, the initial restriction shown on the SwTI tests above is significantly lower than in other tests, and restriction under dust load is also affected. The SwRI tests also include filters that have been washed and dried five times: the changes in air flow between new filters and washed filters are interesting to note.

Nevertheless, the same curves show up in both graphs: similarly to the data in the Spicer/Testand study, there is relatively little spread in the change in air flow between filters until the clogging elbow, except, in this study, for K&N, which remains at very low restriction, increasing its advantage in air flow against the other filters until it hits its own clogging elbow. The AEM filter in this study appears to behave similarly to the AC Delco in the Spicer test, having a very linear curve on air flow vs. capacity. As the two tests use different reference dust (coarse test dust for the Spicer/Testand study., fine test dust for the DwRI study) and vehicles, it is not possible to directly compare them quantitatively.

Since we know that, when they reach clogging, filters definitely impact engine performance, it is fair to say,  at some point as we move towards the right in these curves, we will start seeing some performance impact. It it is possible that a small number of filters might do worse than others. No filter, however, appears to significantly improve performance as dust accumulates compared to the majority average. it is worth noting that capacity impact on performance appears very significant when you get close to clogging.

Conclusion

• There is no difference in vehicle performance between most dirty air filters 

• There may be some differences in vehicle performance between some specific dirty - but not clogged-  air filters

• There is no proof that there is a difference in vehicle performance between any two dirty - but not clogged-  air filters

• Filter Capacity may have more impact on performance than air flow over the life cycle of an air filter

Next ......


<<Previous                                                                                                                   Next>>

Dirty Air Filters And Performance DRAFT

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 9: Dirty air filters and performance

We proved conclusively that there is no measurable difference in vehicle performance (i.e. acceleration, horsepower, torque) between clean air filters. Is it possible that there could be more significant differences in vehicle performance between dirty (but not clogged) air filters?

As filters progressively accumulate dirt, their efficiency improves but their air flow worsens. In fact, Perrin Performance's pitch is that their filters boost performance after 10,000 miles: "We sell our filters by saying all filters when new flow about the same.  But when dirty, foam continues to flow more air. In an example of an OEM filter being replaced with a PERRIN foam filter (on an STI), we see about a 3 wheel Horse Power gain.  This is not much at all and not something you can generally notice.  So its not that your stock filter when clean is super restrictive, but throw 10,000 miles at it, and the story changes."

We have already seen that, close to clogging, there is impact to performance. There clearly is a great difference between filters when close to clogging, as the capacity of air filters varies widely (later in this series of articles we will show conclusive proof of the widely varying capacity between filters). When one filter starts clogging when another isn't close to it,  the clogging filter will certainly worsen engine performance, while the other one should be still able to ensure good air flow. In this case, the advantage will go to filters with higher capacity, assuming that they do not get swapped too early in their life cycle.

What happens when air filters are not clean any more, but not yet clogged, i.e. for most of their working life? Thankfully, we can look at the results from some comparative tests that we review extensively further in our series. The Spicer/Testand study compared multiple different air filters in a series of ISO 5011 tests, using coarse test dust. This is how the tested air filters compare in air flow restriction when accumulating dust:

 

If  we look at the change in air flow restriction between filters, we find out that, among those tested, as long as we remain below the clogging threshold, the difference between filters remains roughly constant, except for the AMSOIL foam filter and the Baldwin filter, where restriction increases significantly faster than other filters. 

We know that, around 2-5" H2O of restriction or 0.5-1.25 kPa - that is, for clean air filters-, XXX there is no difference between air filters XXX. We also know that, as you get to 16" H2O or 4 kPa, you may start seeing significant performance impact. Therefore, somewhere between 6" and 16" we should see the first signs of performance degradation - where exactly we don't know.

Initial restriction for a clean air filter at wide open throttle (here measured at 350 CFM) typically varies from roughly 2" to 6" H20. If we were to encounter performance impact, we would expect a given engine to see performance impact from all filters at the same restriction level, regardless of their initial restriction, but higher than 2-6" H2O. Let's take, for instance, 12" H2O as a hypothetical restriction level where we might start seeing performance impact. For this example, we see that the AMSOIL filter will get in trouble first, at 125 grams of dust, followed by the K&N and the AFE at 205 grams, the Baldwin at 220 grams, the generic (no-brand) filter at 300 grams, the Purolator at 315 grams, the UNI and the AC Delco at 340 grams, and finally by the WIX, performing very well dirty at 400 grams. But - the order in which we see these filters could be different if performance impact started at a different level of air flow restriction.

