Q&A / Tech

Ask Away with Jeff Smith: Micron Ratings and What They Mean for Your Oil Filter

I’ve been looking at aftermarket billet aluminum oil filters with a stainless mesh filter that you can disassemble, clean, and re-use. Are these any good? I don’t mind taking a filter apart and cleaning it. I’m thinking it might reveal problems before they really get bad but I’m not sure I’m getting as good a filter. What’s the story on these filters?

N.R.

Jeff Smith: I’ve looked into the specs of a couple of these filters. As you mentioned, they use a stainless steel mesh that these companies rate in the neighborhood of 30 microns of filtering capacity. Let’s start with what is a micron. In layman’s terms, 1 micron is equal to one millionth of a meter or 0.00039-inch. To put this in perspective, a human hair measures between 40 and 80 microns. The following chart puts microns in inches.

Micron Inches
10 0.00039
25 0.00098
30 0.00118
40 0.00157
ASK-05-02

Fram’s racing filters are designed to increase oil flow through the filter but this comes at the cost of reduced efficiency in terms of dirt below 30 microns. This assumes that the oil will be changed much more often on a race style engine. For this reason, race filters are not necessarily recommended for daily street use.

According to Fram’s Technical Training Manager Jay Buckley, dirt and foreign material measuring smaller than 10 microns will be small enough to be carried in suspension in the oil and will pass through even tight bearing clearances. Material larger than 20 microns is generally considered too large to pass through tight bearing clearances although with performance engines with main bearing clearances of 0.0030-inch, this is certainly possible. But generally speaking, foreign material in the range between 10 and 20 microns will potentially do the most engine damage over time. So a filter that can efficiently filter within this range would offer distinct advantages and certainly reduce engine wear.

There are several tests that some manufacturers perform to test the efficiency of a filter. The original test was a single pass efficiency (SPE) test where the oil passed through the filter once and the amount of dirt captured is rated relative to its efficiency measured. In this case, if  the filter removed 9 out of 10 of the contaminants of a given size –20 microns for example – then the filter would have an efficiency rating of 90 percent. While this test has validity, the more accurate test now generally used is the multi-pass efficiency test (MPE) where the oil passes through the filter more than once. This is regarded as a better measure because most passenger car engines do not filter 100 percent of the oil pumped into the filter. During cold start operation and during high rpm operation, some oil is routinely by-passed around the filter to minimize a serious pressure drop across the filter.

All quality oil filters use an internal by-pass valve. This bypass is designed to avoid a situation where the filter becomes clogged and cannot supply a sufficient volume of oil to the engine. Dirty oil is better than no oil at all. For a typical Fram filter, this valve opens when the pressure on the inlet side of the filter exceeds between 12 to 30 psi more than the outlet side of the filter.

According to Buckley, from a purely legal standpoint, if a filter traps one piece of 30 micron debris, the manufacturer can state it has the capacity to filter at the 30 micron level. Buckley says, “The only way a micron rating means anything is to use the filter efficiency numbers published by the manufacturer.” This filter efficiency rating is listed as a percentage. “For the Fram Ultra Guard filter, it is 99 percent efficient at 20 microns. At 10 microns it’s 94 percent efficient.”

Buckley also made a good point that unless you use brand new solvent to clean a reusable filter, it’s possible you may not be really cleaning the filer as much as you might hope. Remember, a 30 micron piece of dirt is only slightly larger than 0.001-inch.

One reason the Fram Ultra Guard is so efficient is that it uses a higher quality filtering media. A standard Fram Extra Guard, and many other standard oil filters, use a cellulose/synthetic blend filter media. Each company’s design of this media is proprietary. Upgrading to the Ultra Guard filter, this uses a dual layer synthetic glass filter material that is more efficient at capturing debris between the 10 and 20 micron size.

Basically, filters trade efficiency for flow rates. A perfect filter would eliminate all debris with no reduction in oil flow rate, but this – as yet – is not possible. In order for a filter to really clean the oil, it will present some restriction to flow. This means that at any one time, a certain amount of oil is by-passed around the filter. This happens with any filter, regardless of design. If an increased oil flow rate is necessary – as with an oil filter intended for race engines, this will use a material that will allow more oil flow – which generally means it will not filter quite as efficiently as a production car filter. Fram, for example, also makes racing filters that feature a thicker-wall container and a different filter material that allows a higher flow rate. These filters are generally not as efficient in cleaning in the 10 to 20 micron range because the understanding is that a race engine will change the oil and filter far more often than a street-driven engine.

As long as we are on the subject of keeping the engine oil clean, it’s important to mention that the very first line of defense for clean engine oil is to start with  highly efficient air filter. A high-quality air filter goes a long way toward keeping the inside of the engine clean along with a functioning positive crankcase ventilation (PCV) valve that will pull oil vapors out of the engine.

To wrap this up, Buckley offered this quote from GM engineer David Staley taken from a 1988 SAE paper on oil filter efficiency. “Compared to a 40 micron filter, engine wear was reduced by 50 percent with 30 micron filtration. Likewise, wear was reduced by 70 percent with 15 micron filtration.”

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