This article will discuss some of the challenges and inner workings of oil filters. It seems like a simple job, pump dirty oil through a medium of some type and clean oil comes out the other side. In the past oil filters used a simple natural fiber cloth that is not so simple on the microscopic scale of a dirt particle. If you imagine the filter media as a "metal screen" trying to make ever-smaller apertures in the screen has several effects. As the weave becomes denser the greater number of smaller openings can not compensate for the decrease in opening size so the permeability, total flow capacity decreases and the pressure across the media increases.

An oil filter is actually more complex. If a screen filters in two dimensions, a filter has depth or "paper thickness" so a functional model more accurate than that of a single thickness screen would be that of a mile wide thick (on the scale of a 10-micron soot particle) fuzzy-sticky cotton ball. The apertures may be vastly larger than the minimum target size particle but the probability of one particle avoiding capture by filter in depth is small. As more and more particles are captured the media permeability decreases, which increases pressure, decreases flow, makes the average effective aperture smaller, thus doing a better job of trapping ever smaller particles.

In the simplest test SAE J-608 this is exactly what is tested. Small beads 10 to 20 micron are passed through a filter just once - called Single Pass Filtering Efficiency (SPFE) and rated for percent trapped. Particles smaller than 10 micron did not count in the test because they were thought harmless. Particles larger than 20 micron would be trapped in any element media capable of capturing smaller stuff. For this test the advantage goes to larger filters with a lot of very fine media area because the paths are massively parallel. A filter will also do a better job at low pressure and flow rates, not what we require for our SHO engines. Often the high SPFE touted by those marketing oil filters are achieved at very low flow rates and not very useful numbers.

The next advancement in filter design and testing was the perception that a filter could have a relatively large mesh and in time, by way of multiple passes filter out some very small sub mesh size dirt. Say a filter has 20-micron mesh so anything larger than 20 micron gets trapped first pass. With each pass the media load up on more dirt and in each subsequent pass smaller and smaller particles get trapped. Eventually either all the dirt is trapped or the filter is has a full load of dirt and can pass no more oil.

We can run the filter all day long but at some point everything virtually everything bigger than 10 micron will filtered out and the sub 10-micron stuff will never be filtered out unless/until the filter becomes totally blocked.

Now that the ruined the GTP and it burns a lot of oil we can return it from where we rented it and talk about real cars.

How long does it take to run oil 20 times through an oil filter?

Assume 3000 rpm in OD at 80 mph ~ 5.67 GPM oil, 4 quarts per gallon, 6.7 qt oil capacity ~ we empty the oil pan every 17.7 seconds when we book down the interstate at 80 MPH playing "I Love LA" in overdrive or 5^th gear.

With the engine loafing the filter by passes no oil, so it filters 100% of the oil. If we empty the oil pan once every 17.7 seconds, we will do it 20 times in 6 minutes or 8 miles. Assuming we have a good air filter, so we do not add a lot more dirt, after 6 minutes of easy driving our oil should be almost virgin; down to say - 10 microns.

Still with me? What if I have need for speed? The engine pumps 2 GPM for each 1000 rpm. Assume for this example the filter can only flow 6 GPM with the by-pass closed so the by pass will have to open. If one half the oil entering the filter evades the cleaning process and takes the bypass. The other half of the oil will get cleaned at 90%.

(90% x 1/2) + (0% x 1/2) = 45%

We clean out half as much dirt each pass as we did above. Half of nothing is nothing; if the oil was very clean and warm and we drop the hammer to pass someone and soon return to normal operation it is a distinction without a difference because the oil that was bypassed was very clean anyway.