Revisiting the importance of Viscosity Index...

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Patman posted a good topic a while back regarding Viscosity Index: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=000002#000000 Now, Bob explained the "meaning" of Viscosity Index pretty well, IMHO. However, I still think that Patman's question was left unanswered:
quote:
Or does having a higher number not necessarily always equal a better oil?
Let's pick on Mobil SuperSyn for example. 0W40 has the highest VI of 185. Could it be that 0W40 has a high inherent VI, OR did it have the same VI as the others (5W30, 10W30) and Mobil decided to inject a huge amount of VII? If it is the latter, wouldn't the VII be less reliable and more prone to "disappear" since it is just an "additive". I hope I made some sense. Comments? Oz
 
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Actually I don't think it is all that important at all. Besides you can only compare data from oils of the same SAE visc grade.
 
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The way I see it is that it is most meaningful for monogrades as the VI is then more a characteristic of the basestock itself. In a multigrade, the VI can be designed by the blender - a really high VI could just mean that lots of improver is added. In the original post, Patman showed that a Redline oil had a lower VI - this could be a good thing as it shows that very little improver is in the mix and that it is their quality basestock that is responsible for satisfying the viscosity requirements of a multi. Do I have this right? [ August 29, 2002, 10:41 AM: Message edited by: con carne ]
 
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Dixie
A very wide range oil, like a 0w-40 or 5w-50 is going to tend to have a higher viscosity index. However, these grades tend to be less stable over time and have higher evaporation rates. When you start out with a low molecular basestock and add a significant percentage of VI modifier, this is what tends to happen. All things being equal, the 10w-30/15w-40/20w-50 grades will tend to have the best high temp properties, regardless of the type of base oil used. The idea of a 5w-50 is appealing, but in actual practice I favor the narrower range oils. TooSlick
 
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My understanding of the VI is that it measures the amount of change in an oil's viscosity over the 40 to 100 degree C range. So, within the same SAE grade, an oil with a lower VI would demonstrate less viscosity change, ie, less thinning. So, holding all else constant, it is better to have a lower VI. Is that correct? [Patriot]
 
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I've read that the viscosity index is a number showing the vis change from 40c-100c. Well, if this is the case, you have the numbers already with the vis at 40c and 100c in cst - right? what good is the viscosity index? I have noticed that oils with the same cst at 40 and 100 have a different viscosity index. Now i am confused also???
 

Jay

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I'd like to know how VI is calculated. I've heard that it's calculated between 40 degC and 100 degC, but how? I see that Neo's 0w-5 has a VI of 180! [ August 29, 2002, 10:58 AM: Message edited by: Jay ]
 

MolaKule

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The effect of temperature on viscosity is determined by the empirical equation called "Andreas" equation and is u = De^B/T, an exponential equation where: u is absolute viscosity (in centipoise cP), D and B are constants, T is absolute Temperature in Kelvins The kinematic viscosity is v = u/p, where u is above, v is kinematic viscosity in centistokes (cSt), and p is density of the fluid (oil), in mass/volume. The absolute or kinematic viscosity is measured at two temperatures and the constants are then calculated. The SAE and ASTM has another algebraic equation for determining the Viscosity Index. If you're interested, I can show you that one as well.
 

MolaKule

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Oz, I prefer Chicken Alfredo with a lot of garlic. [Big Grin] "The absolute or kinematic viscosity is measured at two temperatures and the constants are then calculated. The SAE and ASTM has another algebraic equation for determining the Viscosity Index." Sorry about the Geek explanation; here is a less Geek explanation. [Frown] What you do is measure the Kinematic Viscosity with a viscosimeter at two temperatures, usually 40 C and 100C. You take the SAE/ASTM algebraic equation and put the two temperature values you just measured into this algebraic equation and the solution to the equation or the answer you get is the Viscosity Index, or VI. The larger the resulting number, the less the oil THINS or THICKENS when the temperature varies or changes. The smaller the number, the more it thins or thickens with temperature changes. I will try and find the equation when I get home tonight. [Roll Eyes]
 

MolaKule

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TooSlick, "A very wide range oil, like a 0w-40 or 5w-50 is going to tend to have a higher viscosity index. However, these grades tend to be less stable over time and have higher evaporation rates. When you start out with a low molecular basestock and add a significant percentage of VI modifier, this is what tends to happen." A lot depends on the basestock's original VI. For high quality true synth bases (Group IV, V), the original VI will be so high that you may not need VII's. In synthetic blending, you might blend a 4.o cSt PAO with a VII of 210 (25%), an 8.0 cSt PAO (say 25%) with a VI of 160, a 30 cSt Pao (say 45%) with a VI of 135, and a 2.0 cSt ester (5%) with a VI of 225, and pour in your additives. Considering the small thickening effect of additives, and you would have a VII of 185, and a multigrade oil of say SAE 5W40. Since each of the base fluids already have an inherently high VI, little or no VII needs to be added. If the base oil is temperature stable (high oxidation resistance) then you have have less volatility (tendency for oil and additives to evaporate). In most dino's, you start out with a low molecular basestock and add VII's to span the temperature range of operation. In blends, the addition of small amounts of PAO's and esters help the base stock's satbility and increase the VI without the addition of high concentration of VII's.
 
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Molecule, I understand the point you are trying to make, but I stand by my previous statement. Here is just one example - I could show you many more: Amsoil 0w-30: Noack 9.2% Amsoil 5w-30: Noack 6.9% Amsoil 10w-30: Noack 6.6% Amsoil 0w-40: Noack 11.6% Amsoil 10w-40: Noack 7.0% Amsoil 15w-40: Noack 6.7% You will find this same general trend holds regardless of the oil manufacturer. The Mobil 1, 0w-30 will have a significantly higher evaporation rate than their 10w-30 synthetic for example. The other reason for this is that some of the VI modifier actually volatilizes at high temps. So minimizing the # of VI modifier also reduces the evaporation rate at high temps. The shear stability of some of the newer 5w-40 grades IS very good however. Delvac 1 in particular is very shear stable - I suspect the VI of the basestock blend is very high as well.
 
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Two questions. With those Amsoil Noaks posted how does the New Texaco Syntheic Group III 10/30 SL oil only have a NOACK a 4 percent? It's a Mineral,not a synthetic. Their 5/30 is 10 percent and the same oil in 5/40 8 percent for compaticon in that family Also,what is the percentage of a PAO added to a dino for the average blends? The more I learn,the less I [Smile] know
 
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It would be very interesting to see which test standards they are using when they come up with this 4% figure. It might still be called Noack but is it performed at the same temperature and time duration?
 

MolaKule

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Jay, The formula is specified by ASTM D2270-64, "Calculating Viscosity Index from Kinematic Viscosity." The actual equation is somewhat more complex, but graphs were developed to aid in determining the viscosity index from the two measurements at the two temps.
 

MolaKule

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TooSlick, What you're seeing is called "compromise." The blender has to compromise between various oils of various viscosities, their inherent VI's, and their associated volatilities. It's a complicated juggling act to say the least.
 

MolaKule

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Con Carne, There are two volatility tests: Here's a swipe from Schaeffer's 10W30 Blend - ASTM D-2887) Volatility 700°F % Evaporation Loss 5.4% NOACK Volatility % Evaporation Loss (ASTM D-5800) 12% Guess which one most oil companies would want to show? Amsoil shows only the (ASTM D-5800) while Schaeffer's shows both. So one should see which test is being preformed before making a volatility comparison. I prefer the (ASTM D-5800) since it is a much tougher test.
 
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