Constructed a test fixture to capture the crack pressure on the K&P type 15 filter, results were a slight dribble <20, > 20 psi the ball became unseated and flow occurred.
Really nice work Don.
Ballpark, the range is 12 to 15 psi for the bypass built in to a standard Lycoming horizontal adapter (use a 48110 filter), and the same for the bypass inside a Champion or Tempest 48108, as used (for example) on a B&C right angle adapter.
Auto filters can vary; assume nothing. The Wix 51515 (NAPA Gold 1515) opens its bypass at only 7 to 9 psi. I think makes it a questionable choice, in particular with 50w oil, where it's probably bypassing a lot when cold. If you wish to use an internal bypass auto filter and your application will accommodate some additional can diameter, look at a Wix 51601 to get a 13 to 19 psi bypass. Look at a Wix 51647 if no internal bypass is desired, i.e. it's going on a standard Lycoming horizontal adapter.
I don't think there is any issue with a 20 psi bypass in the S15 screen filter. I assume the manufacturer is confident the element won't collapse at less than 20 psi, so the worst case is simply unfiltered oil flowing through the bypass if the screen should clog.
There is one interesting variation in design. The aviation filters and the S15 locate the bypass valve at the far end of the element. When the bypass opens, oil flows along the length of the filter OD to get to the bypass. At 7 gallons per minute, that oil is going to sweep trash off the surface of the pleats if it is not firmly embedded. The Wix filter is the opposite, locating a modular bypass valve just inside the filter base, thus there is no flow across the surface of the pleats when it opens. It packages a rubber drainback valve in there too. Module and drainback disk below.
So, would we all feel secure in using these KP filters?
Big picture, I don't think there is a safety question. That said, I sincerely doubt the S15's screen will filter oil down to a micron level as low as any decent fiber filter. And as noted previously, the K&P "technical page" would make P.T Barnum proud, claiming superiority based on an entirely inappropriate standard which few customers will actually read. Here is a link, if you are so inclined:
https://www.danhorton.net/VAF/Reusable Oil Filter/ASTM F-316.pdf
The two major standards for engine oil filtration are SAE J3236 (formerly J806) and ISO 4548.
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