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The K & N... and other aftermarket air-filters,
including the UNI brand.

Copyright 2020, R. Fleischer

The very extensive in-depth FULLY CHARTED laboratory report (from 2004) on the K & N filter has been removed from this article, but I have incorporated LINKS to the charts, etc.   I had, long ago, obtained formal permission to post various charts & tables; but, unfortunately, over the years, the links to the various articles & information & the backup information for my own & others' charts & tables have been rather often either removed from the Internet ...or moved to different addresses or to other sites that I had to do a lot of work to find.  About 70Mb of data was originally posted in charts, etc., here. I do NOT want to post anything I cannot have you easily confirm.   In 2013 I gave up trying to keep up with the internet problems, BUT....  I kept the most pertinent information in the article you are reading.  The copyright to this article you are reading is now mine.

**>>A considerable time after I had removed maybe 80% of the charts, etc., I found the following link! ....which had most of them posted! The following LINK to the 2006 summary commentary was still on-line February 06, 2018, but you may not be able to see the images, which are important:

If that link does not contain all the images from the original, viewable, then the following link should.  Instead, you may want to just go to this link, which is from their archives, and does have all or most of the charts:


K & N was founded by Norm McDonald & Ken Johnson, in 1957.  They were motorcycle racers who met & opened a motorcycle business, called K & N Motorcycles, in Loma Linda, California.  Primarily a service shop, they eventually took on Royal Enfield & then Yamaha.   They hired a teenaged dirt bike rider, Malcolm Smith (yes, the one & the same!!) to work in the shop.   Over the years, they opened other motorcycle shops, selling many brands.  In 1965 Norm & Ken branched out into selling handlebars, fenders, fork braces, etc.  Later they sold insurance, had a beer bar, etc.  In 1966 they started selling the K & N filter, which proved to sell, & still does.  These filters WERE OK for SOME racing purposes, if properly installed (vibration could foam the fuel on carburetors, on the cone model filters, just one of many considerations), and if tuning for them was done properly; this is because the early filters were for racing, and very different from most of their filter uses now.

You must retune, rejet, etc., a BMW Airhead, if you go from the BMW central filter to the individual K & N type (one at each BMW Airhead Boxer carburetor).  You may not like the performance, as the famous BMW mid-range is not at all good with the individual K & N filters; and, you may have fuel foaming and very bad performance too, from the excessive vibration.  This has been particularly bad on the later Airheads.

In 1971 the partners split up, & Norm moved to Tulsa, OK, opening bike dealerships.  Ken kept K & N Engineering.

UNfortunately, K & N filters are used by street riders, & they are not, IN MY OPINION, good for street bikes, letting too much abrasive dirt into the engine.   They also do not flow more air than stock pleated paper filters ....& this air flow & filtering subject was the huge section of laboratory results that I removed the links-to in this article on November 20th, 2013, due to the problems with finding those various links over the years, see top of this article note.  My private files have these charts & tables.  Since 2013, as noted well above, there were other sites hosting the information, but fewer now, but the colorful charts are still  available....AND.....I've posted various charts in the article you are reading, below.  I also, in the below article, explain how I did my own testing, to be sure that the Laboratory testing was accurate for Airheads.  I have the archives link, well above, at the beginning of the article you are reading, to nearly everything in one article, so be sure to study them.

This is from an exchange on the Airheads LIST, edited strictly for clarity:

" ... although the Uni Filter sounded good until they stated that I may have to rejet since their filter provides greater air flow than the stock... Any Hedz have experience with the Uni Filter and rejetting?

