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Nothing will start a good internet discussion like asking about K&N Air
Filters. My take is simple - The benefits are almost zero, the risks are greater than zero.
The K&N "theory" is based on the implication that OEM air filter systems
are especially restrictive. Why do you believe this to be the case? Do you think vehicle manufacturers deliberately design
restrictive intake systems? There are compromises involved in the design of air filtration system, mostly to reduce intake
noise, but these are probably minor. Installing a K&N replacement filter element will not affect many of these these restrictions.
Completely replacing the intake system with a K&N intake system may eliminate some of the restrictions inherent in the
stock system. However, the replacement K&N system may be significantly louder and it will draw in warm underhood
air instead of cooler outside air. For an otherwise stock engine, I doubt if the improvement would be detectable by anyone
without a dyno (except for the noise).
I have links to a lot of references at the bottom of this page. After you
read all these references, you may be more confused rather than less. Claims and counter claims without much supporting evidence
are the norm in this debate. Independent verifiable evaluations are hard to find. You will need to decide
for yourself if the risks are worth the rewards. I have drawn my own conclusions.
My Beliefs:
- A good air filter is key to maximizing the life of your engine. Nothing can wear
out an engine faster than dust entering the engine through the air intake. The incoming air is the major source of oil contamination.
Stopping this contamination at the air filter will go a long way towards protecting your engine against wear.
- If you devise the right test you can "demonstrate" that a properly oiled and
installed K&N Filter will flow more air than a similarly sized paper filter. Whether or not the difference in flow restriction
is great enough to make a noticeable difference is debatable (endlessly). At best you will only see a difference at WOT. I
even doubt this, but it is hard to prove it is not true.
- Claims of greatly improved fuel mileage for K&N Filters are bogus.
- Over oiling a K&N Filter can screw-up your MAF sensor.
- K&N Filters pass more dirt than paper filters, particularly fine particles.
You can debate whether the amount is significant or whether it is bad for your engine. I can't see how it could be good.
- "Savings" based on reusing the K&N Filters are highly exaggerated. A new
K&N Filter for a 2003 Ford Expedition costs $47.50 from Performance Products. The cleaning/oiling kit, good
for 3 cleanings, is $10.50. K&N claims you only need to clean the filter every 50,000 miles. For normal conditions, Ford
specifies a 30,000 mile replacement interval for paper filters. In order to compare costs, I am going to assume you change/clean
the filters at the manufacturer's recommended interval (replace the Ford filter at 30,000 miles or clean and re-oil the K&N
every 50,000 miles). A new paper filter will cost you $9.49 at CarParts.Com (sometimes less at Wal*Mart). So in 150,000 miles
you will spend 5 x $9.49 = $47.45 on paper filters or $47.50 + $10.50 = $58.00 on K&N Filters. So if you follow the
manufacturer's recommended replacement/cleaning intervals, using a K&N will cost you over $11 more than simply changing
the standard paper filter. This does not even include the extra time you are spending cleaning the K&N or make an allowance
for the possibility of damaging a MAF sensor becasue excessive oil was applied to the K&N filter (I know we
would all be careful).
In summary:
For K&N
- possible minor improvement in WOT performance (little independently
verified proof available, but lots of seat of the pants feel reported).
- possible good feeling because you are not throwing 5 paper filters in the
trash over 150,000 miles.
Against K&N
- increased filter cost
- increased maintenance time
- lets more dirt through.
Different people have different ideas about the relative merits of these factors.
I spend very little time at WOT, so the possible improvement in performance at WOT is not very important to me. I spend
a significant portion of time driving on dusty roads, so the likelihood that the K&N Filter lets more dirt
through is a huge negative. For someone who spends most of their time driving on pavement at WOT, maybe the K&N would
make sense, particularly if they were planning on trading vehicles before 100,000 miles.
These are just my thoughts on the K&N debate. I know others have different
opinions. In the end, the person paying the bills has the right to reach their own conclusion.
K&N Filter Size Calculation Method
I will repeat those formulas here -
Pleated K&N filter material will flow 6.03 cfm of air per square
inch. By comparison, a single square inch of the highest flowing paper will allow 4.95 cfm of air to pass and the freest
flowing foam will flow 4.38 cfm. Use the formula below to compute the minimum size filter required for your particular 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 air flows through
this area.
A = (CID x RPM)/20,839
where:
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 Sq.In.
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
where:
A = effective filtering area
H = height
D = outside
diameter of the filter
3.14 = pi
.75 = the rubber end caps
Example:
H = (92.4/12*3.14) + 0.75 = 3.2 Inches
I'll now redo the math for a 5.4 L (330 cid) Expedition Engine. I'll "derate"
the answer by assuming a paper filter (using K&N's value for the paper filter).
A = [(330 X 5500)/20,839]*[6.03/4.95] = 106.1 sq.in. (the 6.03/4.95 factor
is to derate the number to a good paper filter)
The stock filter in an Expedition is conical, with an average diameter 5.25
inches and a height of 7.8 inches (see http://www.pureoil.com/smartlink/?partnum=A34878). Using the K&N formulas, for maximum performance the paper Expedition filter height needs to be:
H = (106.1/5.25*3.14) + 0.75 = 7.2 Inches
So, the Expedition paper filter is already bigger than it needs to be according
to K&N's own formulas.
So even K&N is saying that there is no "significant" performance
advantage in replacing the stock filter with a K&N filter since the stock filter is larger than K&N's own calculations
indicate is necessary for the application.
By the way, I believe the K&N formulas are absurdly pessimistic.
