Air Filter Facts

Facts You Should Know About Air Filters

We believe the primary function of an air filter is to deliver both high airflow and superior dirt protection. We design our air filters to provide minimum restriction allowing high airflow into an engine. In most cases increased airflow will increase engine performance measured in kilowatts and torque (often perceived as throttle response). The performance benefits of maximum airflow are clear, compelling and well documented. That is why so many professional racers are willing to run expensive vehicles without an air filter, as opposed to installing a disposable air filter. They are seeking the additional power and throttle response needed to win the race.

We design our air filters to provide superior filtration of the contaminants that can harm your engine while maximizing the airflow characteristics of the filter in question. The ability of an air filter to protect your engine is generally measured in accordance with testing procedure ISO 5011. We subject a sample of our filter designs to this test procedure using Coarse Test Dust, which includes particles ranging in size from less than 5.5 microns to 176 microns. As a point of reference, a human hair is approximately 50 microns in diameter. The result of the above test procedure is a specific air filtration efficiency number. This efficiency number represents the percentage of test dust retained by the filter and thereby kept out of an engine. Our goal is to design our air filters to achieve maximum airflow while targeting overall filtration efficiency at 98%.

Because no two air filters are alike, the specific airflow and overall filtration efficiency will vary depending on the filter in question. However, you can rest assured that each air filter we sell, has been designed to achieve high airflow while providing superior filtration.


Both airflow and dirt protection are critical to engine performance. For this reason, a consumer should always evaluate an air filter based on both its filtration efficiency and airflow capabilities. It is very easy to design an air filter that exhibits high airflow simply by reducing its filtration to unacceptable levels. As the "look" of a K&N air filter has become popular, many companies have begun offering products that copy that "look." While imitation is said to be the sincerest form of flattery, our own testing has shown that many of these look-alike products do not provide a safe level of engine protection.

What's the Big Deal About Air Flow?

Simply put, Everything! At its most basic level, an engine is an air pump. More air entering the engine increases the efficiency of the combustion process creating more power and torque. The former is a measure of the engine's maximum power while torque measures how quickly you can accelerate.

The K&N Filtercharger is designed to increase engine performance by reducing airflow restriction. 


Maintaining optimal, unrestricted airflow becomes a problem when it must pass through a filtering medium. The level of air resistance varies depending on the size, surface area and physical attributes of the filtering medium.

The Big Deal About Air Flow

A Word About Filtration Requirements

There are few areas more confusing than identifying dirt retention requirements when it comes to air filters. Most vehicle owner's manuals remain silent on the point. In fact, few air filter manufacturers publish any information as to the filtration efficiency of their filters. This stands in marked contrast to oil and fuel filters where there is a relatively large amount of information regarding filtration requirements and capabilities.

Studies have shown most engine wear is caused by particles 10 to 20 microns in size. K&N air filters, like most quality disposable air filters, provide excellent filtration of these particles.

To ensure our air filters provide a high level of dirt protection, we regularly test our designs using the testing procedure described above. Those tests demonstrate K&N air filters generally achieve overall filtration efficiency in the range of 96% - 99%. The fact that our air filters at times reach overall filtration efficiencies as high as 99% while maintaining high airflow is a testament to the quality and capabilities of our oil-impregnated cotton air filter medium.

On occasion we see "new" air filtration products developed and sold under the premise they provide increased levels of dirt filtration. Often, as opposed to quoting specific efficiency numbers, this "increased protection" is described as increased dirt retention capacity, meaning the filter can hold more dirt before requiring replacement. Remember, K&N air filters have always provided a service life more than disposable filters and then only require cleaning and oiling for re-use.


We encourage customers to do their homework and be aware of the filtration capabilities of an air filter before they buy. Our own testing has revealed wide differences in filtering capabilities. We were surprised to see some disposable paper air filters with an overall filtration efficiency as low as 93%. We hope we have provided enough information to ensure consumers know what they are getting when they buy a K&N air filter.

Filtration 101 - A Deeper Cut

Most people believe that all air filters function on a go/no-go basis where dirt particles that are larger than the openings in the filter media are trapped while particles that are smaller than the openings can pass right through. A dry paper air filter does function in this manner. That's why paper filters are so restrictive to airflow. The openings in this type of filter have to be very small to filter efficiently.

