HTHS vs VI vs Temp question

[supercity]

Take these 2 oils with the following information

Oil 1, HTHS 3.9, VI 165
Oil 2, HTHS 3.7, VI 180

As oil temp rises which one is likely to move from hydrodynamic to boundary lubrication first?

 

[GMorg]

It would depend on the lubricated surface. It depends on velocity of the surfaces, the clearances, and the load per unit area. Different bearings will transition to boundary lubrication under different conditions. Even if you state "for the same bearing", it still depends on the bearing..or cam..or ring..etc. There is no answer to the question without specifics of the lubricated surface.

 

[CATERHAM]

Originally Posted By: supercity
Take these 2 oils with the following information

Oil 1, HTHS 3.9, VI 165
Oil 2, HTHS 3.7, VI 180

As oil temp rises which one is likely to move from hydrodynamic to boundary lubrication first?

Presumably you're referring to HTHSV @150C spec's.
In which case we need to know at what operational viscosity thickness in the HTHS measure does the transition zone occur and will it occur below or above 150C?
If it occurs at 150C or below, all other attributes being equal that's a no-brainer; it will be oil 2 since it is lighter at all oil temp's to well above 150C.

 

[A_Harman]

Oil 2, because it has lower viscosity at all temperatures that are likely to be encountered. (Like Caterham, assuming all other factors remain constant.)

 

[supercity]

I was thinking for elevated but "normal" bulk operating temperatures, maybe up to 130c. I see this maybe more simple than I thought. I was however under the impression that HTHSv @150C was an indication of what to expect in the bearings under "normal" bulk oil temps of maybe 80-100C. Are you saying that even with bulk sump temp at say 130C, oil will not exceed 150C in the bearings?

 

[GMorg]

With sump temps even below 130C, oil at the rings could be much higher than 150C. Again, without specifics of the surface in question, we're only guessing.

 

[CATERHAM]

Originally Posted By: supercity
Are you saying that even with bulk sump temp at say 130C, oil will not exceed 150C in the bearings?

I would say definitely not in the bearings; at maximum flow rates the oil is simply flowing through the bearings way too fast to pick-up much much heat from input to output of the crank but there are part's of the engine that will see very high oil temp's in the 250C area such as the rings.

That's one reason why high VI oils provide superior lubrication not just on start-up being lighter but also at very high oil temp's being relatively thicker.

 

[fpracha]

Originally Posted By: supercity
Take these 2 oils with the following information

Oil 1, HTHS 3.9, VI 165
Oil 2, HTHS 3.7, VI 180

As oil temp rises which one is likely to move from hydrodynamic to boundary lubrication first?

All other conditions being the same (for both oils) including vehicle type or surface being lubricated, Oil 2 will reach boundary conditions first.
The more important aspect is which oil will protect as boundary conditions set in.
Of course we have to assume that both oils formulation chemistry is similar, only viscosity being different.
Otherwise there is no one answer to your initial question!