A 350 cid engine at 5000 rpm could only flow around 560 cfm if was operating at 100% volumetric efficiency (i.e. each cylinder
was filled with a complete charge of air at atmospheric pressure on each intake stroke). I think a volumetric efficiency of
60% would be outstanding for an unmodified street engine. A volumetric efficiency of 60% would mean that a 350 cid engine
would require around 340 cfm of air at 5500 rpm. K&N's sample calculation appears to imply that it would require 560 cfm.
So their calculation is already overstating the required flow rate by almost 65%. A standard Expedition filter is at
least as large as it needs to be using K&N's own information which calculates a filter size larger than it needs
to be.
How Often Should Air Filters Be Changed To Maximize Engine Life?
While researching filter I came across the following interesting statement based
on an SAE paper:
"The SAE paper by Marty Barris (SAE Technical Paper No.
952557 titled: ‘Total FiltrationTM: The Influence of Filter Selection on Engine Wear, Emissions and Performance’
) offers two important conclusions:
- "The level of ambient dust concentration can easily overwhelm other factors
in terms of influence on engine wear. It is therefore important to match the air intake filtration system design to the
anticipated ambient exposure. And here's an interesting one-:
- "Too frequent air filter change intervals can double engine wear rate, especially
if changed within the first 30% of the air filter’s life.
"The practice of removing an air filter element to blow it clean
is plain stupid. This malpractice, which can lead to engine wear rates being trebled, still occurs on the false economy basis
of trying to extend the service life of the air filter element. A well meaning but badly trained technician thinks he is being
thorough in servicing the air filter, or replacing it prior to reaching the optimum life of the element. This happens in workshops
without supervisors even being aware of it."
A site visitor pointed me towards information in a reference cited below that has a more detailed explanation
of this phenomenon. The following is from the "Nanofibers in Filtration Applications in Transportation"
listed link below:
"For most engine air filters, the function of the
filter and the corresponding engine wear are based largely on dust cake filtration. The formation of a dust cake on the filter
media changes many important factors in the function of a filter media, including filtration
efficiency and pore size distribution. Engine wear rate has been shown to be significant during the early portion of an air
filter’s design life (Figure 9 in http://www.donaldson.com/en/filtermedia/support/datalibrary/052024.pdf).
"While vehicle manufacturers are specifying longer life air
cleaners and longer maintenance intervals, the actual service interval for air filters varies widely depending on the sophistication
of the vehicle end-user and the maintenance staff. The most sophisticated end-users are large fleet operators with several
thousand vehicles. Air cleaner maintenance is typically performed based on filter restriction using a restriction indicator
on the vehicle. Maintenance decisions are largely made based on economic factors, and trained personnel provide oversight
on maintenance intervals and filter selection. The decisions are based on a complex interaction of factors, including filter
design and efficiency, filter maintenance interval, filter and service cost, and engine wear rate.
....
"Consumers must also make maintenance decisions about the air filters in their cars. This user group is relatively
unsophisticated in their knowledge of filter maintenance and function. Air filters are perhaps the most easily serviced but
least understood parts on a vehicle. Other common replacement parts on a vehicle such as tires, wiper blades, oil and oil
filters work best when new. However, this is not the case with conventional air filters. Since most air filters rely on the
formation of a dust cake to improve the performance of a filter media, over-servicing can lead to dire consequences from inadequate
engine protection.
"Air filter over-servicing is common for light vehicles. Light vehicles are generally not equipped with filter
restriction indicators. Air filters are often inspected by maintenance personnel during oil changes. In spite of the typical
manufacturer’s recommendation of at least a 30,000-mile change interval, it is common for dealers, service stations
and quick lube businesses to recommend more frequent air filter changes. Given the frequency of oil changes and air filter replacements at quick lubes, it is expected many light vehicle
air filters are changed much more frequently than manufacturers recommend.
"While this over-servicing phenomenon is frustrating to those who understand filter media performance, the filter
industry has not sufficiently educated customers how air filters function. Not surprisingly, filter manufacturers, distributors,
dealers, service stations, and quick lubes have economic incentives to change and sell more air filters. It is also understandable
that consumers believe air filters work best when they are new (like other parts) and have a clean appearance."
I appreciate the pointer to this interesting content.
Will a Clean Air Filter Improve Fuel Economy?
K&N (and others) often cite the EPA's "Gas Mileage Tips" (see http://www.fueleconomy.gov/feg/maintain.shtml ) when suggesting that a restrictive air filter can significantly reduce fuel mileage. In fact, the EPA site says "Replacing
a clogged air filter can improve your car's gas mileage by as much as 10 percent. Your car's air filter keeps impurities from
damaging the inside of your engine. Not only will replacing a dirty air filter save gas, it will protect your engine." I don't
agree that either claim is true as stated. The information above regarding too frequent changing of air filters partially
refutes the claim that changing a filter will necessarily "protect your engine." And if you care to investigate the fuel savings
claim, you should click the "Data Sources” link at the bottom of the page. If you do this you will be presented with
the following disclaimer:
"Air filter savings based on Organization for Economic Co-operation and Development
(OECD). 1981. Automobile Fuel Consumption in Actual Traffic Conditions. Paris, France. These tests were performed
before the introduction of computer-controlled, fuel-injection engines. DOE is currently studying the fuel economy effects
of clogged air filters on more modern engines."
So what we discover is that the gas saving related to clean air filters is
based on a 28 year old study from a time when carbureted engines were the norm. This has little or no relationship to modern
fuel injected engines. In other words, the claim is worthless. Even Consumer Reports acknowledges that a dirty air filter
has little effect on fuel economy. The following quote is from the Consumer Reports web site:
"MYTH BUSTERS
"A dirty air filter. 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,
but the Camry accelerated much more slowly with a dirty filter."
Links Related to Air Filters
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