Snowbum stated: "BMW filters have a vastly larger filtering area than required".   This means the BMW filter is not strangling your engine.  This means your engine will demand no more air with a supposedly better flowing filter than it does with the BMW filter & this means you don't have to rejet. ".....  (((note:  need for rejetting for filters replacing the stock filter AT THAT STOCK LOCATION is usually NOT necesssary)))

***Snowbum notes that if you modify the intake system and replace it with individual filters at the carburetor intakes, you WILL have to rejet, and it is due to the disturbance of the intake pulses that BMW so carefully designed-for.  This is very different from air flow obstruction.   A rather complete treatment of intake tuning is here:

OK are what I, Snowbum, Mr. Intense Curiosity, personally did, besides all the various filters I inspected on customer bikes (and the insides of their engines)!  Information on formal testing by an industry-accepted laboratory, follows this section.

I DID TESTING.  Changes to engine & rpm would have to be very big changes for the stock BMW filter to be inadequate for air flow, assuming a reasonably clean, to even moderately dirty filter.    My supercharged /5 went for normal mileages between STOCK filter changes; around 20K for me (I rode in somewhat dusty areas often, otherwise the filter replacement would be at a much higher mileage);....and, I did measurements on it in the same way I did much later to my very first R100RT:

I used an old super-sensitive aircraft gauge that is a dual type (separate elements, twin needles).  I also have a differential altitude instrument that is even more sensitive, & reads out in inches of mercury.  ANY of you with ANY type of manometer, gauges, whatever, can pretty much duplicate what I have done.  With one side of my dual gauges connected to the intake side of the filter, the other side connected to the outlet, I could measure the difference across the filter.  If the 'piping' is properly positioned to avoid venturi & coanda, etc., type of effects, this really works rather well.    I have tested the stock filter on my R100RT when brand-new and when it had over 20,000 mostly road, but some off-road dusty miles. On the SAME R100RT, I also tested a brand-new K & N filter that I specifically purchased for measurements.  I tested the K & N before oiling & after oiling it with K & N FilterCharger red fluid.  I also had an old customer's K & N filter that was quite filthy, & tested it.  I also tested it with a dirt bike foam surround I added.   I also tested a BMW 2002 car filter (round type), that mounted on an aluminum plate, sealed to the top of the lower part of the rectangular R100RT airbox.  That filter was noisy, susceptible to water damage (I never made a hat for it), but the results of ALL my testing was clear in my conclusions. Bottom line:   I personally, and for customers, use the BMW stock filter & have measurements to back me up ....which agree with the formal laboratory measurement reporting, linked-to, in this article.

Some conclusions, notes, advice, etc.:

Our Airhead motorcycles, with the original filter replaced by a standard K & N, do not need rejetting. This means that the stock filter, in the stock location, would be replaced by a K & N mounted at the same location, and fitted the same way as stock.  I do NOT wish you to think that I meant changing to K & N filters mounted individually at the carburetor intakes, which DOES create special additional real problems (besides doing LOUSY filtering).

There are K & N filters that are used in racing (or, on someone's racey-looking street bike), that eliminate the stock filtering setup. These put a mesh filter at the intake of each separate carburetor, usually a racy-looking tapered type.  The use of these types of mesh filters at the intake of each carburetor WILL require re-jetting of the carburetors. It is really not the filter being the reason for the re-jetting, but the elimination of the BMW TUNING EFFECTS by greatly shortening the 'effective pipe length' on the carburetor intake SYSTEM.  These types of filters like the K & N (or, bell-mouthed no filter setups, etc.) CAN & WILL cause problems, such as fuel frothing from vibration, on ALL Airheads, but very particularly more-so on some later Airheads with wrong intake rubber-hose characteristics (information is in my Bing CV-2 article), so ALL carburetors, with the K & N filters at the carb intakes, probably should have a support structure added, which is somewhat tricky.  Such K & N filter use CAN & ALWAYS DO upset the famous BMW mid-range torque curve ...and ADDITIONALLY give NO or almost no FILTERING to very fine particles ...thus, such usage WILL cause a lot of extra wear to your engine.  Again, this is specific to the individual (cone-shaped or not) K & N filters at the carburetor intakes themselves. It also applies to ANY type of filter at the carburetor intakes themselves (where the stock single filter above the transmission is NOT being used).   You are changing the intake tract air flow characteristics for the pulse flow of the air.   The two cylinders NO LONGER work together on intake flow!  This means the power curve of the engine is going to be changed. That change will move the power peak UPWARDS IN RPM, and make the torque curve more peaky.  You WILL have to shift gears more often in many instances, with NO improvement in acceleration being likely.

