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- Sep 28, 2002
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There is one rational reason for different bypass settings for specific applications from my point of view. If the OEM spec is to have the oil pump near or continually in the relief state (some engines start off at 45lb and stay at 45lbs ..they're just in varied states of relief) then a higher bypass setting would assure more filtered flow under that condition. There will be a higher potential for PSID due to viscosity.
Hmmm...I can't see anyone designing an engine like this. I mean, your counting on a commodity part that's continually replaced, and thus susceptible to wrong part selection. I'd rather think they just like to cover transients where other designers may accept a little bypass.
Sure ..mine was only an offered rationale. Something like an Audi/VW with their potential for extreme conditions.... Those are, with substantial distinction ..well above the range that we see most OEM specific bypass settings. One application may have a 16 psi bypass ..perhaps one a 20. Not that far out of bounds with the upper end of Purolators standard bypass. My observations that the PSID is the highest when the pressure/flow output of the pump is out of alignment (xflow to engine y flow to relief) would tend to leave the higher bypass setting applications in bypass less often where pump output and engine flow are not matched. This would happen, in my whacked sensible view (an oxymoron?) in pushrod engines ..where the pump volume may not be directly related to the relief setting. The relief setting may be limited to the long term sustainable stress on the distributor drive ..while volume may be taking long term needs into account. Jeep inlines all have the same OEM MAX relief setting. 75lbs (assumed stress limits of the drive) ..but produced pressures can range anywhere from 13-75 (with qualifications for rpm/temp/etc).
Just to give an example ..and it's totally contrived since I'm not running an OEM pump, my wife's oil pump is in relief with anything over a 20 weight off idle hot. So that means that I'm in various states of elevated PSID due to viscosity 24/7 off idle. Higher when cold (and compounded by visc between 30/40/etc)..reducing as the oil warms. If the threshold is low enough and the viscosity high enough ..I'll have a much higher potential for filter bypass.
You don't see it often ..and, admittedly, I haven't seen it in awhile..but there were/are some generic pushrods that just had a static oil pressure.
Perhaps it would be wiser to state it the other way around. If you've got an engine where the hot normalized pressure the the max cold pressure are very close to one another under most conditions ..the higher the bypass valve setting ..the less likely you are to be in bypass ...
..and leave it at that. The rest is my speculation on the given reasons working "backwards" from observations.