At what level do you see performance impact?

K&N filter minder calibration most gasoline engine powered automobiles 20 inches maximum. 

pic shows 10" H20 85-2444.jpg (JPEG Image, 382x550 pixels) 

As the air filter gets dirty, the yellow position indicator moves in the clear window of the service indicator and locks at the highest point. It can be read even after the engine is turned off. The air filter should not be cleaned until the yellow position indicator reaches the red zone. Reset the position of the indicator to zero by pushing the yellow reset at the end of the service indicator.  

K&N Air filters become more efficient at stopping dirt as they build a dust film. It is recommended that the air filter be cleaned only when the air filter service indicator reaches the red zone.

Donaldson filter service indicator PDF  call?

AEM filter minder PDF : 10"H20...

designed to show a red indication at a pressure of approximately 10 inches of water

We should expect each engine to have a different level of air flow restriction at which it starts showing performance degradation. This level will depend upon the sizing and design of the air intake, and the engine displacement. Unfortunately, we have no data to tell us at which restriction level engines start seeing performance impact. As can be seen from the graph, the set of filters performing well at a given restriction level varies widely with the restriction level: the filters starting with low initial restrictions typically seem to have low capacity, which results in their showing high flow restriction at low with relatively lower dust weight.  As a result, without knowing at which level of flow restriction we see performance impact,  it is difficult to rate filters on "dirty performance" - although we will try later in this series.

These results were all drawn from one single study. Can we find corroborating proof? Next we review supporting evidence on vehicle performance with dirty air filters... So come back soon!

<<Previous                                                                                                                   Next>>

Clean Air Filters and Performance

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 8: Clean air filters and performance- more evidence 


In our last post we reviewed Diesel Power Magazine air filter tests, which conclude that swapping air filters alone does not impact performance (for clean filters). Can we confirm this test's results across other tests? We were not able to find other credible tests.  But we did find information provided within the industry which corroborates, in our opinion, Diesel Power Magazine's results:

• Castrol, of motor oil fame, also sells air filters. Its expert advice section, in a Q&A page ("More Air, More Horsepower) states: "When it comes to modern engines with electronic ignition and fuel control, an air filter upgrade in conjunction with a cold air intake can yield horsepower gains." This statement clearly implies that an air filter upgrade alone, without a cold air intake, is unlikely to yield horsepower gains.

•  Comptech, a manufacturer of performance parts for Japanese cars, writes to one of its customers: "Your ECU [Engine Control Unit] shouldn't be affected by changing the filter only." This implies, of course, that a drop-in air filter change will not affect performance. 

• Perrin Performance, a manufacturer of performance high flow filters, writes in its Filters Q&A: "We sell our filters by saying all filters when new flow about the same [...] In an example of an OEM filter being replaced with a PERRIN foam filter (on an STI), we see about a 3 wheel Horse Power gain.  This is not much at all and not something you can generally notice." In fact, we are no even sure that their measurement (certainly from a dyno) would be statistically significant, as dyno experiments are typically not accurate within 3 hp. What is significant is that the manufacturer recognizes the gain as minor, if any. 

• While many filter manufacturers claim that their filter will improve performance, none actually publish any third party tests proving such increase in performance. We find it unlikely that such would be the case if an improvement in performance could actually be documented.

While we find none of these facts sufficient by themselves, we consider that, together, along with Diesel Power Magazine's tests, they prove conclusively that no notable gain of performance can be obtained from swapping air filters alone. Our conclusion is that changing drop-in air filters does not affect vehicle performance in a measurable way.


Conclusion

• Swapping drop-in air filters alone does not improve vehicle performance for a clean filter

But could this conclusion be different when applied to the same filters when dirty? Next we discuss difference in performance between drop-in air filters when dirty... So come back soon!

