FG uses ST filters typically, IIRC.
Terry's experience may be different then mine. My observations are on one engine. From that/those observations I've constructed my "belief".
There have been customers of Terry that have had lingering HLA noise reduced with the changing of filters. This may just be due to his engine specific knowledge. My observations are applied generically and surely have to be qualified.
I tried my best to get a fitler to twitch in PSID. It rarely happened. Not just bypass threshold ..but virtually anything over 2-5 PSID was never a common reading. I used highly restrictive filters (PureOne) and the smallest size available (the PH3614 size) in my 3.0 Mitsubishi engine.
The only substantial results that I saw was with very heavy oil (20w-50) in subfreezing temps (test performed @28F after an overnight low in the teens) with a 10k old filter (FL300 sized PureOne) that was sent to me by another member. Even that didn't produce enough PSID to break the bypass threshold of 12-16 on a PureOne. The loading did most of the contribution toward the elevated PSID. My newer, smaller, filter hardly even twitched.
HOWEVER, this is just on one engine. My theory, based on observations, is that you will see bypass activity most where the oil pump relief is open (which would be during cold starts and perhaps high rpm speed shifting). Many pushrod engines don't have "matched" oil systems. The pump volume and the pump relief may have no relationship to each other. The relief may be to limit stress on the pump drive ..and have nothing to do with the pump volume.
I would say that a bypass valve would be most active in an engine that has true pressure readings where the cold pressure and the hot pressure have very little difference. That would indicate to me that the oil pump is routinely in relief and then the oil flow doesn't have to resemble the linear models that are almost mandated when the pump is not relieved.
So, Terry can surely be correct, but without qualifying his statements ..there are surely exceptions. I, naturally, tend to believe and follow my self formed rules. Terry, who makes a whole lot more money then I do (not to mention training, higher education, and professional experience) surely follows his.
Keep in mind, aside from my toying around with this homegrown experiment, I've yet to see any SAE paper or other bona fide authoritative article that takes this topic on.
