Filter Efficiency: when small differences aren't small

[Hohn]

Thank you for that post!! I’ll get on those links when time permits.

 

[Mewbs]

1) The ISO standard seemingly uses whatever test flow rate the filter OEM asks them to use. How do you know one filter maker isn't using a wildly different flow rate than another for the same nominal filter? What if one OEM for a typical Honda spin-on says to use 3L/min and another says to use 5L/min? The test duration and injection rate are all scaled to these nominal flows which are not controlled by the standard. By my reading, there's nothing in the standard the precludes a filter manufacturer from sandbagging the test by using a lower nominal flow rate, because face velocity (flow rate per unit of media area) is a major contributor to effective filter efficiency. If you take two filters of identical media but one is twice as large as the other, the latter will have measurably superior filtration efficiency simply because the face velocity through the media is half as much.

This is why I always preferred to run a larger 3600 sized filter over a stock sized 4967 if it fits my engine. More media surface area is always good and also less differential pressure under the same flow conditions. The only downside to running a larger filter is that it needs to be pre-filled prior to install to avoid long oil pressure delays on first start as the pump primes the engine with new oil.

Machinery Lubrication online magazine talks about the benefits of running a larger filter in industrial applications to maximize service interval hours. https://www.machinerylubrication.com/Read/29114/dirt-holding-capacity

Fun fact, Donaldson is one of the few companies that make both stock size and oversized filters for same HD diesel engine and hydraulic systems applications under their BLUE / Duramax Synteq line of filters should space permit for one.

 

[ZeeOSix]

Here's another good Machinery Lubrication article, with some info about filter efficiency and wear. The take-away from Figure 1 is that with tighter journal bearings, it's more important to have better oil filtration. Modern engines do have pretty tight journal bearings, and the film thickness in journal bearings varies with oil viscosity, engine RPM and load, and therefore the bearing film thickness can get pretty small at times. Keeping as many particles out of the oil down to around 10u is beneficial. A filter that's rated at 99% @ 20u is going to be better for this than a much less efficient filter.

https://www.machinerylubrication.com/Read/31963/how-to-identify-and-control-lubricant-contamination