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Equipping the Bike - what's the best gear? Anything to do with the bikes equipment, saddlebags, etc. Questions on repairs and maintenance of the bike itself belong in the Brand Specific Tech Forums.
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Old 21 Jul 2004
Contributing Member
Veteran HUBBer
Join Date: Apr 2002
Location: Slough, UK
Posts: 153
Air filters - the case for oiled foam

This is a response to a post in the XT tech forum, where I asked for part number for a K&N filter for my bike. Brian Parker emailed this to me and asked me to post it:

you don't want a K&N.

i used them on all my cars. when i couldn't get one, i'd get a BMC.

i just bought an R100GS. my friend warned me not to use a K&N. i
wasn't sure whether or not to believe him, but i did realise that
off-road bikes all have oiled foam filters, _not_ oiled gauze. i
searched the net.

don't do it. read this stuff and you'll see.


AUSTRALIA (formerly distributors of UNIfilter, now Australian company;
much better website; exceelent; good helpful people / good service;
technically superior (read about how they make the filters):


Uni High Performance Air Filters Facts

Open cell industrial grade filter foam represents the latest advance in
air filtration technology. This section describes how and why it works
so well when compared to the other three types also in common use. An
optimum air filtering system is very inexpensive insurance against
untimely replacement of rings, valves, bearings, and fouled spark plugs.
These items represent very significant maintenance costs. Additional
benefits are provided by higher performance, better gas mileage, and
lower emission products.

With an engine operating at the optimum air-fuel ratio of 15 or 16 to 1
(air volume to gasoline vapor volume), it normally means that 10,000 to
15,000 "gallons" of air are sucked into the cylinders for every gallon
of liquid gasoline in the tank. The exact volume of air, of course,
depends on the engine displacement, RPM, and miles per gallon. For
example, a 2800cc engine turning 2600 RPM at 55 MPH got 26 miles per
gallon, and used 23,000 gallons of air per gallon of gasoline in the
tank. You can imagine what 23,000 gallons of air and dust going into
your engine every half hour would do if you had no air filtration at

Paper Filtration

Pleated paper elements are used by more vehicles than any other type for
these reasons:

1. They are the least expensive for the manufacturer to install as
original equipment from the factory.

2. For the largest percentage of operating conditions (street and
highway driving) they perform well enough to satisfy the owner/driver at
minimal acceptable levels.

3. The elements are dry, easy to handle, and convenient to replace.

4. Since they are "throw away" items (cannot be cleaned for reuse), they
represent a large and profitable part of the replacement market, while
occupying our landfills.

Paper filters are a stacked matting of fibers creating a random weave
approximately 1/2mm thick, and rely on the "screening" effect to stop
dirt particles. Airflow per square inch is so poor that the paper must
be pleated using many feet of material to make a filter. All dirt or
dust particles must be caught on the surface or not at all. Each time a
particle is caught, it stops up a hole. From the moment you start your
engine, you have a rapidly decreasing air flow rate. Paper also has two
other big drawbacks for off-road use. Any moisture reaching the element
causes the fibers to swell, reducing airflow even more. Another is the
possibility of rupture. Paper is not a very strong material, especially
where it is creased to form each pleat. Intake manifold backfires, or
cleaning attempts with compressed air, usually rupture the paper leaving
the filter ineffective.

Pleated Gauze or Fabric Filtration

This is another screen type that is only 1mm thick. If the dirt is not
stopped on the surface, it is not stopped at all. These filters are sold
on the pretense that they maintain an oil curtain for the air to pass
through, thereby catching all dirt particles. It is impossible to
maintain an oil curtain. The oil soaks the threads of the gauze or
cloth, but does not span the openings; otherwise, the air could not get
through. The dirt particles that do hit the threads have a good chance
of being caught; the others simply go through. The reason the filter
does not look dirty on the inside is because the dirt went into the
engine. You can easily demonstrate this fact yourself by coating the
inside of your housing or carb throat with a thin layer of grease to
trap some of the dirt not caught by the filter or you can place a foam
filter inside the gauze element to prove the same thing.