Regarding the stock location of the air filters on your BMW Airhead Motorcycle:

When installing a K & N filter, there is NO advantage other than reducing the cost of buying filters, and even then, it will likely take YEARS and MANY TENS OF THOUSANDS OF MILES before there is ANY cost savings .... and ending up eventually costing you or the purchaser of your motorcycle $$$ for engine damage.  Use of K & N intake air filters WILL cause you to need an top end job much sooner than with the stock filter.

K & N  use does NOT require carburetor jetting changes if replacing the stock pleated paper filter in the stock place on such as BMW Airheads.  When replacing the stock pleated paper filter For FUEL INJECTED bikes, such as the K series, the K & N does not upset the fuel system for several reasons, INCLUDING the engine being automatically adjusted by the fuel injection components, such as the oxygen sensor or mass-airflow sensor, etc. (depending on model/year). The air filter on a K bike is in a fairly decently protected area, same for the R bikes too.

The K & N filters have two characteristics that make them possibly of value, and that is when the bike is ridden in racing or in off-road or other areas with a very high level of fine silt and dirt in the air ....AND you ARE willing to allow abrasive dirt into the engine.  Conditions would be such that there is a need to clean the air filter QUITE OFTEN, without replacement.  The stock pleated paper filter, instead of lasting 25,000 to 50,000 miles, might only last a fraction of that, even with cleaning, in extreme dirty atmospheric situations, THEN, the cleanable K & N, etc, COULD be useful, but keep in mind that the K & N filters will let abrasive dirt into the engine, and, eventually will also, perhaps the same day, require, somehow, cleaning, and cleaning is no longer that of banging the stock pleated air filter on a rock out in the boonies.

The basic K & N filter does a LOUSY job at filtering fine particles until it is quite dirty (yes, even with proper oiling of the K & N filter).   The stock BMW air filter will last a rather long time, many will go 30,000 miles OR even considerably MORE, in average conditions.   Less if the atmosphere is relatively dirty from such as dust storms, etc.  More if the air is cleaner.   Just because the pleated paper stock filter looks a bit dirty does NOT mean it is not still filtering well!  Contrary to information from paper filter manufacturer's, ETC., they OFTEN can be gently blown-through with compressed air, and they will run many more miles.  As much as 60,000 miles has been safely done on BMW stock filters under good conditions.  The BMW stock filters are quite oversize for the air amount needed by the engine.

Many competitors with air cleaners from K & N will not use the K & N sticky red oil on them, and thus the K & N filters are being used to, essentially, filter LARGE dirt particles only ...and engine wear is greatly accelerated (K & N filter oil helps SOME)!  A partial fix for this filtering problem is to install a FOAM surround, or other type of additional filtering medium, be placed on top of or around the filter (depending on the type and mounting). This creates a slight pressure differential, defeating some of the K & N claims, but the outer foam can usually be washed and wrung-dry-enough.   This is unlikely to be as good as pleated paper filters in actual filtering.   The foam surround, even a thick cloth!, can be removed and washed; an advantage in quite dirty area racing.   CONSIDER that racing engines are rebuilt often, more so if they have K & N filters and the racing is in quite dirty conditions.

The bottom-line, in case you do not wish to read further:
The K & N air filters, used as substitutes for the stock location, stock pleated paper filters, will, in my opinion, greatly accelerate engine wear; &, offer NO breathing advantage under normal paved road, nor most off-road conditions as described by me, for common use.  The K & N filter can do a FAIR JOB of filtering; if coated, and more so if there is a proper foam surround over the filter.  'FAIR JOB' is not really good like the stock pleated paper filters!  You WILL need a top end job sooner if using a K & N filter; and, unsurprisingly, the cylinders themselves may wear excessively fast, not just rings, valve guides, etc.

