Quote:
Originally Posted by madmaxx
Flow stays the same either way. You oil pump is a positive displacement gear pump, it could care less what the temp is, it pumps the same amount at the same rpm every single time. If the oil is thicker the pressure goes up but the volume of oil remains the same. |
This is kind of true, but not exactly. In theory, a gear pump is positive displacement, but that is a relative term. Gear pumps are not 100% effecient.
There are clearances between the sides of the gears and the housing. Some oil is slipping through those gaps (both sides of the gears). Although the volume of oil that the teeth forward is constant, what slips back through the pump is not, and therefore the ouput of the pump at a given rpm will vary with pressure. Imperically I have found that the effeciency drops off by a factor of the delta pressure squared, when the suction side of the pump has adaquate head pressure to completely fill the teeth.
With an oil pump in an engine there is no head pressure, and the pump must suck the oil up out of the pan. Although I have no experiance with thiis, it would seem to me that with vacuum there could be voids or little bubbles with nothing in them. If so, then the density of the oil could vary, further reducing flow.
Imperical proof, put 50 wt oil in a modern overhead cam engine on a cold day and you will ruin the engine, because it will take too long to pump oil up to the cam. Flow is not constant regardless of pressure.