Cold starts and low viscosity

[Leo]

Hey guys.

As I understand, a thinner oil at startup means more lubrication. This because when you start up your car, the pressure relief valve is bypassing some of the oil.

So for a thinner oil, there is more volumetric flow at the same oil pressure? (Whilst the bypass is active).

Eg at 80psi ceiling, the relief valve will be bypassing less oil back into the pan with a thinner 0W than a 10W.

 

[Big Jim]

It's because thick oil does not flow as well as thin oil. The pressure relief valve has nothing to do with it.

 

[Leo]

Well the oil pump is positive displacement so the oil is going to get moved to wherever it needs to go, thick or not? But the volume flowed for the same pressure wouldnt be the same due to the bypass valve..

 

[labman]

As long as the pressure exceeds the bypass cut off, more of the thinner oil will flow to the the engine. The pump pumps nearly a constant amount of oil. If the pressure is high enough to open the bypass, not all of the oil flows to the engine.

 

[Drew99GT]

The pressure relief valve has everything to do with it. If the oil is too thick that the pressure exceeds the relief valve, then you bleed of flow back to the pan. Thinner oil can definitely help in this regard. By the way, why are we discussing cold flow properties of oil etc. when it is 95 degrees out? I'm contemplating going with a 20w-70 at the moment

 

[Winston]

What about the theory that a thinner oil leaves a thinner "film" of oil availible for cold start up?

 

[JAG]

quote:

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Originally posted by Winston:
What about the theory that a thinner oil leaves a thinner "film" of oil availible for cold start up?

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That is true so there are competing factors. Question is which factor is most important? Starting with a little more oil on parts but taking longer to get oil flow or starting with less oil on parts and getting flow sooner? All anecdotal evidence to me seems to indicate that getting flow sooner is much more important. Otherwise we'd be told to run thick oil in the winter for better cold starts and we know that would be the wrong move. You can get the clinging property with esters rather than getting it with higher viscosity. But as Molekule said in another thread, there is a certain amount of polarity that is ideal and above that things get worse. It will cling too much and want to creap through seal gaps. I think that's what he said anyway.

 

[Gary Allan]

I agree that it's probably a situation of "offsetting penalties" about film thicknesses. As far as bearings are concerned, you're under no load ..but your newer valve trains probably benefit the most from quick oil flow since their loads are somewhat 100% while they're running.


Anyway, if your pump is in relief you'll pump more thinner oil to the engine then a thicker oil. That is, 5w-40 in relief ..0w-20 in relief ...big difference in flow to the engine. Naturally it requires that you are in relief. Below relief ..you're at the effiency of the pump. They all leak a little ..but not enough for us to worry about.

My wife's engine is always in relief. Slammed up against it at startup and anything off idle. So she's got a variable flow based on visocity. If she wasn't at double the hot peak pressure that the OEM pump delivered, she would be at reduced flow.

 

[Leo]

Interesting. So one could get a pressure gauge and judge when the engine is in bypass at what RPM. You could say a motor like your wifes might benefit from a thinner oil? Especially at start up?