Re: [MV] Air Filter

Alan Bowes (
Wed, 31 Dec 1997 22:44:25 -0700

Gil Huguley wrote:
> I too, am not particularly fond of the oil bath type air cleaner.
> They are just plain ugly, they look low-tech, and they are difficult
> to keep from being greasy. However, I have been told by several
> mechanics and parts men that the oil bath type is actually superior in
> filtration ability.
> I would like to hear some opinions from list members on this issue.

Hello Gil,

As to filtration ability, the data I've seen is rather limited. One
problem is that there are many different oil bath air cleaner
configurations, each using different filtering media, everything from
rubberized horsehair to stainless steel wool to copper mesh to ???, plus
a lot of different physical configurations.

I've heard that oil bath air cleaners have superb large-particle
cleaning efficiency, but rather poor small-particle efficiency. They
have the advantage of easy cleanability and reusability, plus they don't
load up and cause serious air flow losses, and they give better backfire

The basic operational theory behind an oil-bath air cleaner is quite
different from that of a paper element.

Instead of acting like a strainer (like a paper element), the oil-bath
air cleaner depends on Newtonian physics: A body in motion tends to
remain in motion, traveling in a straight line, unless acted upon by an
outside force. Oil from the reservoir is drawn via airflow up into the
filtering mesh, keeping at least the lower 2/3 of the mesh saturated
with oil. When dirt-laden air passes through the maze of oily fibers, it
tries to carry its dirt particles with it, twisting and turning as it
works its way through the mesh. Dirt particles, with their larger mass,
can't turn corners as sharply as air molecules. They try to go in a
straight line, and the air is only partially successful in pulling the
dirt particles around the many corners. Most of the dirt particles can't
negotiate all the turns and eventually end up slamming into the sticky
oily fibers, where they are trapped. When the engine speed drops or
stops, the oil seeps back into the reservoir, taking most of the dirt
with it. However, smaller dirt particles are better at making sharp
turns with the air and a fair number of them manage to avoid a sticky
fate and pass on through the engine. Perhaps an oil-bath cleaner with a
finer or more convoluted mesh would do a better job with small
particles. There may also be some limited electrostatic effect as the
air passes through the mesh, though to my knowledge, this is a very
minor factor. I've read about "passive" electrostatic air cleaners that
need no external power source, but all of the media that I've heard
about use a dry filter medium, such as 3M's Filtrete.

Oiled foam air cleaners seem to be the best in terms of having excellent
large- and small-particle efficiency, since they have many more and
sharper turns for the air to negotiate, but I don't recall seeing a
foam-type oil-bath cleaner. That is not to say that they don't exist...I
just haven't seen one.

Oiled foam elements (at least the ones I know about) must be cleaned
fairly often, since they don't have the self-cleaning effect of an
oil-bath cleaner.

In terms of flow resistance, it varies widely. I'd say that a clean
oiled foam or oiled gauze filter would have the least resistance to
flow, but they (and paper filters as well) become less efficient as they
load up with dirt. Using a dual medium (coarse, then fine) may extend
their efficiency somewhat longer. This is an area where the oil bath
filters are quite nice. They can trap a lot of dirt before their flow
drops very much.

If you want to compare filters, you can use a sensitive vacuum guage to
measure vacuum inside the air cleaner. A standard automotive vacuum
gauge may not be accurate enough. You can make a simple, but accurate
gauge out of some clear plastic tubing and a board. Just clip the tubing
to a board in a U-shaped pattern, with the U about 18 inches high. Mark
off a vertical scale down the center of the U with tick marks an inch
apart and with the zero point halfway between the top and the bottom of
the U. One end of the tubing should extend out from the top of the U so
that you can connect it to a fitting on your air cleaner. Fill the tube
with colored water up to the zero mark. Check the vacuum at different
RPM settings. Note the total inches of difference between the two arms
of the U. This is a simple gadget, but quite accurate.

I think I'll stick with the oil bath air cleaner, since I'm keeping my
current project stock, but if you're interested in total performance and
clean air, I'd look into an oiled foam air cleaner. There are some
low-resistance, washable elements available from companies like K&N,
etc., that use either oil or some special solution to make the foam into
a dirt trap.



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