<<Previous                                                                                                                   Next>>

Can Air Filters Improve Performance?

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 7: Performance differences between clean air filters

Average atmospheric pressure at ground level is typical estimated at 101.325 kPa. The difference between no filter and the least flowing clean filter, as measured by the Oak Ridge National Lab, is less than 0.7 kPa. This represents less than 0.7% of atmospherics pressure, although it is over 25% of the increase in pressure drop that is considered characteristic of a clogged filter (increase in pressure drop of 2.5kPa). Is this pressure differential enough to cause differences in performance?

Unfortunately, we have no data on this point from Oak Ridge. Diesel Power Magazine, however, commissioned this test from Westech Performance, which compared K&N (typically measured as the best flowing air filter on the market), Volant, AFE, Airaid, True Flow, and S&B aftermarket air filters on a 2004 1/2 Dodge Ram 2500 equipped with a manual transmission:

Diesel Power Magazine's conclusion: "don't bother trying to tell them apart, the average results of the dyno runs ranged between 308.7 and 310.4 hp, and each filter had at least one run that was in the 309s." Differences of 5 hp or less or typically not statistically significant on dynos, due to the sensitivity of the tool, so the differences in results are not significant.  

The test's conclusion, then, is that drop-in air filters do not result in any performance difference when tested clean. Can we get corroborative proof to confirm this conclusion?

Next we look at other supporting evidence on air filters and performance... So come back soon!

<<Previous                                                                                                                   Next>>

Clogged Air Filters Impact Performance

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 6: Do clogged air filters impact performance?

In our last few posts, we proved conclusively, with some surprise, that air flow and air filters do not impact mileage. The next step, for us, is to similarly investigate air flow through air filters and vehicle performance, that is, acceleration, horsepower, or torque.

If the restriction to air flow caused by air filters is going to impact performance, it stands to reason that we would see the worst impact in severely clogged air filters. How do these impact performance?

Highly restricted air filters

Oak Ridge National Laboratory tested several vehicles with highly restricted (clogged) air filters. In fact, one of the filters was so clogged that it got sucked into the intake... Their experiments make it clear that there is severe impact to performance when the filter is clogged:

All three vehicles tested experience significant performance degradation in the presence of a clogged air intake, varying between 6% and 11% in terms of time needed to accelerate from 20 MPH to 80 MPH using a wide open throttle.

There are many supporting anecdotal reports discussing loss of performance with dirty air filters. As an example, columnist Jim Allen reported, on BITOG, some dyno tests on a diesel truck showing loss of performance with a dirty air filter.


Conclusion

• Clogged air filters negatively impact performance in a measurable way

 It is clear that highly restricted air flow, such as one sees in a clogged air filter, does impact performance. What about the much smaller air flow difference that is seen between two clean air filters - is there impact to performance?

Next we review how different clean air filters impact performance... So come back soon!

<<Previous                                                                                                                   Next>>

Air Filters: Does Air Flow Impact Mileage?

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 5: Air flow and mileage

Can air flow improve your vehicle's fuel economy? Many users report improved mileage upon the adoption of an air filter with measurably increased airflow, such as when you change from a stock OEM filter to a K&N filter. They are (mostly) wrong.

We already proved that different air filters can increase (or worsen) air flow (+/- 0.5kPa), and that clogged air filters  significantly impact air flow (up to +6.25 kPa, much higher impact than air filter swap). How does air flow impact mileage?

Fueleconomy.gov  is a federal government site focused on - guess what - fuel economy.  Under their aegis, the famed Oak Ridge National Laboratory published, in February 2009, a investigating the effect of air filter condition (i.e. air flow restriction) on fuel economy. They were able to show, once again, that aftermarket (high flow) filters were able to provide more air flow than OEM filters, measuring a lower pressure loss across aftermarket filters than OEM filters:

The February 2009 Oak Ridge National Laboratory research report on fuel economy measured the difference in mileage between a clean, new air filter, and the severely clogged air filters we showed in our last post. The researchers experimented with both modern fuel-injected engines, and with older carbureted engines. Despite the difficulty at getting very low airflow, the difference in fuel efficiency for all newer vehicles was statistically nonexistent, as shown below for one of the experiments (for this illustrative graph, we picked the Dodge Charger, which showed, in our previous test, the worst clogging):

Oak Ridge National Laboratory's conclusion: "Results show that clogging the air filter has no significant effect on the fuel economy of the newer vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control systems were able to maintain the desired AFR [air fuel ratio] regardless of intake restrictions, and therefore fuel consumption was not increased. The carbureted engine did show a decrease in fuel economy with increasing restriction. However, the level of restriction required to cause a substantial (10–15%) decrease in fuel economy [...] was so severe that the vehicle was almost undrivable."

