Air Filter

[AJ]

On a MC8, 8V71 with standard oil bath type air filter, how can you tell when is the tieme to service the air filter? And what is the easist way to service it? Is it worth to convert to a dry type filter?

[lostagain]

I'm still using the OB filter in my 4-71 that I turboed a couple years ago. It now flows about 1200 CFMs at full boost. I works very well and oil does not get sucked into the intake like some guys were telling me it would. I installed a filter minder gauge that shows 11 inches of vacuum for restriction, which is within accectable specs per DD book. It picks up more dirt than paper filters. The industry changed to paper filters for ease of service. I clean mine once a year (10 000 m or so). Put same 40w oil as in engine. Some guys put used oil in them. It will be the young mechanics that tell you it won't work because it looks prehistoric to them.

[Stan]

I serviced my oil bath filters once a year when I serviced the bus, usually less than 15k miles.  You can drain the oil out of the filter through a piece of cloth and reuse it in the filter after you wash out the bottom of the reservoir.

On some oil bath filters you can remove the top portion of the filter and wash it in solvent but I think all MCI filters have the top as a one piece unit and if it is necessary to wash the filter media, you have to take the whole assembly apart. A vacuum gauge (filter minder) will tell you how much loss in the filter and if it needs to be cleaned.

If you really want to punish yourself, you can remove the entire air filter box in one piece and clean out the inside of the box and then wash the filters.

[lostagain]

Stan, I don't get how the filter minder would read differently when there is more or less dirt in the oil, since the dirt goes to the bottom of the bowl. The air turns sharply just above the  surface of the oil. The dirt is flung at the oil with centrifugal force then sinks to the bottom where you find it when you clean the filter. Am I missing something? There would be less restriction if the oil level went down. But it doesn't. Unless the turbo is sucking oil out of it, which means the filter assembly is too small. That's what I was worried about when I turboed my engine, which doubled the intake air flow from 600 to 1200 CFM. It turns out I'm OK. The OB filter MCI fitted in my bus is big enough. Maybe it is the same size they put in for the 6-71 and 8-71?

[Stan]

The material in the top portion of an oil bath air filter (coarse steel wool) is always saturated with oil when the engine is running. It is carried up into the mesh as vapor with the air and the smaller particles of dirt. The dirt gets trapped in the mesh and the oil condenses and drips back down.  Eventually, the mesh just acts like a paper filter that is plugged with small particles of dirt. Here is a link to a company that sells replacement mesh sections.
http://www.uniairproducts.com/oilbath_air_cleaner.shtml

I am not an air filter engineer (don't even play one on TV) but something to consider on your engine is that the velocity of the air, when it makes the turn at the oil surface determines the size of dirt particle that will be trapped by the oil. BTW: I love that old 4-71 and Courier 96. Best steering bus I had.

[Lin]

The PO of my coach changed out to a replaceable filter.  I was told that the OB filters work great under normal operation but are poorer when idling.  Is that incorrect?

[Stan]

The short story is if you have a Detroit 2 stroke you shouldn't let it idle anyway, so it doesn't matter. As I explained in the post above, the air velocity has a major effect on an oil bath air cleaner and at idle there is very low air velocity through the cleaner.
If you do a search on oil bath air cleaners, you will probably get a million hits and I expect some of them will have all the curves and effect of different oil viscosity and size of dirt particle removed.

[lostagain]

Thanks for the clarification Stan. Good explanation of what happens in a OB air cleaner.
And yes, my Courier 96 does steer very nicely. Mine is very tight that way and a pleasure to drive. Love it!

JC

[Sojourner]

About oil bath filters:
Oil bath filter is what it does. Whenever you see dust or dirt or dried oil crud on screen filter is cause from either too low level or over looked filled crud in pan to allow no oil drawn up into screen mesh via vacuum. At idle or high flow, there is oil soak in mesh to collect foreign material and while oil is rolling like a mushroom cloud to keep mesh cleans at all time while in vacuum. In the mean time it builds up of foreign particles at bottom of pan.

Whenever you remove pan out for inspection via dumping oil into another pan or drain, run your finger at the bottom of filter's pan & slide to see how much crud is there. That will tell you how much you collected while driving in dusty road and how often to clean pan. But you ever purchase bus with dirty filter mesh (screen)...it mean it been sucking foreign particles through that engine....can be damaging with worn rings & pistons.

If you use too light grade of oil, it will try to suck into engine.

Always check the level or it not clean air and foreign particle build up or cake onto screen mesh.

Been there and done many farm tractors and some heavy equipment in dusty condition in the 40's & 50's.

Anyone ever told you that oil bath does not clean air in idle mode does not understand how it works. Or they want you to change to paper type to make a living selling more filters.

However good quality paper filter is fine. But don't get cleanable filter...it not filtering as well as good OEM paper type. Test reports:
http://www.bobistheoilguy.com/airfilter/airtest3.htm

FWIW

Sojourn for Christ, Jerry

[Tom Y]

Jerry, Great site.  Thanks for the imformation. Tom Y

[Stan]

I just did a quick search on oil bath air cleaners and here is a lot of technical information from analysis done at University of Michigan. http://deepblue.lib.umich.edu/bitstream/2027.42/4006/4/bab9791.0001.001.txt