The one advantage that this type of element has over paper is greatly
reduced airflow restriction; however, poor filtration efficiency is the
price you pay. When dirt builds up, filtering action improves, but now
the airflow is poor like paper elements.

Open Cell Filter Foam

The development of this special foam represented a major advancement in
air filtration technology. Foam air filters now combine great airflow
capability, huge dust holding capacity, and very high filtration
efficiency for extremely small particles.

Fully reticulated (open pore) foam is a honeycomb of tiny, interlocking
cells of uniform size, which create an impossible journey for dirt
particles since there are no straight-through passageways. Each
passageway (16 to 25mm long) is like hundreds of very small
centrifugal/oil bath filters connected one to another. In this way, foam
traps and holds the particles throughout the entire volume of foam. This
is why they are referred to as "full depth" filters in contrast to the
paper or gauze elements, which are screens, or "surface type" filters.
The cell strands stop the dirt, while the oil film holds the dirt like
fly paper until removed for cleaning.

The principle of how foam air filters work is simple: "Open Cell"
Polyurethane Foam is wetted with specially developed filter oil. The
"sticky" filter oil is suspended in the path of the dirty air on the
strands of the web-like cell structure of the foam. This makes it
impossible for dirt to pass through the depth of the filter without
sticking to the strands. As the outer strands become loaded with dirt
particles, the wetted strands down stream start trapping dirt, allowing
the entire thickness to be utilized. This prevents surface loading or
air restriction for 80% of the service life of the air filter element.
When the filter is sufficiently dirty, it can be easily washed,
re-oiled, and re-used.

[This message has been edited by Barry Johnson (edited 21 July 2004).]
Yamaha XT600Z 3AJ Tenere
Honda ST1100 Pan European Police
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Old 22 Jul 2004
Contributing Member
New on the HUBB
Join Date: Feb 2004
Location: Price, UT, USA
Posts: 13
You definetly do not want a K&N for your application. For race engines that get rebuilt frequently, they are fine. For street or travel usage, they will cost you most of your engine life. They simply do not take out the fine particles that spell death to an engine. Talk to mechanics at the shops. They love to sell K&N so they get the rebuild work. If you know someone that has a K&N installed, have them remove it and run your finger around the inside top bore of the carburetor and see the amount of grit that is getting through the filter. Then make your own decision.

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Old 31 Jul 2004
Registered Users
Veteran HUBBer
Join Date: Feb 2004
Location: Toulouse, France
Posts: 231
Totally agrees,
Don't buy K&N, better put all those paper bills directly on the box..they'll keep more dirt !
Another comparison would be; buying a K&N is like buying an expensive transparent bikini to your girlfriend for her to walk on the beach with..you don't want that either!
What do you think of exhaust pipes, are they an improvement to the engine or are the OEM guys reason there also?
I heard that an exhaust to "open" will raise the temperature on exhaust valves and they are made to stand a maximum, after that they'll deteriorate.
In the XT600 forum there is Phil from Australia having problems in Greece with his 660; oil consumption, I would guess the guilty is the exhaust, but I could be wrong.


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Old 2 Oct 2004
Registered Users
New on the HUBB
Join Date: Oct 2004
Location: Lisboa, Portugal
Posts: 2
The exhaust valves get too hot, because the extra flow allowed by the open exhaust will lean the mix.... you have to re-jet or you will "burn" your engine...
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Old 26 Jun 2005
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New on the HUBB
Join Date: Feb 2005
Location: scotland
Posts: 11
hey all

i got my bike serviced about 40miles ago and when it came back it was struggling to get over 5000rpm no power at all turned out when they washed the foam filter of which one sides foam the others furry material anywaya, the filter oil was buil up on the furry side and was stopping the air flow since washing, has been fine, never had any oil on it untill that time. on the k&n front avoid at all cost i put one on a car big mistake

cya steve
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