My day job involves working with hydraulic fluids and filtration for a major OEM. I work with filter and fluid manufacturers frequently and get involved with a lot of troubleshooting. Engine oil, fuel, and hydraulic filters typically use the same standard for specifying the filtration capability which is documented in ISO 4406.
The ISO 4406 standard was changed in December 1999 as a result of the availability of test dust. Up to that point, ACFTD (AC Fine Test Dust) had been used for several years for filter standards. This finely ground dust ceased to be available in the early 1990s, and a new test dust standard was required for filter calibration. The old calibrated size for ACFTD was called out in ISO 4402.
ISO 4406 adopted a new standard test dust which is calibrated under ISO 11171 by NIST (National Institute of Science and Technology). When NIST calibrated the size of the dust, they found the ACFTD particles under 4 microns in ISO4402 were actually larger than stated, and particles over 14 microns were slightly smaller than stated.
Since December 1999, most filter companies rate filters using the NIST ISO 11171 dust and if someone is telling you their filter has a 3-micron rating, you should ask them what standard it is based on. It’s either marketing hype or the old standard. Filters previously rated at 3 microns based on ISO4402 are now 5.1 micron under ISO11171. See page 5 of the attached FuelFiltrationRealityCheck PDF.
There’s more to fuel (and other filters) than just micron ratings. A micron rating doesn’t mean much without stating what percentage of a certain particle size is stopped. Beta ratings must be included such as a B75 4 micron filter means 98.7% of 4 micron particles are stopped. Calculation is simply ((75-1)/75)*100 %. You can have 4 micron filter that is 50% efficient too, but B75 is the absolute rating. See details in the attached Beta rating PDF.
Too many get wrapped into micron ratings, where the 3 or 4 micron filter is king over the lowly 5 micron filter. Really? Are we going to split hairs and hang our hat on that alone? 3 microns is 0.0001181102 inch, 5 microns is 0.0001968504 inch and the difference is a whopping 0.000078740200 inch! I agree we need to be clean, but I need to be educated how this alone is significant. My injectors were replaced because of a failed solenoid at 102K, and it ran on the factory fuel filter until then.
So, what is the definition of better, or much better? What is the measure? Are we not going to include burst capability, vibration resistance, manufacturing inconsistencies which can lead to metallic process contamination as well as end cap and seam leak paths? I think overall filter quality is certainly important as I don’t want someone who is making a filter to leave a burr behind that has a good chance of doing a lot of damage. Cat, Fleetguard, Racor, Baldwin, and Donaldson all make good filters and I’ve never had a failure using them. I’ve been in filter manufacturer R&D labs observing exhaustive tests of competitive filters, there’s more to it than efficiency tests. I’m more concern about getting fooled some counterfeit junk, and they are out there.
With respect to high flows - I want as much reasonable flow potential as possible and I am not really worried about maximum filter flow capability. A system uses what is needed, the pump regulates the flow to maintain desired pressure and excess flow is returned to tank. Early common rail systems by more than one manufacturer had injector problems from heat due to insufficient cooling and cavitation was also an issue. Increased flow was a great help in some cases. Ensuring sufficient flow gives me some peace of mind that I am reducing the potential of cavitation as well as quite possibly improving my injector’s lives by keeping them just a bit cooler. The choice for me wasn’t difficult, and after doing so it was the first time in 7 years I was able to get 23 highway mpg hand calculated. Yes, the Fleece pump is not cheap, but I have no regrets.
Attention: TDR Forum Junkies 
