While a relief valve with a lower pressure setpoint would open sooner, it would not have any effect on oil pressure BELOW the relief pressure setpoint. Raising the setpoint reduces the blow-off flow to the sump, which in turn increases the flow to the galleries. Higher oil flow has more pressure loss from fluid friction, which means the pressure "seen" at the pump is higher.
As explained earlier, the oil pressure relief valve only limits the maximum oil pressure and oil pressure is a reflection of pressure losses in the oil system in response to the oil flow generated by the oil pump. You were able to raise your hot oil pressure at idle by shimming the relief valve spring because your high volume oil pump is putting out so much flow at idle that it is causing the relief valve to continuously blow off excess pressure.
Since andyf found that he was not able to measure a significant variation in pressure at various points in his oil systems, the oil pressure losses in the galleries is normally small from the oil pump to the oil pressure gauge port. In your case, because the oil pump develops so much flow, the oil pressure at the pump is always at the relief valve's setpoint or greater.
Somewhere in your system, the oil flow at one or more branches from the oil galleries is relatively large compared with the flow supplied by the oil pump to the engine. The normal big block pressure gauge location is pretty much at the far end of the oil system and most of the pressure losses occur upstream of the gauge.
If I'm reading the oil system diagram correctly, this is the order that the following branches receive oil flow between the pump and the pressure gauge:
- crankshaft + camshaft bearing #1
- exhaust valve lifter #2
- intake valve lifter #2
- crankshaft + camshaft bearing #2
- intake valve lifter #4
- exhaust valve lifter #4
- crankshaft + camshaft bearing #3
- exhaust valve lifter #6
- intake valve lifter #6
- crankshaft + camshaft bearing #4 + L&R rocker shafts
- intake valve lifter #8
- exhaust valve lifter #8
- crankshaft + camshaft bearing #5
- left main oil gallery (lifters for cylinders 7,5,3,1)
I think you mean there is very little loss of
pressure because the oil galleries carry the
entire useful flow output of the pump.
