Air Filter Facts

Air Filter Facts

Precision Airflow Engineering

The primary function of an air filter is to deliver both high airflow and excellent protection from contaminants. K&N air filters are designed to provide an increase in airflow—up to 50% more airflow than a disposable filter—and allow for improved engine performance and protection in the form of greater horsepower, torque, and contaminant capture. For five decades, K&N has been perfecting the design of High-Flow Air Filters™—offering your vehicle advanced protection and exceptional performance.

The Science Behind K&N Air Filters

Depth Loading


An airstream will always take the shortest path possible, and as air is forced around the fibers of K&N filter media, particles in the air stream make contact with the filter fibers and are captured—a principle known as interception. Particles are then held in place by oil or other filter media properties.

Depth Loading


Larger or heavier particles experience a phenomenon known as impaction, in which inertia or particle momentum causes a particle to deviate from the flow path and collide directly with filter fibers.

Depth Loading


Small particles are highly affected by forces within the air stream, such as velocity changes, pressure changes, or turbulence. As a result, the behavior of extremely small particles is typically random and chaotic. These particles do not follow the air stream, and their erratic motion causes them to collide with the filter’s fibers. This particular phenomenon—diffusion—enables a K&N air filter to capture contaminant particles that are much smaller than the openings within the filtration media.

Depth Loading

Depth Loading

K&N air filters also collect particulate matter in a manner very different from disposable filters. A paper filter exhibits a principle called “surface loading”—dust collects only on the surface of the media. In contrast, K&N filters utilize “depth loading”, in which multiple layers of cotton fibers provide particle retention on multiple levels, and the media also features pleats to increase surface area over disposable filters. As a result, K&N filters are able to hold significantly more dirt and particulate matter per square inch of media over an average paper filter.

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Frequently Asked Questions

Why is airflow critical to engine performance?

Under normal circumstances, an increase in airflow to the engine also increases the efficiency of the combustion process, creating more horsepower and torque. K&N High-Flow Air Filters™ are designed to increase both horsepower and throttle response by improving airflow.

What’s the difference between K&N filters and other filters that look similar?

Both airflow and filtration efficiency are critical to engine performance. Some companies design filters that provide high airflow by reducing filtration efficiency to unacceptable levels, which risks engine damage—unlike K&N filters, which are designed to provide high airflow while maintaining exceptional levels of filtration. As the "look" of K&N filters has become popular, many companies have begun offering products that mimic the appearance of K&N filters, but do not provide the same level of quality and protection.

How efficient are K&N filters at trapping contaminants?

Laboratory tests for dust capacity demonstrate that K&N air filters generally achieve overall filtration efficiency in the range of 96% - 99%.

What are the filtration requirements for my vehicle?

Most vehicle owner’s manuals do not identify dirt retention requirements for air filters; in fact, few air filter manufacturers publish any information as to the filtration efficiency of their stock filters. Studies have shown that most engine wear attributed to contaminants is caused by particles 10-20 microns in size, and K&N filters provide exceptional filtration for those particles.

How is airflow efficiency measured?

The ability of an air filter to protect an engine is generally measured in accordance with testing procedure ISO 5011. Filter designs are subjected to a procedure involving 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 results of the test procedure are measured by a specific air filtration efficiency number. K&N filters are designed to provide exceptional airflow while simultaneously targeting an overall filtration efficiency of 98%.

Are K&N air filters better for the environment?

K&N High-Flow Air Filters™ are designed to last for the life of your vehicle. If you assume an engine life of 175,000 miles—and replace your disposable air filter every 15,000 miles—only one K&N air filter will be used during the same period in which eleven disposable air filters are discarded. High-Flow Air Filters™ also feature a cotton filtration media; cotton is a renewable, sustainable resource.

Why not use a disposable paper air filter?

Most dry paper air filters function on a go/no go basis, in which dirt particles that are larger than the openings in the filter media are trapped, while particles that are smaller than the openings pass through. For this reason, most disposable paper filters restrict airflow significantly, as the openings in the filter media have to be extremely small to filter efficiently.