If clogging the air filter - which increases restriction to air flow by 6.25 kPa -  does not have an effect on mileage,  then clearly, swapping air filters - which varies air flow restriction by +/- 0.5kPa -  certainly does not impact mileage either.

There is an important note to add to the Oak Ridge's results: the vehicles tested were all gasoline engines. Normal diesel engines are unthrottled in normal use after warm-up, which means that, even under light loads, they operate at high air flow, and throttle cannot be used to adjust fuel fix. It is possible that air filters could impact fuel consumption for modern diesel engines.  

We found multiple confirmations for the conclusions of the Oakridge National Laboratory's tests. For example, Consumer Reports, in 2011, published a post on fuel economy, and listed dirty air filters under myth busters (...): "Our tests show that driving with a dirty air filter no longer has any impact on fuel economy, as it did with older engines. That's because modern engines use computers to precisely control the air/fuel ratio, depending on the amount of air coming in through the filter. Reducing airflow causes the engine to automatically reduce the amount of fuel being used. Fuel economy didn't change [...]." Offroad Adventures tested a high flow drop-in filter replacement and found no impact to mileage: "Flow capacity is 15-20 percent higher than stock, so it gives you a little over-capacity for mods. We ran the panel in the stock housing for several months and noted no change in fuel economy."

All the scientific evidence clearly points at the same conclusion: air flow in modern vehicles does not impact mileage. Everything else being equal, high flow filters with higher air flow do not improve a vehicle's flow efficiency. In fact, several studies (e.g. Fodor & Ling, "Friction reduction in an I.C. engine through improved filtration and a new lubricant additive", Lubrication engineering, 1985, vol. 41, no10, pp. 614-618; Andrews, Li, Jones, Hall, Rahman & Saydali, "The Influence of an Oil Recycler on Lubricating Oil Quality with Oil Age for a Bus Using In-Service Testing,"SAE 2000 World Congress, Paper 2000-01-0234) have shown better mileage resulting from better oil filtration. As a result, is possible that filters with higher air flow but worse filtration (these two traits often go hand in hand), which contribute to more wear and more particles in the oil, may ultimately result in worse fuel economy.


Conclusion

• The choice of an air filter can materially impact air flow

• Air flow does not affect fuel economy in gasoline engines

• Drop-in high flow air filters have no effect on mileage in gasoline engines

But does air flow impact performance? Next we investigate the effect of clogged air filters on vehicle performance... So come back soon!

<<Previous                                                                                                                   Next>>

Do Clogged Air Filters Restrict Air Flow?

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 4: Do clogged air filters restrict air flow?

In our previous post, we proved that different air filters can improve (or worsen) air flow when clean. But what is the impact, if any, of a clogged (or very dirty) air filter?

Our friends from Oak Ridge National Lab also investigated the impact of a clogged air filter. To simulate the appropriate level of reduced air flow, the researchers ended up stuffing shop towels (!!!) on the upstream side of the air filter's air box, or wrapping the filter itself in shop towels:

 

The shop towels had to be stuffed so hard to get to the right level of reduced airflow that the one of the air filters was quite damaged in the process:-) This is representative of the difficulty of getting severely restricted airflow... In the end, the study was able to restrict flow significantly:

 

It is worth noting that filters are typically considered clogged when pressure drop across the filter increases by 10" H2O, i.e. 2.5kPa. What this means is that the tests were run using significantly higher air flow restrictions. These are truly worst case tests.

Conclusion

• Clogged air filters significantly reduce air flow

Does this very significantly reduced air flow (to the degree that it sucked a shop towel into the intake) actually worsen fuel economy? Next we investigate how air flow impacts mileage... So come back soon!

