Al,
hydraulic pressure is exactly the same throughout any container that is being pressurized if the fluid is not moving. Think brake system. Doesn't matter if it's a round pot (oil filter) or a long system of pipes like the oil passages in a motor or a master cylinder and wheel cylinders. If it is moving, then friction along the pipe walls and at corners and such reduces the pressure the farther along it moves. Net effect is that once the pressurized oil leaves the pump, it can only decrease in pressure as it moves through the system. So if the guage reads 40 psi, and it's located at the back of the block near the end of the cam galley, then EVERYTHING between it and the pump has to be AT LEAST 40 psi. That includes inside the oil filter.
A good example to consider is the air-cooled VW engine. It has no spin-on filter (stock) and no bypass valve, only a pump and a pressure relief valve. You can purchase filter kits that mount where the stock oil cooler bolts to. BUT in order to use them, you MUST install a different relief valve spring, or you will explode the new filter. That's because the stock pump puts out about 100 psi or more. The filters will split open around 80 to 90 psi, so they use pressure relief springs set aywhere from 60 to 80 psi.
The same theory applies to our L6's. The only difference is that the L6 has the bypass valve which opens if the flow through the filter is restricted. That means nothing more than the oil will go through the valve (and bypass the filter) if the pressure drop through the filter exceeds the valve unseating threshold. The backside of the bypass valve is the same passageway as the filter outlet passage.
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David
newbie #4153
