OK, I'll give this a shot. Someone please correct me if I'm wrong...
DD91: First, click the link below to check out Britney Spears in a traditional "spread eagle" position, although those rose pedals covering her privates really **** me off... (Well, actually it's three photos of a dissected Mitsubishi oil filter).
http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=6;t=000577
In the middle of the BOTTOM image is a dime-size circle, which looks like it's part of the larger stamped steel cap. It's not -- it's actually a coil spring-mounted plunger which progressively moves into the cartridge when oil pressure pushes against it. In the MIDDLE image, inside the cartrige you'll see a small steel dome which houses the coil spring for the plunger.
So here's how it works, with a bit of beer-fueled speculation on my part. Oil is forced into the outside of the filter housing through the numerous small holes along the base. Oil under pressure pushes against the media and permates it, as well as pushes against the OUTSIDE of the plunger. Oil flows through the media and into the cartridge interior. Once there, it both flows into the engine's threaded inlet pipe, but also presses against the INSIDE of the plunger. If the filter media has zero flow resistance (which is impossible), the two pressures on opposite sides of the plunger would cancel each other out. But since the media restricts flow a bit, the pressure is greater on the OUTSIDE of the plunger than on the INSIDE, but not great enough to overcome the 8-22 psi strength of the typical valve's spring, so the plunger doesn't move. As the filter becomes clogged over time, there's less flow through the media, and less pressure against the INSIDE of the plunger while pressure on the plunger's OUTSIDE remains the same as before; therefore the plunger opens a bit. Twelve months later (lazy %#/* owner!!), the filter is almost fully clogged with little flowing through it, and therefore little pressure pushing against the INSIDE of the plunger. So the pressure on the OUTSIDE of the plunger fully overpowers the spring, and the plunger fully opens. So a "8-22 psi bypass valve" means that the valve will open once the pressure INSIDE the cartridge is 8-22 psi less than the pressure OUTSIDE the cartridge, caused solely by media clogging and restriction. As for the significance of an engine's designed pump pressure, I don't see any great relevance to the filter clogging process described above, correct...?
Geez, time for another Sam Adams, maybe two after all that thinkin'. Hopefully I got this thing right...
P.S. The stamped steel spring in the TOP photo simply locates the filter cartridge in place within the shell (some people mistake this for a bypass spring), and the large black gasket in the MIDDLE image is the anti-drainback valve. You can see how the gasket rests against the five small filter inlet holes, preventing oil from draining back out of the filter, emptying it out, and causing lag-behind oil pressure and lifter chatter at start-up. When operating, the oil pump forces oil through the five holes and past the gasket. This quick photo shows how an ADBV interfaces with the base plate:
http://www.ntpog.org/reviews/filters/napas4.jpg
[ November 07, 2003, 08:34 PM: Message edited by: TC ]