My original article had the results of ISO 5011 testing of many types of air filters. The testing was independently performed under controlled conditions using a $285,000 machine at Testand Corp of Rhode Island (manufacturer of the machine). Arlen Spicer organized the test. Ken, an employee of Testand, offered to perform the tests at no charge. (These tests typically cost ~ $1700.00 per filter and would be MUCH MORE now).  Ken shared Arlen's interest in performing an accurate unbiased test of different types & brands of engine air filters. The filters used in the test were purchased retail & donated by Arlen & others. The detailed reports from the testing were compiled & originally were presented as part of this article you are reading.

ISO 5011 Test:
The ISO 5011 Standard (formerly SAE J726) defines a precise filter test using precision measurements under controlled conditions. Temperature & humidity of the test dust and air used in the test are strictly monitored & controlled. As Arlen learned in attempting his own tests, there are many variables that can adversely affect filter test results.  A small temperature change or a small change in humidity can cause the mass of a paper filter to change by several grams. To obtain an accurate measure of filter efficiency, it’s critical to know the EXACT amount of test dust being fed into the filter during the test.  By following the ISO 5011 standard, a filter tested in Germany can be compared directly compared to another filter tested 5 years later in Rhode Island. The ISO 5011 filter test data for each filter was contained in two test reports; Capacity-Efficiency and Flow Restriction.

Capacity and Efficiency:
The Capacity and Efficiency test report presented the test results of feeding an initially clean filter with a PTI Course Test Dust (dirt) at a constant rate and airflow. The course test dust has a specific distribution of particle sizes ranging from less than 2.5 microns to greater than 80 microns (see table below). Every filter is initially tested at 350 CFM and the Initial Restrictor differential pressure across the filter is recorded in IN-H2 0 (Inches of Water). The filter is then tested by feeding test dust at a nominal rate of 9.8 grams per minute with a constant airflow of 350 CFM. The test is continued until the flow restriction exceeds the Initial Restriction + 10 IN-H2 0. At this point the test is terminated and the amount dust passed through the filter - Accumulative Gain - is measured. Dirt passing through the filter is captured in the Test Station’s Post Filter. The exact amount of dirt passed is determined by measuring the before and after weight of the Post Filter. Similarly, the amount of dirt retained by the Filter under test,  Accumulative Capacity, is measured by taking the difference between the before and after weights of the Filter. From these results the overall % Efficiency of the filter is calculated.  This test also indicates how long a Filter will last before replacement is required (or cleaning for reusable filters).

Flow Restriction:
This report presented flow restriction of a clean filter resulting from an increasing airflow. The differential pressure restriction across the filter is reported in inches of water (IN H2O) versus Air Flow in cubic feet per minute CFM.

Filter Efficiency:
Filter efficiency is a measure of the filters overall ability to capture dirt.

Accumulative Capacity:
This is a measure of dirt holding/loading capacity before reaching the maximum restriction limit - Initial Restriction + 10 IN-H20.

Accumulative Gain:
The total amount of dirt that passed through the filter during the test.

Initial Restriction:
The Filter under test's resistance to flow at 350 CFM.

Dirt Passed Versus Total Test Time:
The graph showed each the duration of each filter's test versus dirt passed (Accumulative Gain).

Dust Loading:
The dust loading curves showed graphically how each filter responded to a constant 9.8 gms/min dust flow before reaching the maximum restriction limit.  It's interesting to note the shape of these Dust Loading Curves. Some filters had near linear responses until reaching maximum restriction. Restriction for these filters increased at a constant rate versus the 9.8 gms/min dust feed rate. The other filters, most notably the oiled reusable types, had an exponential loading response before reaching maximum restriction.  These filters had a lower initial restriction, but they became exponentially more restrictive under a constant flow of dirt.

Restriction to Flow:
The Restriction to Flow curves graphically showed how each "clean" filter responded to a steadily increasing flow of air up to 350 CFM.