The oiled cotton filter media used in the K&N air filter functions in an entirely different manner. There are scientific principles that determine how an air filter removes dirt particles from the air stream. The first of these principles is known as interception, which applies to dirt particles traveling with the air stream. Airflow will always take the shortest path and as the air is forced to flow around the filter's fibres some of the particles will contact the sides of the fibres and be captured. These particles are then held in place by the oil or tacking agent in the fibre.

Another principle is known as impaction, which mostly affects larger or heavier dirt particles. Impaction occurs when the inertia or momentum of the particle causes it to deviate from the flow path. In other words, the heavy particles do not follow the air stream around the filter's fibres but instead they run straight into the fibres and are captured. 

The most important principle for our use is diffusion, which deals with the laws of physics that govern the motion of very small dirt particles. Small particles are highly affected by the forces in the air stream. Forces such as velocity changes, pressure changes, turbulence caused by other particles and interaction with the air molecules cause these very small particles to become random and chaotic. As a result, these particles do not follow the air stream and their erratic motion causes them to collide with the filter's fibres. This phenomenon enables an air filter to capture dirt particles that are much smaller than the openings in the media.


In addition, the way that dirt collects or loads on the K&N filter is very different. A paper filter exhibits surface loading which means dust collects only on the surface of the media. In contrast, K&N filters exhibit depth loading. The multiple layers of cotton fibres provide many levels of dust retention. This characteristic allows the K&N filter to hold significantly more dirt per square centimetre of media than the average paper filter. Utilizing these scientific principles, K&N has been able to design an air filter that is very free flowing while also being highly efficient at removing dirt from the air.

Independent Laboratory Test Results

In order to verify that our filters maintain filtration levels necessary to protect your engine, we test our filtering media through independent laboratories. The testing procedure used in the past was the SAE J726 air filter test procedure established by the Society of Automotive Engineers, however this procedure has been superseded by testing procedure ISO 5011.


We have included a detailed example of test results using the SAE J726 procedure. These results are for two individual air filters that each demonstrated among the highest overall filtration level we have achieved with our media.

Independent Lab Flat Filter Test Results

Independent Lab Round Filter Test Results

Paper Air Filters vs K&N Air Filters

To meet minimum filtration standards, paper air filters must be thick and/or the fibres must be tightly compressed and dense. Therefore, paper elements that provide adequate filtration are more restrictive to airflow by design. Additionally, as a paper filter becomes clogged, the pressure inside the filter drops while the atmospheric air pressure (approximately 14.7 psia at sea level) outside the filter remains the same. It's like using your lungs to draw the air out of a plastic milk bottle. When the pressure differential becomes too great, the bottle will collapse. The same thing could happen to your paper filter, although it is unlikely. But what will happen could be just as severe. An excessively high-pressure differential created by a restricted filter can literally pull dirt particles through the paper medium. In other words, the performance of a paper filter – in other words airflow through the filter and its ability to protect your engine - DECREASES near the end of its service life.

Disposable Paper Air Filter Chart

Disposable Paper Air Filter

Pleated wood pulp bonded together.


As dirt builds, passages are blocked and filter must be replaced in approximately 16 000 kilometres.

Irregular passages filter out dirt on a go/no-go basis.

As fibres swell from moisture or oil blow-by vacuum pressure increases and airflow decreases.

Turbulent filtered air.

Typical Foam Air Filter Chart

Typical Foam Air Filter

Lack of surface area hinders airflow and dirt holding capacity. Open cell foam usually saturated with oil.


Dirt builds on outside and blocks the openings.

Higher vacuum pressures distort the cells drawing dirt deeper into the filter.

Airflow is reduced as cells become blocked.

Turbulent filtered air.

The K&N air filter is somewhat more complex. The unique design features multiple layers of oiled cotton fabric which captures the airborne dirt particles. These dirt particles cling to the fibres of the filter and become part of the filtering media. This process, known as depth loading, allows the K&N air filter to retain significantly more dirt per square centimetre than a paper filter. The cotton fabric is sandwiched between pleated aluminium screen. Pleating increases surface area which in turn prolongs service intervals. Pleating exposes substantially more surface area compared to a flat element like foam.

