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  7. The Truth About Engine Air Filters
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  7. The Truth About Engine Air Filters

The Truth About Engine Air Filters

What are they and when should you change them?

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The basic job of an engine air filter is to supply the cleanest possible air with the least airflow restriction. Air filters don’t stay clean long, though, and as the filter loads up with dirt, restriction increases. Sooner or later, there’s enough restriction to impact power production. Around town, the operator of a lightly loaded vehicle may not feel that loss of power for a long while. An RVer with a full load out on the road will notice it quickly.

Engine Filter Types

Today’s light-truck air filters are often a mix of several media, including cellulose, non-woven synthetic fiber, nano-fiber, foam and oiled cotton gauze. You should not automatically dismiss any one material as worse or laud another as better, rather, look closely at how it’s employed.

Cellulose media

Cellulose Media is plant-based fiber treated with resin to make it hold shape and resist deterioration. Sometimes treated with a tackifier, a sticky substance, to attract dirt, it’s efficient at filtering, inexpensive and easily made. Because it’s a surface loading media (collecting dirt on the outer surfaces), it gets restricted more quickly than synthetics. Per square inch of media, cellulose doesn’t flow air as well as synthetics.

Synthetic media

Synthetic media is man-made fibers of various types. Fiber size can be precisely controlled to manage airflow and filtration and it’s easy to form into a graduated density material. It’s considered depth media, meaning it carries dirt inside the media as well as on the surface, and it can carry more dirt than cellulose with less restriction. The profile of the fibers can be engineered to further enhance filtration and tackifiers can be added as well. It’s approximately equal to cellulose in efficiency, but flows better and can be cleanable. Nano-fibers are a synthetic fiber but much smaller in diameter. Typically they are blended with larger fibers to enhance filtration and capacity. The main downside to synthetics versus cellulose is the expense.


Foam, more precisely open-cell foam, is most often a polyurethane material. Open-cell foam has a labyrinth of pores, the size and density of which can be controlled during manufacture. Varying densities of foam can be layered. A tackifier, usually oil, is needed to increase efficiency but foam filters are usually cleanable. The downsides to foam are finding a good compromise between airflow and efficiency, plus cost.

Oiled Cotton Gauze

Oiled cotton gauze uses sheets of pleated cotton, which is fibrous, sandwiched between layers of wire mesh. By itself, cotton efficiency is low, but efficiency increases greatly when an oil tackifier is added. Oiled cotton gauze flows very well, can carry a high dirt load and is cleanable. Its efficiency is average. The main downsides are a need to be careful during cleaning so that the fibers are not damaged, and the cost.

Air Filter Efficiency and Capacity

As filter media loads up, filtering efficiency improves. The filter is given an efficiency percentage and rated two ways. The initial percentage, which measures the efficiency after 20 grams of dust have been applied to new media, and the final percentage, which is taken at a specific restriction (2.5kPa/10 inches of water vacuum) over the initial new filter reading.


Dust cake, such as the one in this picture, collects and becomes filtration media in its own right.


The auto industry is now shooting for an initial efficiency of around 98 percent, meaning the filter stops 98 percent of the dirt it ingests. Jon Wake, air filtration product manager at Parker Filtration (home of Racor), said “a filter might start at an initial 98 percent but within just a few thousand miles, it’s gone up a full percentage point and by the time it reaches 20 to 25 inches of water vacuum, the usual maximum restriction range, efficiency has reached 99.9 percent.”

Two percent doesn’t seem like much on paper, but to your engine it’s a big deal. Going from 98 to 99 percent is a 50 percent decrease in the amount of dirt that makes it into your engine. This is why early or overly frequent filter changes are not advised because, according to Wake, 90 percent of the lifetime amount of dirt that passes through a filter does so in the first 10 percent of use.



The filter on the left looks nasty but when tested for airflow, was still below 20 inches of water vacuum. The one on the right has many years and thousands of miles to go.


How much and what size dirt particle is harmful to your engine?

Often that depends on the engine, and the motor industry has not yet reached a general consensus. Modern engines have tight tolerances and emission controls have added new wrinkles, making air filtration more of an issue.

John Concialdi, chief engineer at AEM, pointed out numerous studies that show any dirt is potentially harmful, but high concentrations of particles in the 1 to 20 micron range are the most harmful to modern engines because they are small enough to get into places where they can do the most harm. In many ways, the city environment offers the most potential for harm because the air has higher concentrations of fine materials, such as diesel soot (which is essentially carbon and very abrasive).

Poor air filtration often shows up in the oil. A high silicon reading is one of the main indicators when the oil is analyzed. Dirt is highly abrasive, especially as it works down the cylinder bores, but also in the bearings after it migrates into the oil. By then, it may be small enough to go through the oil filter (which typically can capture particles in the 20 to 30 micron range) but it’s still big enough to increase wear.

Capacity is an issue unto itself. It’s usually measured in grams and indicates the maximum amount of dirt the filter can hold before reaching a restriction limit. Capacity is relative to the physical size of the filter, more precisely the area of the filter media, which varies according to the number of pleats. More area equals more capacity. If you compare two filters for the same application and of the same material, the one with the most pleats usually has the higher capacity of the two.


The dirty filter (right) was so packed that the valleys in the pleats were full.


Air Filter Sealing

One indicator of filter quality is how well it seals in the filter housing. As Jere Wall, test lab engineer at K&N, is fond of saying, “An air filter that doesn’t seal properly in the housing is not a filter at all.” Wall suggests inspecting each new filter for seal integrity. Sealing failures due to increased restriction or vibration can also occur in service.