<<Previous                                                                                                                   Next>>

Air Filters: Can They Improve Air Flow?

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 3: Can air filters improve air flow?

In the next few posts, we will investigate if you can improve automotive mileage and performance (i.e. acceleration and torque) by swapping drop-in air filters that have higher air flow. We already know that air flow through an air filter does not impact mileage in modern cars. Because so much opinion on air filters is based on assumptions rather than facts, we should ask - can we measure meaningful differences in air flow between air filters?

Air flow differences across air filters

Fueleconomy.gov  is a federal government site focused on - guess what - fuel economy.  Under their aegis, the famed Oak Ridge National Laboratory published, in February 2009, a research report investigating the effect of air filter condition (i.e. air flow restriction) on fuel economy. The researchers experimented with both modern fuel-injected engines, and with older carbureted engines. They showed conclusively that some aftermarket (high flow) filters were able to provide more air flow than OEM filters, measuring a lower pressure loss across aftermarket filters than OEM filters:

 

These tests were done on the vehicle itself, at wide open throttle for 10 seconds. There is a clear difference  between no filter, high flow aftermarket filters, and the OEM style filter, which experiences the highest pressure drop (meaning the least air flow, but also, probably, the best filtration). So, clearly, it is possible to increase air flow (and decrease pressure drop) by using a different drop-in filters. We will see later than the K&N filter is particularly successful at that. However, the differences, when compared to atmospheric pressures, are very small - so it is not clear if these will result in actual changes in mileage or performance. 

Conclusion

• Non-stock air filters can improve air flow

How much worse does air flow get when your air filter gets dirty? Is there a measurable difference? Next we discuss the impact of dirty air filters on air flow... So come back soon!

<<Previous                                                                                                                   Next>>

Car Air Filters: What To Look For

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 2: What makes for the best air filter

What makes for a good automotive air filter? To a degree, it depends upon your usage. Rural drivers in a very dusty environment will look carefully at capacity, while Friday night racing aficionados will look more at air flow. We look at criteria for an average user, who does not race, and whose environment is only moderately dusty. For our average user, we present below the most significant criteria for air filters, by decreasing order of importance.

Filtration efficiency

We already discussed here the importance of filtration, which simply cannot be overemphasized, as dust is the primary cause of engine wear: both oil and air filtration are the most critical factors to making an engine last.  Lycoming, the manufacturer of aircraft engine, states in its maintenance manual: "Dust or some form of dirt is frequently the principal factor in premature piston ring, ring groove and cylinder wear. If a worn or poorly fit air filter allows as much as a tablespoon of abrasive dirt material into the cylinders, it will cause wear to the extent that an overhaul will  be required. Evidence of dust or other dirt material in the induction system beyond the air filter is indicative of inadequate filter care or a damaged filter." The difference between filtration efficiency of 90%, 99%, and 99.9% is very significant:

The average capacity of all the filters surveyed in our series is 234 grams. If we assume that the life cycle of the filter is approximately 40% of its capacity (you will change it before it gets clogged), we can assume that the filter will see roughly 100 grams of dust at the intake. If the filter has 90% cumulative efficiency, what will pass through it is the amount of the first jar. If it has 99% efficiency, it will pass through the content of the second jar. With 99.9% efficiency, it will pass through the content of the third jar. Given that whatever passes through the engine goes straight into the cylinder chambers, which one would you like it to be for your car?

It is worth noting that the typical way to measure filtration efficiency is by calculating how much dust is filtered over the theoretical lifetime of the filter (i.e. until it reaches capacity), which is really measuring cumulative filtration efficiency. Why is this important? Filters get more efficient as they start accumulating dust. This means that, if you always use clean filters, your efficiency will be lower than that measured in ISO 5011 filtration tests! Amazingly, is actually better, for filtration, to have some dust in your filter. What this means is that, unless you keep your filters until they clog, your true filtration efficiency will be lower than that measured for your air filter.

We should mention, in passing, that we have frequently seen manufacturers' specs for filtration efficiency be proved wrong by third party tests. Unfortunately, it appears that we cannot trust air filter manufacturers to give us their own filters' specs unless the actual tests are conducted by third parties.