To meet minimum filtration standards, paper air filters must be thick and/or the fibers must be tightly compressed and dense. Therefore, paper elements that provide adequate filtration are more restrictive to airflow by design. As a paper filter becomes more and more clogged, the pressure inside the filter drops while the atmospheric air pressure outside the filter (approximately 14.7 psi at sea level) remains the same. An excessively high-pressure differential created by a disposable filter can begin to pull dirt particles through the paper filter media. The performance of a paper filter typically decreases substantially near the end of its service interval.

A few facts on paper disposable filters:

  • Disposable paper filters are typically composed of bonded wood pulp
  • Irregular passages filter out dirt on a go/no go basis—requiring very small openings that reduce airflow
  • As dirt builds, air passages are clogged
  • Must be replaced approximately every 15,000-20,000 miles
  • As fibers swell from moisture or oil blow-by, vacuum pressure increases and airflow decreases
Why not use a disposable foam filter?
  • Lack of surface area hinders airflow and contaminant holding capacity
  • Open cell foam is usually saturated with oil
  • Dirt builds on the outside and blocks openings
  • Higher vacuum pressures can distort the cells, drawing dirt deeper into the filter
  • Airflow is reduced as cells become blocked
How many times can I wash and reuse my K&N High-Flow Air Filter™?

K&N High-Flow Air Filters™ can be cleaned and re-oiled using K&N Recharger kits as often as is reasonably necessary. Under laboratory conditions, one K&N air filter was washed and re-oiled more than 100 times, and still provided a high level of engine protection and airflow. K&N High-Flow Air Filters™ require servicing once every 50,000 miles under normal highway driving conditions.

How often should I service my filter?

A K&N High-Flow Air Filter™ is a high-performance air filter, both in terms of airflow and filtration. However, the service interval can vary greatly depending on the severity of the driving conditions—from 100 miles in a desert-racing environment to 50,000 miles for normal highway driving conditions. The proper way to determine when an air filter needs servicing is by using an air restriction gauge, which measures the pressure differential inside and outside the filter. As the filter collects dirt, the restriction value increases. Installing a restriction gauge will optimize service intervals and take the guess work out of your maintenance schedule.

Will a K&N® filter cause my vehicle’s mass air sensor to fail?

If somehow K&N air filter oil were to make contact with a mass air sensor, the oil would not cause the sensor to fail. We have intentionally coated mass air sensors with our filter oil, and monitored the sensors' output when installed in daily driven vehicles. These sensors continued to function normally, even after being sprayed and submerged in K&N air filter oil.

Is it possible to over-oil a High-Flow Air Filter™?

When servicing your High-Flow Air Filter, be sure to follow the oiling directions carefully. Excess oil can migrate into the intake system and coat electronic sensors, which some manufacturers claim may hinder a sensor’s operation and result in a repair not covered under warranty. Use only K&N-recommended oil, and never saturate the filter. If oil drips from the filter, wash it and start over. Follow the oiling instructions included with your filter, or refer to the K&N air filter cleaning Instructions on our website.

Can I use anything other than K&N® cleaner and oil?

We do not recommend using other detergents or solvents to clean your air filter, or other types of oil. Products other than those manufactured by K&N have not been tested and may deteriorate the performance and filtering ability of your air filter.

Are K&N® filters emissions legal?

Yes, K&N replacement air filters are emissions legal in the United States.

Air Filter Selection


When fitting a conventional round filter on top of the engine—such as a carburetor, central fuel injector, or throttle body fuel injector—a large diameter, short filter will provide more airflow than a small diameter, tall filter. For example, a 10-inch diameter, 2-inch tall filter will provide more airflow than a 5-inch diameter, 4-inch tall filter. 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 vehicles. Typically, these vehicles will use a flat panel filter, or a conical/cylindrical shaped filter with a rubber mounting flange designed to be mounted on the end of the inlet hose.


Use the formula below to compute the minimum size filter required for your particular application. The usable portion of the filter—the effective filtering area—is calculated by multiplying the diameter of the filter times pi (3.1416), multiplying by the height of the air filter in inches, and 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 air flows through this area.

Panel Filter Equation

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

Round Filter Equation

Referencing the K&N application search shows the proper filter for this application would be an E-1500, which is 3.5 inches 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.

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.