Oil pressure and Oil viscosity

[PLDRIVE]

Originally Posted by olddog

The oil pressure reaches a maximum and never goes higher no matter how high the engine revs. This is because the oil pressure has lifted the pressure relief valve (PRV) in the oil pump. The PRV allows oil to flow back to the pump inlet or back to the pan depending on design, but the point is, the oil flowing out the PRV is not flowing through the engine. Once the PRV lifts, oil flowing through the engine (to the bearings) never increases.

The rpm, at which the oil pressure reaches it maximum, is the rpm where the volume of oil flowing through the engine reaches its maximum. So if the oil pressure hits a maximum of 65 psi at 2000 rpm the amount of oil flowing through the engine has reaches its maximum. You can rev the engine all you want and the flow will not increase. This means that at 8000 rpm the oil in the bearings will see 4 times as many revolutions as it did at 2000 rpm.

Now if you put a thinner oil in, and the engine hits its maximum oil pressure at 3000 rpm, instead of 2000 rpm, then there is 50% more oil volume flowing through the engine, at high rpm.

Oil thins by the shear action in the bearing (basically the speed difference of the inner and outer surfaces). The oil also heats up from this shear action. The longer the oil spends in the bearing the hotter and thinner it gets. It is quite possible to put a thicker oil in an engine and end up with thinner oil in the bearing at maximum rpm, because the lower flow of oil is allowing the oil to stay in the bearing too long.

It is my opinion that more engines have failed because the oil viscosity was too high than failed because the oil viscosity was too low. That said 60 wt oils is a waste of energy pumping oil through a PRV, at best. It may even cause a bearing failure, if the oil flow volume drops too low.

Very well put olddog.