The dirt particles collected on the surface of a K&N element have little effect on airflow during much of its service life because there are no small holes to clog. Particles are stopped by layers of crisscrossed cotton fibres and held in suspension by the oil. As the filter begins to collect debris, an additional form of filter action begins to take place because air must first pass through the dirt particles trapped on the surface. That means a K&N air filter continues to exhibit high airflow throughout the life of the filter while it is accumulating dirt.

At the same time, the airflow for an average paper air filter can decrease dramatically as the paper element gets dirty. So as dirt accumulates, the performance advantages of a K&N air filter can increase. Tests performed by an independent laboratory commonly known as the Frazier Permeability Test have shown that the medium used in K&N air filters flows more than 300% more air than paper air filter medium when compared on a square centimetre per square centimetre basis.


A square centimetre comparison is not directly proportional to the increase you can expect from installing a K&N air filter in replacement of a paper air filter due to the effect of such things as filter size and the number and depth of pleats. However, you can be assured a K&N air filter will provide dramatically more airflow which can enhance engine performance.

Typical Foam Air Filter Chart

K&N vs. Disposable Filter Life


Stop Throwing Away Your Air Filter!

One K&N air filter will last the life of your vehicle. Assuming you drive your vehicle 240 000 kilometres and bought and installed a disposable filter every 24 000 kilometres, you would throw away 10 disposable air filters.

Washable and Reusable Air Filter

All our air filters are washable and reusable. They can be easily cleaned and oiled using our K&N Recharger kits as many times as reasonably necessary. In our testing laboratory, we have washed and re-oiled one K&N Air Filter more than 100 times and it still performed up to specification. We recommend you check your air filter every 48 000 kilometres, however, under most street conditions the filter will not require cleaning until 80 000 kilometres of continuous use. And yes, we've heard the stories of customers who ran their K&N filter for 160 000 kilometres without a cleaning, but we believe cleaning after 80 000 kilometres to be the most beneficial service life without sacrificing airflow.

Stock Replacement Air Filters

We manufacture stock replacement air filters to fit most vehicles. These filters are designed to replace the factory air filter that came with your car. They fit into the factory air box and are engineered to seal tightly with no air leakage. These filters are made with the same filter media used in our racing filters and put a little bit of performance into your everyday driving experience. Our stock replacement filters are backed by our Million Mile Limited Warranty.

Our stock replacement air filters are washable, reusable and can handle all driving conditions. Water will not damage their performance and with proper cleaning, they will last the life of your engine. And of course, they offer high airflow and that means performance! 

Our replacement air filters are designed to provide up to a 4% increase in power and torque. The amount of performance gain varies from vehicle to vehicle based on the overall factory air intake design. The greater the restriction created by the stock paper element, the greater the potential performance gain when you switch to a K&N.


The airflow comparison charts below are for two individual stock replacement air filters sold by K&N. The tests were performed in a dust-free environment on laboratory equipment. Flow comparison results will vary depending on part number, vehicle application and barometric air pressure. For a complete description of the airflow testing method used by K&N to develop the information in the charts below and other published airflow results, see the K&N Airflow Testing procedure here:

Air Flow Comparison Charts

Beyond the Stock Air Filter and Air Box

Addressing the next area of restriction, K&N engineers looked at the vehicle's airbox (the container that houses the air filter) and any hoses or duct work connecting it to the engine. Because the air must first travel through this system before it reaches the carburettor or throttle body, the overall size and shape of the system has a profound effect on airflow. Air, like water, does not like to turn corners nor does it react favourably when confronted by an obstruction such as a sharp bend in a hose or a baffle. In many cases, the air box and/or the hoses and duct work used to create the air filtration system is just as restrictive as the original paper filter element. In some extreme cases, the air box and/or the air delivery system is the greatest source of restriction. The inlet to the air box is a good example. In many instances this opening is one half the cross-sectional area of the throttle body or carburettor opening. It would be like trying to run a marathon while breathing through a straw.

An original-equipment cylindrical air cleaner box covering a throttle body or carburettor is another example. Most often, these round air cleaner housings are sealed to the outside air. The engine must then breathe through a snorkel attached to the perimeter of the housing. At times the snorkel is fed through a network of hoses and scoops.


To directly address the problem, we introduced a line of air intake kits, the most popular of which is our Fuel Injection Performance Kits (better known by the acronym, FIPK). These kits replace both the air box and the restriction. Our FIPKs vary in design because they are application-specific, meaning each kit is engineered to fit a particular make, model and year of vehicle. FIPK air intakes utilize a 360-degree filter design which provides increased surface area to promote airflow.

