Oil pressure and Oil viscosity

[PLDRIVE]

Originally Posted by CHANMADD

Wait a minute......a multi garde oil ....10w60....is a SAE.10.oil with additives to make it lubricate like a SAE 60.When the additives break down you're left with a 10 weight oil. Very thin, 5 weight is like water. In our old engines I prefer 20w50. This is why with multigrades oil changes are essential.

Most cobra owners change oil once, or twice a season (1500 to 3000 miles) and never see 230 Degrees oil temp. Do u really think he is running his engine hard enough to create enough heat to break down his oil to 10w in this time period??????? Not even close.

Oil viscosity is based primarily on bearing clearance not because u have an "old engine" I would bet the farm he does not have a loose bearing package.

Cobra 53
Do yourself a favor and call a couple of lubricant specialist (Joe Gibbs racing, Brad Penn). Then report back on what they say.

[Dwight]

- Bob is the Oil Guy

[Turbonut48]

I have a 418W with 530 hp with a high volume oil pump.

I run 10-40w. My cold pressure is 65-70. Idle pressure when hot is never below 45. At any higher RPM its 65.
The engine builder initally recommended 20-50W.
I explained my climate in Wis. and they then said 10-40W would be good.

The Nut

[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.

[mrmustang]

For street driving, I'd suggest a good quality 10W40 and leave it at that. Synthetic or non synthetic is your choice. 20W50 if you are in a hotter climate while driving your car would not be out of the question.


Bill S.

[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.