Originally Posted By: used_0il
Temporary shear can be described as the lining up of conventional VIIs in the bearing space caused by a high shear rate, reducing the viscosity of the engine oil.
The shear rate is determined by rpm, shaft diameter and bearing clearance.
If I have this understanding of temporary shear correct, would the following be true?
1. Under high loads during the temporary shear condition, the engine oil in the bearing space is reduced to it's base oil viscosity, which is the base viscosity of the complete engine oil blend, including additives and the un-activated VIIs.
2. A vehicle that is shifted under load, such as a drag race car, may have a higher viscosity requirement than an engine that is operated at a steady high load condition, such as a boat.
3. Engine oils containing VIIs should be avoided in vehicles that shift during a high load condition to a lower rpm.
Shear rate is also influenced by bearing radial load, as that affects the oil film thickness.
1. I think that's generally true, but I wouldn't go as far as saying the sheared viscosity is reduced to that of the base oil. Some VII's are better than others at resisting temporary shear.
2. I wouldn't want to make that conclusion until running an analysis. There are just too many variables to consider.
3. You're generalizing too much. Many engines are shifted under load, such as those that are bolted to automatic transmissions. All of them are subject to upshifts at WOT. No manufacturer specifies a different HTHS for engines based on the transmission it's teamed with.
Don't forget that HTHS is the defining characteristic that indicates which oils provide better protection under high operating stress. Temporary shear just tells the difference between viscosity under low and high shear conditions. If somebody tells you that the oil in the bottle he's trying to sell you doesn't shear at all, you still don't know if it has high enough HTHS to form a thick enough oil film for your engine.