Which is better: High Viscosity Index or Low Viscosity Index?
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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.
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).
Also, lower weight multis (5w30) require a higher VI base oil than higher weight (15w40) multis with the same spread (25). That is if they use the same amount of VI improver.
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.
I believe the VI Improvers in the 0w30 are probably the most shear stable in the industry. Otherwise the Noack would be higher like Mobil 1 and the viscocity at 100C would be lower like Mobil 1.
Please feel free to correct me if I am mistaken.
You might have said the same thing I'm about to say, but I'm not sure, since you confused me a bit.
I think that a good synthetic basestock has a naturally high VI but it's not possible to get a wider spread like a 5w30, 5w40, 0w30, etc. without using greater quantities of VI Improvers.
As an example, Redline 10w30 has a VI of 137 and a NOACK of 5%.
Redline 5w30 has a VI of 153 and a NOACK of 6%.
Redline 10w40 has a VI of 159 and a NOACK of 6%.
To me that indicates that the 5w30 and 10w40 are using more VI Improvers to acheive their higher VI and are thus more prone to evaporation loss under high heat conditions.
[ August 07, 2003, 08:36 AM: Message edited by: harrydog ]
I think that a good synthetic basestock has a naturally high VI but it's not possible to get a wider spread like a 5w30, 5w40, 0w30, etc. without using greater quantities of VI Improvers.
As an example, Redline 10w30 has a VI of 137 and a NOACK of 5%.
Redline 5w30 has a VI of 153 and a NOACK of 6%.
Redline 10w40 has a VI of 159 and a NOACK of 6%.
To me that indicates that the 5w30 and 10w40 are using more VI Improvers to acheive their higher VI and are thus more prone to evaporation loss under high heat conditions.
[ August 07, 2003, 08:36 AM: Message edited by: harrydog ]
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.
IMO, the shear stability of the VI improver used has little to do with the overall volatility of the finished motor oil.
Maybe the high NOACK numbers are not directly related to the amount of VI Improvers. The thermal stability of the base stock itself determines the evaporation rate I think.
The use of VI Improvers is related to shearing down to a lower viscosity (ASTM D4624)and to thermal cracking which causes varnish deposits.
The use of VI Improvers is related to shearing down to a lower viscosity (ASTM D4624)and to thermal cracking which causes varnish deposits.
The VI number really doesn't tell you a whole lot about an oil except about how its viscocity behaves between 40deg C. and 100 deg C with respect to two reference oils at those temperatures.
To get a better look at how well formulated an oil is you have to look at the Noack, pour point, flash point, kinematic viscocity at 0C and 100C. HT/HS
Large spread, low Noack usually means higher quality base stock and/or more shear stable VI improvers.
You can make a 5w30 with a 100 VI group I base oil but it won't end up with a 180+ VI finished oil.
Also, just because 2 oils may have the same VI, they may not behave the same at extreme low temperatures nor last the same length of time at high temperatures.
To get a better look at how well formulated an oil is you have to look at the Noack, pour point, flash point, kinematic viscocity at 0C and 100C. HT/HS
Large spread, low Noack usually means higher quality base stock and/or more shear stable VI improvers.
You can make a 5w30 with a 100 VI group I base oil but it won't end up with a 180+ VI finished oil.
Also, just because 2 oils may have the same VI, they may not behave the same at extreme low temperatures nor last the same length of time at high temperatures.
G-Man II,
You are correct in your definition and wulimaster was actually agreeing with you until he added the Noack piece. I think the most important thing in your post is the last part. If you have a choice between two oils of the same viscosity spread you want an oil that achieves the spread with less VI improver. The reason: an oil that depends heavily on VI improver will, over time, thin out, become more volatile and will put contaminants in the oil as the improver shears off and becomes detritus. What I don't believe is necesarily true is your statement that lower VI oils within a viscosity spread (e.g. 5W30) can be assumed to have less VI improver. Logically, that would seem to be the case, but VI merely addresses resistance to viscosity change with temperature...without knowing the blenders "raw material" specs you don't know for sure if he got the high VI with a "magic" base oil or with lots of VI improver. I'm thinking what wulimaster brought up may be a key to this: if the oil has a high Noack that makes me think that the oil is dependent upon more VI improver which over time is shearing off, leaving the oil thinner and more volatile and so you get more burn off.
My understanding is that a high VI is important for cold start performance. High flash point and low volality typicals are important for hot performance.
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