Monday, March 7, 2011

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.

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!


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