OEM Style Air Filter Chart

Off-Road and Racing Off-Road

An air filter element becomes an insurance policy when used in off-road applications. Competitors will sacrifice a high-tech engine for a chance to win a race — but to win, they must finish. If the engine ingests too much dirt and debris, it may die an ugly death before the vehicle can cross the finish line.

You might think this would be the perfect application for an inexpensive, throw-away paper filter. But remember, in competition a little extra power can mean the difference between coming in first or finishing second. The air filter now becomes an important part of the performance package. 

K&N air filters are designed to provide minimum restriction long after disposable air filters have begun choking an engine. In other words, due to its characteristics, the restriction of a K&N replacement filter increases at a slower rate when compared to a disposable filter – in other words a  K&N filter will last longer under the same conditions. That's why most off-road competitors choose K&N filters.


In a hypothetical 24-hour off-road race to further the point, a properly sized K&N filter will see the racer through to the end with cfm to spare. The equivalent disposable air filter, on the other hand, will need to be replaced with a fresh element to ensure the engine has an adequate supply of air to complete the course. A K&N will provide excellent filtration without sacrificing airflow for a longer period of time — that's performance with value.


One might consider a circuit or tar oval track as a clean air zone. After all, how much dirt and debris could be hovering above an asphalt track?


Subscribing to that theory, a road racer may elect to forgo an air filter in favour of large volumes of unrestricted air. However, testing the theory using an air filter enclosed in a vented housing should dispel the myth. The filter and housing will trap particles of loose dirt kicked up by other race cars during the heat of battle. Dirt, small stones and pieces of shredded rubber expelled from soft compound racing tyres can be found inside the housing after even a short race. Once a driver, car owner or engine builder realizes just how much debris is thrown around during a normal race, few would expose their expensive engines to unfiltered air in future events.

Down Draft Figure

Whenever possible, performance enthusiasts should install a K&N 360 degree open-element filter. A correctly sized conical or round filter will deliver virtually unrestricted airflow. And, as we have learned, providing the engine with the air it needs promotes optimum performance. In a high-speed application, a K&N filter will straighten the air which counteracts turbulence. 

Straight cut velocity stacks, for example, pose a unique problem. Exposed to the outside air, velocity stacks experience a phenomenon that hinders performance at high speed. We are referring to stacks and air horns that protrude through the bonnet and extend into the air stream so the direction of the air rushing over the car is at a perpendicular angle to the length of the tube.

Air moving rapidly over these stacks create turbulence inside the opening. At high speed, the rushing air tends to create a partial vacuum inside the tube. The condition is counterproductive to airflow. The phenomenon also affects open carburettors. The higher the ground speed, the greater the problem. Vacuum created by the engine is trying to coax air into the cylinders and the high-speed air flowing over the open end of the stack is causing resistance.

Reversion creates other problems. In an automotive application, reversion refers to reversed airflow, or in simpler terms, it's when air in the intake runner reverses direction for a split second. The condition is caused when a burst of pressure escapes into the intake runner from the cylinder during valve overlap.

Reversion creates resonance shock waves inside the tubes which exit the open end of the tube at various rates depending on engine speed. It has also been proven that these shock waves interfere with each other when the stacks are in close proximity.


Installing a free-flowing air filter on top of each stack or over the carburettor air horn eliminates these conditions. How? The solution is simply explained. The filter creates a plenum over the opening. Air entering the filter is slowed, smoothed and straightened. The filter then becomes an endless source of calm, clean air. Shock waves dissipate within the confines of the plenum without interfering with the shock waves emitted from an adjacent stack.

A Better Choice For The Environment

K&N cotton air filters have always been washable and reusable, designed for the life of an engine. If you assume an engine life of 240 000 kilometres in which a disposable air filter must be replaced every 24 000 kilometres, only one K&N air filter would be used during the same period in which 10 disposable air filters were discarded. Considering there are millions of vehicles throughout the world, the volume of disposable air filters that could be eliminated from our landfills is a staggering number.

Air Filter Cartoon Figure

Air Filter Selection

If maximum power is the objective, the size and shape of the air filter element is paramount.