(Arlen) SPICER wrote (September 2004):

"Now that I am not doing the tests & my objectivity is not necessary, let me explain my motivation. The reason I started this crusade was that I was seeing people spend a lot of money on aftermarket filters based on the word of a salesperson or based on the misleading, incomplete or outright deceiving information printed on boxes and in sales literature.  Gentlemen and Ladies, Marketing and the lure of profit is VERY POWERFUL!  It is amazing how many people believe that better airflow = more power! Unless you have modifications out the wazoo, a more porous filter will just dirty your oil! (dirt will DAMAGE your engine, not just dirty your oil.)  Some will say " I have used aftermarket brand X for XXX # years with no problems. The PROBLEM is you spent a chunk of ching on a product that not only DID NOT increase your horsepower, but also let in a lot of dirt while doing it! Now how much is a lot?  ANY MORE THAN NECESSARY is TOO MUCH!

Others are persuaded by the claims of aftermarket manufacturers that their filters filter dirt "better than any other filter on the market." Sounds very enticing. To small timers like you and me, spending $1500 to test a filter sounds like a lot. But if you were a filter manufacturer and you believed your filter could filter dirt better than any other media on the market, wouldn't you want to prove it? Guess what. Test your filter vs. the OE paper. It will cost you $3000 and for that price you will have the data that you can use in your advertisements. Your investment will be returned a thousand fold! EASIER than shooting fish in a barrel! So why don't these manufacturers do this? Hmmm? Probably not because they would feel guilty about taking more market share.

Now I am not saying that ALL aftermarket filters are useless. A paper filter does not do well if directly wetted or muddy. It may collapse. This is why many off-road filters are foam. It is a compromise between filtering efficiency and protection from a collapsed filter. Now how many of our trucks collapse their filters from mud and water? However, if a filter is using "better airflow" as their marketing tool, remember this....Does it flow better? At very high airflow volumes, probably. BUT, ..... CAN'T flow that much air unless super-modified, so what is the point? The stock filter will flow MORE THAN ENOUGH AIR to give you ALL THE HORSEPOWER the engine has to give. And this remains true until the filter is dirty enough to trip the air filter life indicator. At that point performance will decline somewhat. Replace the filter and get on with it.

Hopefully the results of these tests will do 2 things. Shed some light on the misleading marketing claims of some aftermarket manufacturers and/or give us new insight on products already on the market that are superior to our OE filter. I stand for truth and will eat my words publicly if my statements prove wrong. I appreciate all of the help and support that you members have offered in this project. It would simply be impossible without your help.

A huge thanks to Ken at Testand for his willingness to take on this project. I would be spinning my wheels from here to eternity without his help... SPICER"


The full article was previously numbered 12B, but was somehow lost from the Technical Articles LIST Page LINK and LISTING, a long time ago. I recovered it, renamed it 12C, and made a new place for it on the articles list.  I had continuing labor intensive problems maintaining this article, hence the 11/20/2013 changes:

11/20/2013:  Completely revise and simplify.  Keep much from some years ago, such as 2009.  Remove all dead linking charts and databases.  Remove all prior revision notes. In particular remove this link:  which no longer links directly to that article. Added at the top of my revised article is this:    which contains many of the more pertinent parts/charts of the original article.
12/24/2015:  Update slightly, meta-code area for W3C. Check links too.  Narrow the article to better accommodate small screens. Increase font size.
03/05/2016:  Update meta-codes, layout, clean-up.
08/11/2016:  Extensive update to metacodes, scripts, H.L.  Update to modern HTML A/R.  Minor other changes to layout. NO technical information changes.
02/06/2018:  Go through entire article and clean up excessive html, colors, fonts, layout.  Add 10pxl margins. Make sure the nicoclub ARCHIVES still contain the charts, and add notes in that section about nicoclub links.

Copyright 2020, R. Fleischer

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Last check/edit: Monday, December 07, 2020