It would be pretty easy to test for the impact of an internal leak on efficiency with an official ISO 4548-12 test. And you wouldn't need anything close to 30 or more samples. People that design and test these filters for efficiency don't test 30 or more. Ascent tested one sample of 5 different oil filters and the ISO 4548-12 efficiency data pretty much matched what the manufactures claimed, and they also ranked in the same order as their claimed ISO efficiency from the manufacturers. Now if one or more came out not matching close to what the manufacturer claimed, then another one or two would have to be tested to see if it tested differently. And the one that didn't pass would have to be cut and inspected of course. If you read that big Ascent testing thread, you would also see that a bubble point test is done before conducting the ISO 4548-12 test. Ascent bought two filters of each model, and used one set for the bubble point test. It think the pairs were the same date code or very close to each other - I'd have to go back and read the thread again. I have way more confidence in an official ISO test on one filter vs a test like BR does on one filter.sorry to be "that guy".... but you need a bare minimum sample of 30. Preferably >100. Ideally 500.
For fleets, it's a no-brainer to get a "scientific" answer. For Normies, we only have flame fights on the internet, unless someone is willing to start a new season of "Mythbusters"
If you wanted to be certain of the correlation, you could take four filters, open them up and verify at least two had measurable gaps at the leaf spring, and take the other two and 100% seal the leaf spring to end cap interface. Put the filters back together and test them per ISO 4548-12. Fram has done this type of testing in their lab on competitor's filters, and one competitor's filter that has pretty low efficiency was found to be leaking internally. When they fixed the leak and put the guts back in the can the efficiency was improved.
The ones with the internal leak are going to have a lower efficiency vs particle size curve. There is not way one that is leaking internally past the media is going to test as good as one that's not leaking.
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