Shearing and HTHS #'s
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"" what causes more shearing on oil X vs. oil Y?""
??? if you mean what causes one oil to shear more than an other not counting fuel then it is the VII's used both the shear stability or SSI othe VII and also how MUCH is used.
bruce
??? if you mean what causes one oil to shear more than an other not counting fuel then it is the VII's used both the shear stability or SSI othe VII and also how MUCH is used.
bruce
Are they both the same labelled viscosity, i.e. 10w30 vs 10w30?
base oil, HTHS, VII all play an important role in how an oil performs in an engine in regards to shearing.
HTHS will always be a function of how shear stable the oil is, not the reverse.
The quality of the base oil, as well the quality of the VI improvers (if any are used), will determine how shear stable the oil is. The term "shear stable" itself can have various meanings.
Permanent shearing results when the molecules of the VI improvers are so compromised that they are unable to regroup and continue their ability to thicken the oil at higher temps. Permanent shearing can also occur (to a much lesser degree) when the base oil molecules themselves are chewed up. This phenomenon generally occurs in transmissions and gear boxes. Temporary shearing occurs at critical high load areas in the engine such as bearings, the cam/lifter interface, and the upper ring area. Here the molecules of the VI improvers can align themselves in such a way that that the viscosity the friction area sees is significantly less than what the oil would be elsewhere in the engine.
IMO, the ideal oil is one with the highest VI possible without the use of VI improvers, no matter what the desired finished viscosity may be. This is the path taken by the blenders of oils used in Formula 1.
The quality of the base oil, as well the quality of the VI improvers (if any are used), will determine how shear stable the oil is. The term "shear stable" itself can have various meanings.
Permanent shearing results when the molecules of the VI improvers are so compromised that they are unable to regroup and continue their ability to thicken the oil at higher temps. Permanent shearing can also occur (to a much lesser degree) when the base oil molecules themselves are chewed up. This phenomenon generally occurs in transmissions and gear boxes. Temporary shearing occurs at critical high load areas in the engine such as bearings, the cam/lifter interface, and the upper ring area. Here the molecules of the VI improvers can align themselves in such a way that that the viscosity the friction area sees is significantly less than what the oil would be elsewhere in the engine.
IMO, the ideal oil is one with the highest VI possible without the use of VI improvers, no matter what the desired finished viscosity may be. This is the path taken by the blenders of oils used in Formula 1.
If you are concerned about sheer stability, then you should also be concerned about temporary sheer as described by G-man:
It is my understanding that this flaw of temporary sheer (a.k.a. temporary viscosity collapse) is used to advantage by oil companies in order to get many of their oils to pass the energy conserving test. As such, I would think anyone who is concerned about viscosity sheer should avoid any oil labeled "Energy Conserving" with the exception of those synthetics that meet the EC requirement without using temporary sheer.
Compare HTHS between energy conserving oils and high mileage oils and I believe you will see generally higher HTHS for the high mileage oils as they typically don't carry the EC designation.
It is my understanding that this flaw of temporary sheer (a.k.a. temporary viscosity collapse) is used to advantage by oil companies in order to get many of their oils to pass the energy conserving test. As such, I would think anyone who is concerned about viscosity sheer should avoid any oil labeled "Energy Conserving" with the exception of those synthetics that meet the EC requirement without using temporary sheer.
Compare HTHS between energy conserving oils and high mileage oils and I believe you will see generally higher HTHS for the high mileage oils as they typically don't carry the EC designation.
According to Royal Purple technical folks shearing occurs when you run too thick of an oil in your motor. In other words...your motor is spec's for a 10-30 and you run a 10-40 or a 5-40. That was their "position" when my Racing 41 sheared to a low 30 weight
/semi-thread hijack.
When is shearing bad?
I see Blackstone comments all the time on Subaru turbo's and they are, for example, telling an STi owner to extend their OCI by 1,000 miles on a 5W30 4,000 mile OCI where the oil sheared to a 20 grade.
If the wear metals are normal for a trend are you increasing the chances of the oil failing in a turbocharged car? Or does this temporary shearing not really increase the likelihood of an engine problem due to insufficient film strength.
Footnote: All N. American Subaru's (turbo and n/a) list 5W30 as preferred for fuel economy and 10W30, 10W40, 30 grade, 40 grade and 20W50 are allowed for high temps and severe conditions. 5W30 is probably the most commonly used grade. This is probably part of the reason that Subaru of America is requiring 3,750 mile OCI's on all turbo's effective MY2008.
-Dennis
When is shearing bad?
I see Blackstone comments all the time on Subaru turbo's and they are, for example, telling an STi owner to extend their OCI by 1,000 miles on a 5W30 4,000 mile OCI where the oil sheared to a 20 grade.
If the wear metals are normal for a trend are you increasing the chances of the oil failing in a turbocharged car? Or does this temporary shearing not really increase the likelihood of an engine problem due to insufficient film strength.
Footnote: All N. American Subaru's (turbo and n/a) list 5W30 as preferred for fuel economy and 10W30, 10W40, 30 grade, 40 grade and 20W50 are allowed for high temps and severe conditions. 5W30 is probably the most commonly used grade. This is probably part of the reason that Subaru of America is requiring 3,750 mile OCI's on all turbo's effective MY2008.
-Dennis
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