Like anything else, there are quality variations that can dictate the useful seal life. It’s vital to always ensure a good fit in the air-filter housing no matter what filter you use. It’s best not to remove and replace the filter for inspection any more than necessary. Not only can you introduce dirt into the intake tract doing so, you may damage the seals. Seals may also deteriorate over time due to heat aging. Filter grease, such as the product K&N sells, can be particularly useful in sealing filters into the housing and preventing seal damage during inspection.



A new fuel injection programmer installation was being tested and tuned on a late-model Chevy Duramax, but performance in stock and tuned form was sluggish, until it was noticed that the filter restriction gauge was tripped to red.


Shattered Air Filter Myths

The Oak Ridge National Laboratory ran a series of tests in 2009 on the effect restricted air filters had on gasoline engine fuel economy. It restricted the air filters on a small group of cars and ran them through the same test routines used to determine the EPA fuel economy ratings.

Fuel economy was not affected for the fuel-injected cars until the filter was at a very high restriction and then only by a small amount. Some cars were not affected at all. With the most affected fuel-injected car, using the most stringent test, the Highway Fuel Economy Test (HFET), the drop was only 1.7 percent. In the easier urban test, the drop was only 1.1 percent on the same car. The single carbureted car in the test showed a greater loss of 2.5 percent during HFET.

The fuel-injected systems, via oxygen sensors, airflow or manifold pressure sensors, constantly adjusted the fuel mixture. The 20-to-80-mph acceleration of all the cars showed a definite performance loss with severely restricted filters. The tests also showed a highly restricted filter can collapse and be sucked into the engine.

Diesel fuel economy, especially with older mechanically injected diesels, is very much affected by air filter restriction because the fuel system is calibrated for a specific volume of air. If that volume isn’t received, it still injects the same amount of fuel and more goes out the tailpipe.



Filters come in every shape and size. Here are just a few from K&N.


When To Change Air Filters

The OEMs list an air filter change interval based on averages. If your driving situation matches their averages, the factory interval will serve you well. If you fall outside those norms, as many drivers do, you will either toss a filter away too soon, or too late. Extensive studies have been done by the automotive industry and there is no one-size-fits-all recommendation, hence the overly conservative OEM intervals. One average statistic we found is 60 grams of dirt per 30,000 miles for the average street vehicle. If filter capacity is 300 grams, then the service life “should” be in the 150,000-mile range. But this is all guesswork and a restriction gauge makes it a science.

As the pores in the filter get blocked with dirt, restriction increases and creates greater vacuum behind the filter. A restriction gauge is essentially a recording vacuum gauge. Some simply trip from green to yellow to red at predetermined points of restriction. Others deliver a read-out in inches of water vacuum or Kilopascals (kPa). The beauty of a restriction gauge is that it tells you exactly the right time to change the filter. That point could come in 5,000 miles of harsh conditions or 100,000 miles of easy driving.

Cleaning Air Filters

While it’s possible to “clean” a non-cleanable filter and increase its operational life, it’s just as possible and quite easy to unknowingly put holes in it, allowing dirt into your engine and slowly harming it. Cleaning is more a temptation for farmers and heavy equipment operators that count air filter life in days or weeks of operation. The average driver or RVer is normally looking at years of operation and tens of thousands of miles. If you get tempted, remember it’s aged in other ways. You may be able to clean it sufficiently and do so without damage, but when you place that filter back into the housing, you might end up with sealing issues.

Some filters are cleanable but how you do it is very important to avoid damage and filter efficiency reduction. The big no-nos usually involve compressed air and harsh cleaners. Most cleanable filters are washed in water with mild soaps and laid out to dry in the shade. Our best advice is to always follow the instructions.



Whether it’s a new or used filter, a quick inspection is worth the time before the unit is fitted into the air box. Look for cracks or holes.


Air Filters and Performance

An engine needs a specific volume of air to generate its maximum rated power and torque. The required amount of air often increases according to tuning changes or other modifications you might make to the engine. Even things like a free-flow exhaust could increase airflow needs on the intake side.



This filter has 100,000 miles on it and still doesn’t trigger a restriction gauge set at 25 inches of water.


Concialdi at AEM said that engines seldom have all the air they can use. “We often find the air intake system lacking in airflow and that the engine can use more. Less restriction results in lower pumping losses (the power it takes to draw air against a restriction) but in the case of a tuned system, we can often take advantage of length and diameter tuning of the intake duct to increase power.”

A tuned intake may also result in a lowering of intake air temperature. Cooler air is more densely packed with oxygen and the fuel-injection system will add more fuel in the right amount for a little more power and can maintain more ignition timing advance. Cooler air also reduces EGT (Exhaust Gas Temperature) in both gas and diesel engines.

Simply replacing a filter element with one that has less restriction seldom results in a noticeable power increase, but if filter area is increased with an upgrade, the filter can carry more dirt and maintain lower restriction over a longer period. That isn’t exactly a power increase but it reduces power loss from restriction for a longer period. Fuel economy doesn’t often increase, or increase much, from a performance intake on a gas engine, but gains are more possible with diesels.

The proper evaluation of a performance intake/filter should begin with its dynamometer test results and include total airflow (versus stock), reductions in intake air temp, improved filter efficiency, increased filter area/capacity and serviceability. With many low-cost, price point products on the market, though, cost is a useful comparison only on products with similar performance characteristics.

The best air filter for your purposes often comes down to a decision on priorities. Now you know enough to make that choice with confidence.


AEM Induction Systems
800-992-3000, www.aemintakes.com

952-887-3716, www.donaldson.com

Filter Minder
319-234-0231, www.filterminder.com

800-858-3333, www.K&Nfilters.com

Oak Ridge Fuel Economy Test

Parker Filtration
800-272-7537, www.parker.com

University of Northwestern Ohio
419-998-3120, www.unoh.edu

Jim Allen
+ posts

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