PRESSURE VERSUS FLOW

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Mini Oil Pump 2000-2006
Let's consider Pressure versus flow inside an engine. The size of any oil pump is designed to supply the correct amount of oil to meet the engine requirements. It produces a specific amount of flow at a given RPM. The resistance to the oil flow is from the bearing and lifter bore clearances.

When an engine is new, the clearances are tight and the pressure is good. As the bearings and lifter bores wear, clearances increase, resistance to flow decreases and oil pressure starts to drop.

When the pressure drops, we get our first signal that something is wrong in the engine. With the increased clearances/decreased resistance, the flow from the oil pump is at its maximum.

A few comments about the relief valve in an oil pump. The spring pressure behind the valve determines when it will open. If it is designed to open at 60-PSI, it does not have an effect on anything below 60-PSI, unless it sticks open. If it does stick open, the pressure is low at idle, but builds up to 60-PSI at the point the valve.

If an engine has excessive clearances in anything but rod bearings, the oil pressure will be uniformly low throughout the RPM range. If the rod bearings have excessive clearances, the oil pressure will be low at idle and will get worse as the RPM increases. Rod bearings turn in a circle rather than on axis, so they are subject to centrifugal force trying to pull the oil out of the bearing. The amount of loss will vary depending on the actual clearances in every bearing and lifter bore.

This gives you an idea of how much flow increases with a small increase in clearances and how pressure can drop when we exceed the flow available from the pump.
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Oil Pumps

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