Originally Posted by SubieRubyRoo
It's probably highly filter-specific; but I'd say generally as the delta P goes up, efficiency likely goes down slightly.
For sure. Look at Figures 3 and 4 in this Machinery Lubrication article (link below). Then look at the efficiency graph the Purolator/M+H got in their efficiency testing vs media loading. You can see in all cases there is a point where the efficiency goes down with more media loading. That's because - depending on the media design and performance - as the pressure across the media increases, at some point the media starts to shed captured particles, which makes the downstream efficiency go down due to dislodged particles contaminating the downstream fluid.
That's why people who always throw out the "
oil filters get more efficient as they load up" haven't looked into it enough to be able to make that blanket statement.
https://www.machinerylubrication.com/Read/564/filter-beta-ratios
Figures from the Machinery Lubrication article:
Figure 3: Starts off with high efficiency when new, then loses efficiency as the delta-p increases. Similar to the Purolator/M+H data.
Figure 4: Starts off with low efficiency when new, then increases efficiency until the delta-p hits a certain point, then loses efficiency with increased dealt-p.
They also talk about "Beta Stability" which is used to define how much efficiency a filter loses as the delta-p increases across the filter.
In both graphs, there's a point where the delta-p starts dislodging captured particles in the media. I think all filters will have this characteristic to some degree - some worse than others. No filter is going to be able to hold every single particle that is initially captured, given the design of the media and enough delta-p.
IMO, full synthetic media tends to produce less delta-p across the media, and that also helps with keeping captured particles from breaking lose.
As mentioned before in these discussions, the ISO 4548-12 efficiency rating is the average efficiency over the length of the test (ie, new filter to nearly clogged filter). So if a filter has a high ISO 4548-12 efficiency that probably also means by definition that it does not shed particles as badly as it loads up as a filter that has a low ISO 4548-12 efficiency. If a filter sheds particles badly as the delta-p increases, it's not going to be stellar in the ISO 4548-12 test. A low ISO 4548-12 rating is a combination of less efficient media along with particle shedding on top of that.