Viscosity and Viscosity Index (VI) Calculation

nicrfe1370

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Ferrari, That is the trick of VI, from what I understand. The higher the viscosity index the less the viscosity changes over a range of temperature, since Amsoil's 0W-30 has a VI of 196, it will be thicker at higher temps than an oil with a lower VI and the same grade. [ April 09, 2003, 01:20 PM: Message edited by: nicrfe1370 ]
 

nicrfe1370

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It seems counter-intuitive, but when the effects of VI are understood and taken into account, then it can be quite logical. Amsoil 10W-40 has a VI of 183, 10W-30 has a VI of 167, so all else being equal, the 10W-40 will be thinner at low temps and thicker at high temps (particularly the high temps since it's a different grade). I have been making up some spreadhseets with graphs, I'm a visual learner so seeing graphs really helps me. I will try to post my graphs shortly for anyone to look at.
 
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quote:
Originally posted by nicrfe1370: Amsoil 10W-40 has a VI of 183, 10W-30 has a VI of 167, so all else being equal, the 10W-40 will be thinner at low temps and thicker at high temps (particularly the high temps since it's a different grade).
This isn't the case at all, since there are other variables that affect the VI of the oil. An oil that has a high VI because of a heavy dose of polymer VI improvers will be thicker at lower and higher temps than an equivalent Xw grade (with the same or lower VI) where the VI is achieved with a higher VI base oil blend and less VI improver. For oils where the finished VI is achieved primarily with the VI of the base oil blend and with equivalent Xw grades, the one with the higher VI will be thicker at lower temps. If your example of comparing the CCV of Amsoil 10w30 and 10w40 held true for all oils of equivalent grades, then Mobil 1 0w40 would be thinner at low temps than Mobil 1 0w30, but that isn't the case. Look at the spec sheet and you'll see the CCV of 0w40 (with a VI of 185) is 4301 @ -35*C, while the 0w30 (with a VI of 176) is 3800 @ -35*C. So with these two oils, the oil with the higher VI is thicker in the cold than the one with the lower VI.
 

Jay

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Originally posted by Ferrari: According to that calculator, the Amsoil 10W-40 is THINNER than the 10W-30 at -30*C. I don't believe that. Any thoughts?
But it is true. The 10w-40's kinematic viscosity is less at that temp and it's CCV is also less. [ April 09, 2003, 06:56 PM: Message edited by: Jay ]
 

Jay

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<b>G-Man II</b>, The VI is calculated from kinematic viscosity so the type of VI improvers, etc. don't come into play. CCV is dynamic viscosity where non-newtonian factors do have an effect.
 
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Originally posted by Jay: G-Man II, The VI is calculated from kinematic viscosity so the type of VI improvers, etc. don't come into play.
Of course they do if they affect the viscosity. Polymer VI improvers make a base oil thicker at ALL temps, not just high temps.
 

nicrfe1370

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I'm less sure about the VI being the sole determinant of how an oil will react at a given temperature. Here's an interesting tid bit that I found when I was exploring the possibility of using Mobil 1 0W-40. It has almost the identical viscosity as Amsoil 10W-40 at 40C, 100C, and has an almost identical VI. the Mobil's VI is slightly "better" (186 vs. 183) so this must explain how it can be classified a 0W while Amsoil is a 10W.
 
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Originally posted by nicrfe1370: I'm less sure about the VI being the sole determinant of how an oil will react at a given temperature. Here's an interesting tid bit that I found when I was exploring the possibility of using Mobil 1 0W-40. It has almost the identical viscosity as Amsoil 10W-40 at 40C, 100C, and has an almost identical VI. the Mobil's VI is slightly "better" (186 vs. 183) so this must explain how it can be classified a 0W while Amsoil is a 10W.
Not quite. The viscosity of the Mobil 1 0w40 at -35*C is how it can be classified as a 0wXX oil.
 

nicrfe1370

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G Man, yeah, that's what I was getting at but didn't say. Despite being identical to Amsoil 10W-40 at 40 and 100C, at very low temps (where they measure for xW grade) it would be thinner.
 
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If you plot viscosity as a function of temp, you get a curve and not a straight line. You can't take the VI, which is simply the change in viscosity between 40C and 100C and extrapolate the data to temps << 40C and >> 100C. It simply doesn't work that way! TooSlick
 

nicrfe1370

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TooSlick, Yes, that fact I have recently become aware of, the viscosity being a curve, not a straight line. But if you take the numbers Ferrari has put in a table based on the calculations from the linked page, and graph them, you do indeed get a curve, an exponential curve with the oils getting drastically thicker at colder temps.
 
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As I recall, the Amsoil 10w-30 is approx 3440 Cp @ -25C, compared to 4500 Cp @ -25c for the 10w-40 (the same spec sheet as the 10w-40 Motorcycle oil). The 10w-40 uses a heavier basestock, so this makes perfect sense. Ted
 

MolaKule

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Of course they do if they affect the viscosity. Polymer VI improvers make a base oil thicker at ALL temps, not just high temps.
The main purpose of VII's is to thicken the oil at higher temperatures where the oil would normally thin out. The VII adds about 0.5 to 2 cSt to the base oil's viscosity at 40 C, and adds higher viscosity at higher temps. At about -10 F and below, the VII has little effect on overall viscosity. Also, the VII's effect is highly dependent on the base oil's solvency.
 
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Originally posted by MolaKule: At about -10 F and below, the VII has little effect on overall viscosity.
MK, my point was that because polymer VI improvers are themselves a very high viscosity additive (from what I've read) the addition of them to the base oil will increase its viscosity at any given temp over the viscosity of the neat base oil. Is that not correct?
 
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