Oil filters exposed

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Straight Talk About Oil Filters

 Information provided courtesy of Purolator Filters


Let’s face it. Automotive technicians can perform some pretty complicated procedures. That's why they're called technicians. You can work for years and earn certicfications and gain experience, but the basic procedures are where you started out in the beginning.


One of those basic procedures is spinning an oil filter onto an engine. In most cases, performing that procedure is probably the equivalent of asking your helper to pick up an oil filter and quickly put it on so we can send the finished job out to the customer.


But are we really doing justice to the customer and, in fact, to our careful work? After all, does it really matter which filter we screw onto a fresh everyday motor? Well, sure it does. That first filter must protect our fresh engine from whatever chips and machining debris might linger in the recesses of the oil galleries and crankcase. And, probably more importantly, the filter we install is likely to establish the precedent for the brand of filter that the car owner will continue to use. So our choice is more long-lived than we might think.

Are all filters the same?

And so we come to the next question: Aren’t all oil filters pretty much the same?[PAGEBREAK]

 As it turns out, they’re not.

 For the straight skinny on oil filters for everyday engines we went to the experts at Purolator, since they’re the ones who invented the first automotive oil filter (in 1923!), and also the ones who invented the spin-on oil filter in 1955, as well as a highly respected supplier of OEM filters. Ramon Nuñez, Director of Filtration for Bosch, joint venture owner of Purolator told us that oil filters are deceptively simple and yet deceptively complex.


Here’s the deceptively simple thing about oil filters: the two most important aspects of oil filters are:

What size particles can they filter out?

How much debris can a filter hold before the bypass valve quietly directs unfiltered oil to your new bearings, lifters, and cam lobes?

Sure there are other features to consider, like the integrity of the housing and crimping, the design and reliability of the bypass valve, and the durability of the filtering media itself. But if the filter isn’t safely capturing and storing debris, then the other features don’t matter much.

Purolator’s Nuñez suggests that we first consider particle size. After all, it’s the big chunks we’re worried about, right? Well that’s true if all you’re worried about is the first few hours of operation. But if you’re interested in the longevity of the engine, then size really does matter.


How the filtering media works

Here’s a surprise: the filtering media does not actually filter.

Yup, that’s right. The filtering media does not actually filter. Remember the oil bath air cleaners of the ’40s and ’50s? If you’re old enough to remember them, you may or may not be aware that they worked by forcing incoming air to follow an “S”-shaped route past a trough of engine oil. The concept was, the air could easily follow the turns, while the heavier particles of dirt and debris couldn’t make the turn, and momentum would carry the grit straight into the oil, where it was held hostage until someone dumped the dirty oil and replenished it.

Believe it or not, that’s actually how modern oil filters work. The filtering media does not act like a screen, but rather traps contaminants that cannot change direction easily. Of course, explains Purolator’s Nuñez, today’s oil filter media is very sophisticated, and much science has gone into its design and construction. The design of the media determines how small a particle can be held, and how much capacity the filter has for holding debris.

By the way, Nuñez also notes that the configuration of the size, shape, and number of pleats in a given oil filter are actually mathematically calculated to allow for the maximum amount of media to be exposed to the oil flow.

So back to particle size and filter capacity. Let’s deal with particle size first.

Here’s the short answer: if you can see it, it’s too big, it’ll damage the engine, and it needs to be filtered out.

OK, OK, here’s the long answer. The term “efficiency” is used in the filtration world to describe the amount or percentage of contaminants that are caught and held as oil passes through the filter. There are SAE standards for “test debris” and for testing procedures. Purolator Classic oil filters are 97.5 percent efficient, and Purolator PureONE filters are 99.9 percent% efficient – the highest efficiency ratings on the market. Both filters will essentially remove their respective percentages of particles 20 microns or larger in diameter — including virtually all particles large enough to be visible to the naked eye.[PAGEBREAK]

Picture this: a human hair can measure as little as 30 microns (one micron is a millionths of a meter) in diameter (don’t forget we’re talking an end view here…). A human bacteria can measure 20 microns. And the soot found floating around in cigarette smoke can measure nearly one micron. So a filter that removes virtually all particles 20 microns or larger is offering phenomenal protection to your engine.

Here’s more food for thought. A single chip from your rotary broach or milling machine often runs around 0.007”, which translates into about 175 microns. And a stray piece of casting flash is certainly much larger. You can well imagine, and have probably seen, the devastating effects of having a 0.007” piece of debris trying to squeeze itself into the 0.002-0.003” clearance between a rod or main bearing shell and a crankshaft journal. And who among us can claim with certainty that they’ve never left a single, or modest collection of, machining chips hidden away in an engine block or cylinder head?

There are two ways to measure filter efficiency – single-pass and multi-pass. Which test do you think better represents real-world oil filtration – a single-pass test that runs just 45 gallons of oil through a test filter, or a multi-pass test that passes more than 2,500 gallons through the filter? Why it’s the multi-pass test of course.

But won’t a filter that’s more efficient get clogged sooner, causing the bypass valve (if the filter has one…) to open and direct unfiltered oil to the crankshaft, bearings, and other critical components? Ah, that brings us to what the filter engineers refer to as capacity. Capacity represents the amount of contaminants a filter can remove and hold before flow is restricted. Capacity is usually measured in grams.

Some filter makers don’t advertise capacity if it’s not a favorable number for them. In the case of Purolator PureONE oil filters, they have a capacity of at least 13 grams. In real-world terms that means that a PureONE filter will hold the equivalent of 31 standard-size paper clips before it becomes blocked. And that’s a whole lot of debris.

Beyond efficiency and capacity, there are other features you should look for in your choice of an oil filter – things like a steel center tube for reliable support of the media, a one-piece anti-drainback valve (less likely to leak than a multi-piece design), and a flat sealing ring, shown by SAE tests to provide greater sealing surface area, higher blowout resistance, and longer life than O-ring or P-ring designs. Purolator oil filters provide all this and more. Some filter companies use paper or felt end caps versus the steel end caps used by Purolator. 

In cold climates where cold start-ups can cause huge momentary spikes in oil pressure, you should look for a filter design that’s been tested for burst strength. Our Purolator expert tells us that Purolator Classic and PureONE oil filters, for example, are tested 25,000 cycles at pressure pulses of 0-100-0 psi to validate filter housings’ mechanical strength.

If you ever wondered about how an oil filter does it’s job, now you know.

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