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Willys Air Cleaners
How they work and upgrades
By Todd Gabrielson

In order to understand air cleaners, we have to understand dust. That’s a big topic, so here’s an abbreviated course.

Dust size is measured in micrometers (usually shortened to microns), and the actual shape and weight of the dust particle is dependent upon on the type of dust it is. Chaff, road dust, snow, all have unique characteristics that affect their behavior in the air stream and their effect on engines and air cleaners. A micron is one millionth of a meter. So for comparison, the diameter of a human hair varies from about 40 microns to 120 microns. Copier paper is about 100 microns thick. Humans lose the ability to see dust particles at about 40 microns. It’s important to filter the small dust particles even if you can’t see them.

Dust has weight, mass, hardness and shape. Air cleaners, no matter what type, exploit these characteristics to capture dust. In a diesel engine, machine tolerances are much tighter than they are in a gas engine; new gas engines are tighter than older units originally supplied with the vintage Willys, which makes them more tolerant to dust than a newer engine. Dust larger than 15-20 microns, depending on its characteristics, is generally not so much of a problem either, as it is typically too large to be airborne long enough to get to your engine. Therefore, the dust that is most important to catch is the 3-15 micron dust, as this dust will stay airborne and can also damage your engine.

Most heavy duty air cleaner systems consist of more than one component; this is true of the Willys air cleaners as well. The picture below is the Donaldson FAE07 air cleaner; this model was used on most Willys Jeeps at one time or another. Oakes air cleaners were also utilized, as was a Fram air cleaner that had a depth loading dry type media, however, these were eventually replaced by Donaldson.

The critical parts of the air cleaner are described in this section: The inlet section, the pre-cleaner section, the oil cup, mesh assembly and outlet. Dust has weight, therefore when it is moving it has inertia. The inlet section of the air cleaner does two things, sending air in the direction we want it to go, and accelerating the air prior to the pre-cleaner section. In this air cleaner the pre-cleaner creates a tortuous path, a 180 degree turn that uses the inertia of the dust to separate it from the air stream and drop it into the oil cup where it is captured.

The above arrow shows the inlet section, which is designed to provide the following functions:

The air stream is then directed up into the mesh assembly. This mesh is coated with oil from the oil bath, the tortuous path (through the mesh) is again used to separate the dust from the air stream and where the dust is impinged on the oily mesh, the cleaned air then goes through the outlet section to the engine inlet.

The mesh screen is an aluminized steel mesh captured in a steel frame, in some oil bath units this screen is removable for cleaning, but in the Willys Donaldson air cleaner it is not. Cleaning is accomplished by following the directions in a Donaldson Service Bulletin.

Above is the outlet section, as you can see it has nice rounded corners and smooth transitions from the air cleaner body, where the air was slowed down (low velocity), to the outlet tube which has to match the carburetor intake diameter. Smooth transitions reduce the overall restriction in the air cleaner and allow for longer service life.

So here is an important tip: The air is now nice and clean and the most important thing you can do from here on is make sure it stays that way. The hose connections, rubber sleeve and boot at the carburetor need to be in great condition in order to seal around the inlet/outlet tubing and keep dust out. The level of vacuum pressure inside the ducting will increase as the diameter decreases, at any given air flow, consequently if you have a weak spot in the seals or clamps, a dust leak may occur and it will go right into the engine.

In addition to making sure the seals and clamps are tight, it’s important to service the air cleaner regularly. (See the link above for the Donaldson service bulletin for these air cleaners) Most of us, however, are restoring vehicles that have been ignored for some time. It would be prudent to take the air cleaner off of the vehicle, remove the oil cup and clean it completely, check for holes and leaks as water often sits in these and will corrode them. JB weld is a good way to repair the oil cup.

Service Suggestions
After removing the oil cup, submerge the rest of the air cleaner in a bath of cleaning fluid, such as Stoddard Solvent or Kerosene. I would use a low volatility bath, don’t use gas, please; I like you guys too much. Let it rest for a while to loosen the built up grime and junk. The mesh screen does not come out of these cleaners, so don’t try - it will ruin the air cleaner.

Most of us are lucky to drive more than 1000 miles a year in our jeeps, and unless these miles are all off-road, you shouldn’t need to change the oil in the cup more than once per year. It’s pretty easy to remove the cup, check the bath and then decide if you need to change it or not. Check it a couple times a season; make a good decision based on how you use your jeep.

These oil bath air cleaners are 98-99% efficient on dust larger than 5 microns, which is an acceptable level given the tolerances in a L134 engine. Newer tighter tolerance engines required higher levels of efficiency, that’s why the dry type air cleaner was developed.

Some of us are looking for system upgrades to the oil bath air cleaner. Donaldson makes a dry paper type air cleaner that is a great substitute for the original, the FPG05.  This is a very flexible air cleaner, as it can be mounted horizontally or vertically, has the same outlet diameter as the original so it will be a direct connection.  The FPG air cleaner is a two stage air cleaner with an inertial separator, again taking advantage of the mass and velocity of the air stream, and it also utilizes a high efficiency paper media as the final stage.  In addition to the air cleaner a host of accessories, such as rain caps, ducting, and elbows are available to upgrade that fine old Willys. Click here for the Donaldson webpage.

Credits to:
Donaldson Company Inc for technical information
Sarah Swenson
Randy Thompson
Tom Miller
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