Fit

This is a frequently neglected factor. The variety of shapes and sizes across all cars is very, very broad, and there are many cases when a filter brand which performs in general well does not have perfect fit for a given car's intake. Big air filter manufacturers, such as Wix or Purolator, will often design and manufacture filters themselves for vehicles with wide distribution, but may sometimes factor out filters for less common vehicles, making fit and quality problems more likely.

As a result, it is not possible to evaluate fit on the basis of the air filter brand, since it varies by vehicle. If you are not going to actually check out the fit yourself, you are better off requesting an OEM filter for your vehicle, which is more likely to have good fit.

Capacity

The capacity of an air filter is the amount of dust that it takes to increase the resistance of the filter to air flow by 10" H2O, and is supposed to be a good measure of what it takes to clog a filter. The larger the capacity, the longer you can take before you change your air filter. Of course, larger capacity is also preferable when driving in a dusty environment, where a lot of dust may end up in your filter in a short amount of time. While we do not believe that 20-30% difference in capacity makes much of a difference to the average user, we found out that filter capacity varies by a factor of 1 to 10. We do believe that a difference of one order of magnitude makes a significant difference.

Build quality

There are many ways to build a cheaper air filter - but how much do then also cheapen build quality? What material is the frame made of? How well dimensioned is it? What is the filtration medium? How are the pleats built? A better-built filter is likely to be more expensive, although not always better performing. Given that air filters can last through tens of thousands of miles (while oil filters only last thousands of miles), some difference in price may not be significant to the overall life cycle cost of your vehicle, while filtration improvement definitely will make a difference.

Quality control

The best filter does not amount to much is quality varies enough between units that fit is not guaranteed, or that filtration is not maintained. As air filters have become commoditized, quality control and build quality have become progressively worse.

Air flow

It is the last criterion on our list, and the most controversial as well. It is the least important of all criteria, and we will discuss why. The next few sections will investigate the influence of air flow on mileage and on performance.


Next we discuss if different air filters can impact air flow... So come back soon!

<<Previous                                                                                                                     Next>>

K&N/ High Flow Air Filters: Worth it?

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 1: What you need to know                                                                                                                                   .

Performance air filters: K&N (aka KN or K and N), Purolator, Amsoil, AEM, Green Filters, AFE make high flow automotive air filters, K&N being the most popular choice. Do these drop-in high performance automotive air filters make a difference to your mileage or performance? Their manufacturers claim that they let more air in through the system by decreasing flow restriction in the filter itself, and therefore increase engine power along with mileage.What are the right air filters to buy for your car? Many of the net discussions over air filters today are based on opinion rather than facts. What hard data do you have to be able to make well grounded decisions?

In this thorough Best Air Filters Review, we will explore air filter facts that are truly backed with data, then discuss all of the existing comparative tests that can be found on air filters, and finally review all of the air filter brands for which we have data.


What you need to know about automotive air filters

Why air filters
Your combustion engine burns fuel (gasoline, diesel, others) with oxygen (extracted from atmospheric air) in a sealed chamber to violently expand the mixture in it and provide motion to a shaft which ultimately drives the car wheels. Dust in the air could make their way into the combustion chamber and progressively degrade the metal surfaces and damage the seals, ultimately shortening the life of the engine very significantly. This is why your car manufacturer puts in an air filter on the air intake, so that as little dust as possible can make its way into the combustion chamber.

Replacing your air filter
Air filters eventually get contaminated by dust, and need to be periodically changed, typically on a schedule specified by your car's manufacturer. If they do not get changed, they will eventually let more dust into the engine, shortening its life. They will also let less and less air in, worsening car performance. Periodically changing both oil and air filters are critical maintenance operations to prolong the life of your car.

Filtering media
Standard OEM air filters are made of paper and/or other fibers, using many pleats for filtering (AC Delco, Wix/ Napa, Purolator Classic/ PureOne). So-called performance air filters typically use foam (Jackson Racing, Racing Beat, AMSOIL previous generation), synthetic fiber (AEM, AFE Pro Dry),  oiled cotton gauze (K&N, Green Filters, S&B, AFE ProGuard), or oiled cotton- synthetic mix (Airaid) as the basic filtering media. AMSOIL, previously using foam, now uses nanofiber.  Wire mesh with oil bath is a technology that has practically disappeared but that may make a come-back.

