VW -- More on Air Filters

(written in 2002)

I've just finished reading the thread on air filters. If you haven't read it, you probably should. Marvelous stuff. Laughed my ass off.

Yes, there is a standard for automotive air filters. SAE J726 (or maybe 826... there's more than one).

Read the specs? Okay, here's pop quiz: Other than the Beta test (ie, ratio of particles that make it through a filter as compared to the number of particles which go into a filter), have you ever wondered how they arrive at those "efficiency" figures? Come on, think about it. No filter is 100% perfect for all sizes of particulate contaminant. So what are they comparing things to? What is the nearest thing they have to a perfect air filter?

Would you believe it's a multi-stage kinetic type filter, usually having a water-bath catchment... covered by a thin film of silicon or mineral oil to reduce evaporation?

Although we're talking laboratories here, the principle is the same as the oil bath air cleaner on your veedub. Not small, though; you can walk inside some of them. And except for HVAC applications they're almost always used with pre- and post-filtering, including electrostatic precipitators. (Anyone who has ever worked at a chip plant knows about this stuff. Even old sailors, like me because the government has its own chip plant, inside the fence at Ft. Geo. Meade, home of the world's longest indoor shooting gallery... and one of the cleanest 'clean rooms' on the planet.)

But the subject of comparison also applies to the air filters for cars and the point a lot of folks missed is the fact treated paper filters (ie, a non-kinetic labyrinth-type filter) are compared to an oil bath filter (which typically scores about 99.9%).

And those marvelous K&N filters? Last time I checked, they're compared to paper filters. (Go on; lookit up; K&N has a web site and is justifiably proud of their product, which works at least as well as a paper filter, so long as it receives the proper periodic maintenance and liberal doses of K&N's magic elixir.)

All of which means that clunky, old fashioned oil bath air filter on your bug or bus is still a hands-down winner... for anyone who cares to do a bit of reading :-)

As for all that 'flow-rate' stuff... Anyone care to take a WAG at the flow-rate for your 1600cc engine, roaring along at 3500 rpm? No? Howzabout a 2550cc big-bore stroker turning five grand? (Hint: Be prepared to be underwhelmed.) But bigger is always better, right? So more flow has gotta be good, right? Horseshit. The fact a particular filter can flow more air doesn't mean your engine can use it. Those counter-top ‘comparison’ units are just a sales ploy; a kind of IQ test for the technologically naive.

Read the specs. Flow is relative to the pressure differential and restriction. The bottom line is that your oil bath air cleaner does just fine, folks. Always has and always will, so long as you give it the maintenance it requires. For stock engines an oil bath air cleaner has no problem providing more air than the engine can use. (If you think about it for a minute you'll understand why that has to be so.) In fact, the stock oil bath air cleaner works just fine for engines up to about 2200 cc so long as they don't spin over 5000 rpm, a fact you can work out for yourself with nothing more exotic than a #2 yeller pencil and basic arithmetic. Working it out will also give some idea how much you'll have to shorten the service interval when you use an air filter designed for a small engine on a big one. And the service interval of any air filter is a critical factor in determining its practicality.

Indeed, the service interval is one of those things the tea-cozy crowd doesn’t like to talk about. As with any labyrinth-type filter, their effectiveness is a function of surface area. Under harsh conditions, where you might need to service an oil-bath air cleaner as often as once a day, you may need to wash & re-oil a gauze & window-screen air filter every few minutes... which is why you don’t see such filters used for real-world applications.

Don't take my word for any of this. After all, nobody is right all of the time :-) Think for yourself. Go look it up. Work it out. It's your ride and it's entirely up to you what kind of filter you use. You're the Mechanic-in-Charge, not me.

In fact, lemme repeat that: You are the Mechanic-in-Charge. Making the right choices is up to you. All I've done here is express an opinion, albeit one based on experience. In effect, I've described a particular path. The decision to explore it -- or not -- is yours.

The only reason I've bothered to say anything at all is because automotive engineering is not subject to the democratic process. Having a big mouth doesn't automatically make you a good singer, nor a loud voice a good speaker (although it helps :-) The fact someone posts a thousand messages a month does not mean the messages contain anything worth reading. Apply that logic to the fact all your buds are using gauze & tea strainers for air cleaners and you'll see it only means its a popular idea, not necessarily a good one. The task of determining the merit of something is up to you. And I'm not just taking cars here. Every person is capable of error. That fact extends to every organization as well because organizations are nothing more than collections of people. And if you think an individual can screw up, our personal faults pale by comparison to the outright evil of which groups are capable. (And yes, you can look this up too if you wish. Although since 9-11 I've got a hunch most folks can figure it out for themselves.)

The wiser course, in my opinion, is to always think for yourself, even if doing so sets you apart from the majority. Because when push comes to shove, it's your life. And your ride. Not theirs.

-Bob Hoover

PS -- Oil bath air cleaners fell into disuse for passenger cars because of economic factors, environmental issues and the shear size of the damn things.

The size issue had to do with their height. To work properly an oil bath air filter must have a certain minimum height. If you had a 'tall' engine yet wanted a low hood line you had to move the oil bath air filter over to the side, as Volkswagen did with the Karmann-Ghia, Hudson with their dual-carbed 'Hornet' and others. But doing so increases the parts count and complexity of the system and such things carry an inherent increase in maintenance cost. Which leads to...

The key economic factor in the demise of oil bath air cleaners in cars had to do with the time needed to properly maintain them. I can't remember the Flat Rate Manual for servicing an oil bath air cleaner but it was on the order of 0.5 m/hr. No big deal when making 'good money' meant a dollar an hour. But nowadays spending half an hour to clean an air filter would be economic suicide for a dealer paying his line mechanics $37.50 an hour plus full medical, dental and so forth. (What's it take to change a treated paper air filter? Maybe a minute... if you've never done it before. Less, for an experienced mechanic :-)

The environmental issues with regard to oil bath air filters wasn't so much the oil you poured out of the thing. Hell, recyclers will pay you for the stuff. What really sets a tree-hugger's hair on fire is seeing that drum of sandy sludge -- the scrapings out of the bottom of the oil pan -- accumulate over in the corner of the lubrication bay. A high percentage of the particulate matter found in the air above most roadways is toxic. And you've just concentrated 55 gallons of the stuff (and usually hauled it off to the local dump). The solvent-filled parts washer you ran it through was another E-ticket ride for tree-huggers. Not only did it generate its own quotient of sludge, in normal use it pumped a few pounds of hydrocarbon aerosols into the air every day. By the time you've shifted to a water-based high pressure detergent washer, or added a vapor recovery system to your solvent tank and negotiated a HazMat contractor to collect & dispose of the sludge, servicing an oil bath air cleaner has become a very pricey proposition.

There's plenty of justification for doing away with oil bath air cleaners. But their ability to clean the air wasn't one of them. Indeed, such filters remain a standard method for many industrial applications, as a bit of research will show. - rsh