An engine oil system is very similar to a hydraulic brake in that pressure will equalize throughhout the system. In the case of engine oiling, where the pressure is monitored will have a slight, but measureable difference in the gauge reading. The oil gallery that feeds the main bearings on a stovebolt six is the one that also contains the pressure gauge port, so the pressure going to a bypass filter will be virtually identical to the pressure seen by the rest of the system.
To assure that the oil pressure is sufficient on some of the Chevy V-8 round track race engines I build, I tap into the center oil gallery just above the timing cover, and install an adjustable grounding switch that turns on a red tractor-trailer clearance light on the dash when pressure drops below 20 pounds. The very early small block engines used this location to supply oil to the bypass filter on 55 year model 265's that did not have a block-mounted filter. This point in the oil system is as far as possible from the oil pump, so it gives a means of monitoring pressure after all the leakage past bearings and the oil supply to the valve system has had its effect. It's generally 10 to 15 pounds lower at racing speeds than pressure monitored at the port beside the distributor. The switch in the front port also warns of pressure drop caused by oil sloshing away from the pump pickup in a turn before the gauge can respond, and the big light gets a driver's attention in a hurry.
Jerry