Cold Air Intakes
The high flow, "performance" air filters we are discussing are supposed to be drop-in replacements for stock air filters. There is, however, one possible wrinkle on drop-in air filters for performance improvement, which involves some modifications to the air intake. Cold air intakes bring in cold air from outside the hot engine bay, through  a filter, and into the combustion chamber. Because cold air is denser, everything else being equal,  more air coming into the engine will theoretically enhance engine performance and mileage, although, with modern engines automatically adjusting the amount of injected fuel to the amount of air, mileage improvement may not be realized. Cold air intakes are not drop-in replacements, and we are not evaluating them in this review.

Throwaway vs. reusable
Standard air filters, made of paper or fiber-based material, are generally throw-away parts that you toss upon change-up - AC Delco is the benchmark. Cotton, synthetic or foam filters can be cleaned, water-washed, dried, then reused. Cleaning the filter typically requires an aerosol can with a cleaning substance (sold by the filter manufacturer), after which the filter is carefully reverse-washed, then dried for up to two days, and sometimes oiled (also using proprietary oil from the filter manufacturer) before it is put back in. The procedure is, in general, more delicate for foam filters  than it is for cotton gauze. It is best to have two alternating filters, so that there is no need for the car to be idle while the filter is drying.

Mass Air Flow (MAF) sensors
Modern, non-carbureted, fuel-injected engines use mass flow sensors to measure the amount of air getting into the engine to automatically adjust the air/fuel mixture to optimal levels.

Cost
Standard OEM air filters are significantly cheaper than their alternatives, but reusable filters eventually make up their increased costs and more, even with the additional cost of the cleaning aerosol cans and filtering oils.

Filtration efficiency and engine wear
The seminal paper connecting engine wear and filtration efficiency, Correlating Lube Oil Filtration Efficiencies With Engine Wear, was published by the Society of Automotive Engineers in 1988, and convincingly proved that engine wear could be reduced 70% by going from 40 micron oil filters to 15 micron filters. While the research was conducted on oil filters, since particles can reach the engine through oil or air, conclusions are likely to be valid for air filters as well.

                   Graph 1: Filter efficiency vs. particle size (SAE tests)

The graph above shows the filtration efficiency for eight different filters. Below we see the effect that these eight filters have on engine wear. There is clear and total correlation between the filter's effectiveness at removing smaller particles and the lesser amount of wear, wear being measured in metal loss from critical engine parts.

                               Graph 2: filter effect in diesel engine wear (SAE tests)

                                    Graph 3: filter effect on gas engine wear (SAE tests)

High flow car filters have their very strong supporters on many automotive forums, in particular among speed aficionados. The concept that high flow filter supporters push is that, if you can reduce the air flow constriction caused by the filter, by using filtration media which are not so constraining, you will improve engine performance and mileage. In fact, many high flow filter supporters feel that their engine is peppier and their mileage improved: "2 miles per gal better fuel economy", " more responsive to the gas pedal", "improvement in acceleration." We were not able, however, to find relevant scientific information to validate improved mileage claims, and ended up discounting them. The extra horsepower obtained from aftermarket sensor is also doubtful: "you aren't going to pick up any power from a filter swap alone," writes Jeep Magazine - many experts agree.

We had expected, when initiating this review, to quickly find conclusive research:  high flow filter companies had to have commissioned independent research labs to produce studies conclusively proving higher mileage, better performance, and/ or better filtration. We were very surprised to find absolutely no research data proving the advantages of high flow filters. AMSOIL, in particular, is well known for commissioning many third party tests for its motor oils - we were shocked to find that it had not commissioned a single test, that it was willing to share, comparing the effectiveness of its air filters. We found a lot of articles, from prestigious sites such as edmunds, extolling the virtues of high flow air filters. Unfortunately, they were not supported by scientific proof.

So we kept on digging for more facts. And - we finally found a few. But the results were not quite what we expected.

Next we discuss what makes a good air filter... So come back soon!

                                                                                                                                  Next>>