Let's first consider shape. When fitting a conventional round filter on top of the engine; such as a carburettor, central fuel injection or throttle body fuel injection, we have found a large diameter, short filter will flow more air than a small diameter, tall filter. For example, a 25-centimetre diameter filter 5-centimetre tall will flow more air than a 12.5-centimetre diameter filter that is 10 -centimetres tall. Where space permits, the height of the filter should be between 1/5 and 1/4 of its diameter.

The shape of the filter is less important if the application calls for a remote mounted filter, which includes many late model fuel injected models. Typically, these vehicles will use a flat panel filter or a conical or cylindrical shaped filter with a rubber mounting flange designed to be mounted on the end of the inlet hose.

That brings us to size.


Use the formula below to compute the minimum size filter required for your application. The usable portion of the filter is called the EFFECTIVE FILTERING AREA which is determined by multiplying the diameter of the filter times Pi (3.1416) times the height of the air filter in inches, then subtracting .75-inch. We subtract .75-inch to compensate for the rubber seals on each end of the element and the filter material near them since very little airflows through this area.area.

A=(CID X  RPM) / 20,839
A = effective filtering area
CID = cubic inch displacement
RPM = revolutions per minute
at maximum power

Example: A 350 CID Chevy engine with a horsepower peak at 5,500 rpm.
A=(350 X  5500) / 20,839 = 92.4 square inches

If you are sizing a panel filter, multiply the width of the filter area (not the rubber seal) times its length. If you are sizing a round filter, use the following formula to determine the height of the filter.

H=(A / D * 3.14)+0.75
A = effective filtering area
H = height
D = outside diameter of the filter
3.14 = pi
0.75 = the rubber end caps

H=(92.4 / 12 * 3.14)+0.75 = 3.20 inches

Referencing the K&N catalogue shows the proper filter for this application would be an E-1500 which is 9 centimetres tall. Keep in mind, this is the minimum size requirement. To extend the service interval and to provide an even greater volume of air to the engine, install the largest filter that will fit in the space allotted. If the space above the engine is restrictive, perhaps a remote filter arrangement could be used to gain space.


Off-road conditions require added filter area. A filter should be sized 1-1/2 to 2 times larger than normal for any conditions that could be considered severe. In this case, the E-1500 used in our example should be replaced by an E-1120 or an E-1150. For long distance off-road events, two double-size remote mounted filters would be best.

Some important tips about K&N Filterchargers

Service Interval

A K&N Filtercharger is a high-performance air filter, both in terms of airflow and filtration. However, the service interval can vary widely depending on the severity of the driving conditions. The service interval can be from 160 kilometres in a desert-racing environment to 80 000 kilometres for normal street use.


The proper way to determine when an air filter needs service is with an air restriction gauge. Such a device is commonly used on heavy duty trucks and construction equipment. A restriction gauge measures the pressure differential inside and outside the filter and gives the information in different forms of measurement. As the filter collects more and more dirt, the restriction value increases. At a predetermined point or rate of restriction, the filter is serviced. The maximum allowable restriction for a K&N Filtercharger is 38 centimetres of vacuum (water). If the restriction can go higher, the filter media might become so restricted that the element could distort, allowing dirty air to bypass the filter and enter the engine. Conversely, cleaning a filter too often will shorten its serviceable life expectancy. Installing a restriction gauge will optimize service intervals and take the guesswork out of your maintenance schedule.

Service Life

K&N air filters are washable and reusable. They are easily cleaned and oiled using a K&N Recharger kit. They can be cleaned and oiled as many times as reasonably necessary. In our testing laboratory, we have washed and re-oiled a K&N Air Filter more than 100 times and it still performed up to specification.

Limited Warranty

All K&N stock replacement air filters are backed by our famous Million Mile Limited Warranty. See our warranty page for stock replacement and off-road warranty details.


When servicing a K&N filter, take care not to over-oil the element. Besides impeding airflow, excess oil can migrate into the intake system where it can coat electronic sensors, which some OEM's claim may hinder the sensors' operation and result in a repair that will not be covered under warranty. Although K&N disagrees with such claims, as explained in more detail here: , in order to avoid a dispute with an OEM over the denial of a warranty claim, we suggest that you be careful not to over-oil your K&N air filter. Never saturate the filter. If oil drips from the filter, wash it and start over. Use only K&N oil. For example, an E-1500 filter has 235-centimetres of surface area requiring 50 millilitres of oil. Follow oiling instructions included with your filter or refer to the K&N cleaning instructions.

