Primer on Filtration

Allow us to offer this primer on filtration.

Filter media has a range of particles that it will remove. The smallest particle size it can catch is usually referred to the “nominal” size. The largest particle size it lets through is referred to as the “absolute” size. The sizes between those two are usually expressed as a beta ratio, or percentage of efficiency. In other words, if you introduce 100 particles of the 15 micron size and 60 of them get caught, you have a percentage of efficiency of 60. It is interesting to note that some companies are very liberal with their advertised absolute numbers.

There are many different test procedures that can be used to test the particulates a filter media will remove. There are single pass, multiple pass, SAE, ASTM, and even European specific tests. What gets difficult is how to compare the numbers used for marketing filters. Picture yourself standing in the auto parts store in front of a rack of filters trying to decide which filter is “best”. You see a 22 micron filter with some sort of efficiency rating attached to it (for example 94% efficiency) and another with a 15 micron number printed on the box with no efficiency rating. And then you see one that has printed on it that the filter will catch down to 5 microns. There is no way to accurately compare those numbers. From a marketing standpoint it is better to print lower numbers since most consumers don’t have a clue about efficiency ratings, and there is more of a chance of the customer purchasing the lower number filter. Problem is the “5 micron” filter may be letting more particles through the filter than the one that has 22 microns printed on it. A good example of this is the Harley Davidson Twin Cam filter. On the box it says it will filter down to 5 microns. When tested at the lab it actually passes 40 micron particles. Before you condemn the filter, I would note that it pretty much tied with the Dodge Cummins diesel filter as the tightest OEM filters tested. As an fyi, most paper filters range from 50 to 90 microns as their absolute number. Ours is 35 (and yes, we can catch 5 micron particles as well).

We use the ASTM test because it offers the smallest amount of manipulation of the testing process. We test for worst case, or absolute numbers, partially to be able to compare apples to apples between filters, and partially because of FAA testing.

As filter media gets tighter, the flow rate gets slower and the resistance to flow increases. This resistance is referred to as differential pressure (the difference in pressure between the outside and inside of the filter element). As the differential pressure increases, the pressure against the bypass valve increases and the chance of the bypass opening increases.

K&P Engineering focuses on maximizing flow rates while meeting or exceeding OEM filtration. Not all stainless steel filter cloth is created equal. We specify the wire diameter, thread count each direction and the type of weave to use to create our filter cloth which is how we meet the filtration and flow goals. We can get between 5 and 7 quarts of oil through our filter element in the time it takes to get 1 quart through an OEM filter element while still meeting or exceeding OEM filtration. The reason for this is that paper and synthetic filter media is made of fibers that are pressed or glued together and require a certain thickness to get to the filtration objective. This type of media inherently has more flow restriction that the filter cloth that K&P Engineering uses. We tested a pro stock engine on the dyno with a gage on both sides of the filter cavity. At wide open throttle we measured 20 lbs of differential pressure on the paper filter (0w at 180 degrees). The K&P Engineering filter measured less than 1 lb. This means the bypass on the paper filter is open letting unfiltered oil through the filter and into the engine. Ours has a lot further to go before it opens. The result is more filtered oil getting to the engine, and getting there faster. The reduction in differential pressure also results in less backpressure against the oil pump which has the potential to improve performance and or fuel mileage.

Some will argue for a very tight filter media, and concentrate on the smallest particles a filter will catch (even though it might be a nominal number). An analogy we use is to think about the towel you are about to dry your freshly washed new car with. Are you more concerned about the dust that has settled on the towel, or the rock that is in the towel left over from when the towel was dropped. It also becomes a moot point on how tight the filter media is if the bypass is open and the debris is flowing right past the filter media. Some filter companies argue that they are ok with the bypass being open because the debris will eventually get caught as the oil seeps through the media and the oil will get to a cleaner level at some point in time. Our concern is what damage is being done while that debris is passing through the engine over and over again waiting to get caught in the filter media. We have customers that do want to get to that finer level of cleanliness, and they usually go with a bypass filter setup. They use our filter as primary filter, and a very tight filter on the bypass circuit. That way the oil is cleaned to OEM levels with our filter, and as it dribbles through the tighter media of the bypass it will get even cleaner.

The stainless steel filter cloth technology has been around a very long time. It is used in military aircraft (fighter jets, helicopters) and even used for filtering blood. K&P Engineering filters are approved by the FAA for aircraft use, and have just recently been approved by the European equivalent of the FAA (EASA) for use on aircraft in Europe. We are consistently monitored for quality assurance as a part of these approvals.

Having said all this, K&P Engineering appreciates the people that question the quality provided by different filters. There are some cheaper counterfeits of our filter out there that use the K&P Engineering information and numbers to promote themselves. When we sent the media of those knock off screen filters out for testing at the lab, they came back as filtering worse than the worst paper filters we have tested. It is kind of like the Harbor Freight versions of Snap On tools.

K&P Engineering