VI is nothing more than an "indicator" of an oil's ability to resist viscosity change with change in temperature. In general, the wider the vis spread of a multi-vis oil, the higher the VI.quote:
Originally posted by Ken4:
I notice most 10w30s are around VI=150 while 0w40s are around VI=190. Does higher viscosity index mean better? Or lower viscosity index mean better for cold starts?
Your first sentence was correct but from there you went 180 degrees off. Higher VI base oils can cover a larger spread than lower VI base oils and therefore require less VI improver (modifier).quote:
Originally posted by G-Man II:
VI is nothing more than an "indicator" of an oil's ability to resist viscosity change with change in temperature. In general, the wider the vis spread of a multi-vis oil, the higher the VI.quote:
Originally posted by Ken4:
I notice most 10w30s are around VI=150 while 0w40s are around VI=190. Does higher viscosity index mean better? Or lower viscosity index mean better for cold starts?
When it comes to synthetics, my theory is that if an oil of a certain grade, say 0w30, has a lower VI than the 0w30 of another brand, the oil with the lower VI is relying more on the VI of the base oil blend to achieve the vis spread than on VI improvers. Hence, the oil with the lower VI uses LESS VI improvers. Ergo, the oil with the lower VI is the "better" oil.
I think my theory is correct. The shear stability of the VI improver and the overall volatility of the oil (as reflected in the NOACK number) are not, IMO, interrelated.quote:
Originally posted by YZF150:
Who is correct, G-Man or wulimaster?
The VI numbers you've posted are not the "base oil VI," they are the VI numbers of the finished motor oil. If you built a 5w30 with a base stock blend that had an inherent VI of 196, not only would you need absolutely NO VI improver, the finished oil itself probably wouldn't be a 5w30.quote:
Originally posted by wulimaster:
That is why you see Amsoils base oil VI go up as the spread increases.
10w30 VI 167 Noack 6.6%
5w30 VI 182 Noack 6.9%
0w30 VI 196 Noack 9.2%
Were the 5w30 to be made with the 196 VI basestock you would have seen a lower Noack than 6.9 %. A lot of the difference in Noack is the amount of VI Improver that is burner off.
G-Man II,quote:
Originally posted by G-Man II:
VI is nothing more than an "indicator" of an oil's ability to resist viscosity change with change in temperature. In general, the wider the vis spread of a multi-vis oil, the higher the VI.
When it comes to synthetics, my theory is that if an oil of a certain grade, say 0w30, has a lower VI than the 0w30 of another brand, the oil with the lower VI is relying more on the VI of the base oil blend to achieve the vis spread than on VI improvers. Hence, the oil with the lower VI uses LESS VI improvers. Ergo, the oil with the lower VI is the "better" oil.