Fit and Finish

Air Filter Installed

When installing a K&N filter, check all gaskets, clamps and seams for damage and/or deterioration. Check the filter's sealing surface. Do not install a filter if the seal shows signs of damage or deterioration. Check for cracks in the air box, particularly at the seams and around the corners. Such defects could cause air leakage around the filter. Also, check to ensure the filter is sealing properly in the air box. A plastic air box can warp from age or continuous heat cycling.

Apply a thin layer of filter grease on both sides of the seal each time the filter is installed. An impression in the grease will indicate a positive seal. Check any lines or hoses connected to the air box or adjoining hoses leading to the engine. Engine oil in the air box, resulting from excessive blow-by, will cause the filter to shrink and possibly lose its seal. Clean any dust or debris out of the air box with a damp cloth, making sure nothing enters the air inlet while the filter is out.


Never start the engine without the filter in place. Use only K&N Air Filter Recharger® to clean your filter. Harsh household cleaners can damage the cotton material and/or the rubber seal. Check the outside of the medium for broken wires, rips or tears. Do not use an element that shows signs of damage or wear. Use K&N Air Filter Sealing Grease on the sealing surface that contacts the air box. Do not use sealing grease on clamp-on type filters. If a filter is especially difficult to install, it will come with an instruction sheet explaining the correct installation procedure. Keep the instruction sheet with the vehicle for reference anytime the filter is removed. It is very important the filter be installed correctly. The filter can be positioned in the base or the lid, whichever is more convenient and makes the installation easier. Do not use excessive force to install a filter.

Vehicle Warranty

Contrary to what you may have heard or read, in the USA, a K&N Filtercharger will not void your vehicle warranty. The Magnuson-Moss Warranty Act, passed in 1975, prohibits a manufacturer from conditioning its warranty of a consumer product upon the consumer using any article or service (other than one provided without charge under the warranty) which is identified by brand, trade or corporate name, unless expressly authorized by the Federal Trade Commission. If the manufacturer does not provide air filters free of charge, they cannot void the vehicle's warranty simply because you have installed an aftermarket air filter.

Letter from K&N Regarding Vehicle Warranty and K&N Replacement Filters


We offer thousands of stock replacement applications, hundreds of universal clamp-on filters and dozens of Fuel Injection Performance Kits. Our goal is to provide the highest quality component at a competitive price. A K&N Filtercharger will be the last filter you will ever buy for your car, bakkie, motorhome, motorcycle, ATV, boat or jet ski - Guaranteed.


Emission Warranties on New Vehicles

THE MYTH: A manufacturer's new-vehicle warranty is automatically voided once an aftermarket part (non-original equipment) is installed.

THE TRUTH: Rarely does the use of aftermarket parts violate a new-vehicle warranty.

THE RULES: Federal law, (the Clean Air Act), requires two emissions warranties: a "defect" warranty and a "performance" warranty.

"Defect" warranties require the vehicle manufacturer to produce a vehicle which, at time of sale, is free of defects that prevent it from meeting required emissions levels for its useful life, as defined in the law.

"Performance" warranties require that vehicle manufacturer make repairs - at no cost to the owner - should a vehicle fail to meet certain levels of emissions performance during the warranty period. This period ranges from 2 years (or 38 000 kilometres) to 5 years (or 80 000 kilometres) for most parts, and up to 8 years (or 130 000 kilometres) for certain emission-controlled parts, specifically, the catalytic converter, the electronics emission-control unit and the on-board diagnostic device (check owner's manual for specifics on your vehicle).

Consumers are protected under a parts self-certification program administered by the Environmental Protection Agency (EPA).

If a parts maker self-certifies its parts under this programme, the vehicle manufacturer cannot void the emissions warranty even if the certified part fails and/or is directly responsible for the emissions warranty claim. In this situation, the vehicle manufacturer must arrange a settlement with the parts manufacturer, but the new vehicle warranty is not voided under the law.


If a parts maker chooses not to self-certify its parts, the only case where a vehicle manufacturer can void the emissions warranty is if a non-certified aftermarket part is proven to be responsible for an emissions claim.