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Indeed, the oil filter does not determine system oil pressure. There is usually something of a regulator on the output side of the oil pump. Generally and ball & spring affair in the main oil gallery.
 

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Please correct me if i am wrong.

Oil pump>oil filter>engine parts.....and back to the oil pump.
But, any resistance in the system can affect the flow and pressure. In this case if the oil pump pumps the oil into the oil filter and the filter has less resistance, then the oil pressure must drop upstream regardless of any other resistance before or after the resistance(high flow oil filter).
See Bella749's answer and then also consider that the filter has FAR less resistance to oil flow than the places that the oil eventually goes to....

Let's say that the oil pump is pumping into a 1' diameter pipe going to the filter....

the filter is a 5 gallon can with 10 sq ft of window-screen as the filter medium....

the filter output goes into another 1" diameter pipe (the oil gallery) which has 6 holes along it's length. Each hole is 1/16" diameter, and all 6 together flow 3/4 of a quart per minute, and they each spray the oil onto some bearings in the motor.

At the end of the pipe is a 1/2" hole covered by a 3/4" ball bearing (pressure relief valve) that is held in place by a 30 lb spring.

The oil then free falls somewhere inside the engine cases, down into the sump where the oil=pump pick-up sucks it back into the pump.


If the pump puts out 1 quart per minute at up to 60 psi.... and you accept that a 1" pipe can carry 2 quarts per minute you will understand that:

1. the 1" pipes contribute no resistance to oil flow.

2. the oil filter is so large that it already also provides essentially no restriction (thus no contribution to oil pressure) to the system because it is designed to be too big to do that. It can only reduce down-stream pressure/volume if it gets plugged with crud.
It can increase the pressure from the pump to the filter however, if it gets plugged. In fact, up here in the great white north, it is not unknown for an engine to blow up it's oil filter in the winter if the oil is too heavy and dirty when it's -40F.

3. the 6 "oil delivery holes" produce most of the restriction to the oil flow from the oil pump while passing only 3/4 qt per minute.

4. the oil-pressure relief valve will be passing the other 1/4 quart of oil all the time (given that the 6 holes can't flow 1 quart per minute, and thus maintaining a constant 30 psi internal oil pressure. This allows for the oil filter to get dirty enough to restrict the through-put up to 25% (1/4 qt per min) before the engine sees a drop in internal oil pressure.

Hope this helps.... it kinda made me dizzy!
 
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