What is VISCOSITY INDEX ??
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Viscosity Index is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range. Higher numbers indicate a low change, lower numbers indicate a relatively large change. The higher the number the better. This is one major property of an oil that keeps your bearings happy. These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown.
You can only compare VI of like oils, that is XW-X to another XW-X. Foe example,you cannot compare a 5W-30 to a 10W-30.
Viscosity Index is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range. Higher numbers indicate a low change, lower numbers indicate a relatively large change. The higher the number the better. This is one major property of an oil that keeps your bearings happy. These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown.
You can only compare VI of like oils, that is XW-X to another XW-X. Foe example,you cannot compare a 5W-30 to a 10W-30.
All oils thicken when they get cold. All oils also thin out when they get hot. Higher viscosity index means less thickening when cold, and less thinning when hot.
So, let's say you have two oils that have the same viscosity at room temperature, but:
Oil A has a VI of 200
Oil B has a VI of 100
Now let's say you increase the heat to 100 degrees C. Oil B gets real thin. Oil A also gets thin, but not as thin.
Now, let's say you drop the temperature down to 30 below freezing. Oil B turns into goo. Oil A still flows.
Don't take this as literal truth (i.e. not all 100 VI oils turn to goo at 30 below), but that's the general idea of how VI works. Hope that was clear.
So, let's say you have two oils that have the same viscosity at room temperature, but:
Oil A has a VI of 200
Oil B has a VI of 100
Now let's say you increase the heat to 100 degrees C. Oil B gets real thin. Oil A also gets thin, but not as thin.
Now, let's say you drop the temperature down to 30 below freezing. Oil B turns into goo. Oil A still flows.
Don't take this as literal truth (i.e. not all 100 VI oils turn to goo at 30 below), but that's the general idea of how VI works. Hope that was clear.
Originally Posted By: FrankN4
You can only compare VI of like oils, that is XW-X to another XW-X. Foe example,you cannot compare a 5W-30 to a 10W-30.
Actually, VI would be a good way to compare a 5W-30 and a 10W-30.
It wouldn't be as meaningful with a 10w-30 and a 10W-40
You can only compare VI of like oils, that is XW-X to another XW-X. Foe example,you cannot compare a 5W-30 to a 10W-30.
Actually, VI would be a good way to compare a 5W-30 and a 10W-30.
It wouldn't be as meaningful with a 10w-30 and a 10W-40
I am not sure if this a fact so check it out , but usually equal oils like 10w-30 to use the grade for example . The higher V.I. probably has more viscosity improvers in the formulation.
Viscosity index is calculated by the 40C to 100C spread of the oil. A 10W-40 would have a different spread compared to a 10W-30, which would have a different spread compared to a 5W-30.
A well known conventional 5W-30 has a 40C cSt of 69.74, and a 100C of 10.67, and a VI of 141.
A well known synthetic 5W-30 has a 40C of 64.8 and a 100C of 11.3, and a VI of 169.
A well known synthetic 10W-30 has a 40C of 62 and a 100C of 10, and a VI of 147.
A well known synthetic 10W-30 HM has a 40C of 78.2 and a 100C of 11.79, and a VI of 145.
The synthetic 5W-30 starts lower and goes higher than the conventional so has a better/higher VI.
In the two 10W-30's, the HM starts higher and goes higher, has a greater spread, yet a lower VI. It also has a better spread than the 5W synthetic, yet a lower VI, so you could not use the VI to compare a 5W-30 and a 10W-30. The VI only tells you the spread of a oil within a given range.
Any time you know the 40C cSt and the 100C cSt, you can calculate the VI. It depends only upon the 40C to 100C spread, at least of American sold oils. Others may/can use a different spread.
If you know the VI, you can calculate the 40C or the 100C.
A well known conventional 5W-30 has a 40C cSt of 69.74, and a 100C of 10.67, and a VI of 141.
A well known synthetic 5W-30 has a 40C of 64.8 and a 100C of 11.3, and a VI of 169.
A well known synthetic 10W-30 has a 40C of 62 and a 100C of 10, and a VI of 147.
A well known synthetic 10W-30 HM has a 40C of 78.2 and a 100C of 11.79, and a VI of 145.
The synthetic 5W-30 starts lower and goes higher than the conventional so has a better/higher VI.
In the two 10W-30's, the HM starts higher and goes higher, has a greater spread, yet a lower VI. It also has a better spread than the 5W synthetic, yet a lower VI, so you could not use the VI to compare a 5W-30 and a 10W-30. The VI only tells you the spread of a oil within a given range.
Any time you know the 40C cSt and the 100C cSt, you can calculate the VI. It depends only upon the 40C to 100C spread, at least of American sold oils. Others may/can use a different spread.
If you know the VI, you can calculate the 40C or the 100C.
Originally Posted By: FrankN4
Viscosity index is calculated by the 40C to 100C spread of the oil. A 10W-40 would have a different spread compared to a 10W-30, which would have a different spread compared to a 5W-30.
A well known conventional 5W-30 has a 40C cSt of 69.74, and a 100C of 10.67, and a VI of 141.
A well known synthetic 5W-30 has a 40C of 64.8 and a 100C of 11.3, and a VI of 169.
A well known synthetic 10W-30 has a 40C of 62 and a 100C of 10, and a VI of 147.
A well known synthetic 10W-30 HM has a 40C of 78.2 and a 100C of 11.79, and a VI of 145.
The synthetic 5W-30 starts lower and goes higher than the conventional so has a better/higher VI.
In the two 10W-30's, the HM starts higher and goes higher, has a greater spread, yet a lower VI. It also has a better spread than the 5W synthetic, yet a lower VI, so you could not use the VI to compare a 5W-30 and a 10W-30. The VI only tells you the spread of a oil within a given range.
Any time you know the 40C cSt and the 100C cSt, you can calculate the VI. It depends only upon the 40C to 100C spread, at least of American sold oils. Others may/can use a different spread.
If you know the VI, you can calculate the 40C or the 100C.
Excellent answer and examples!
Viscosity index is calculated by the 40C to 100C spread of the oil. A 10W-40 would have a different spread compared to a 10W-30, which would have a different spread compared to a 5W-30.
A well known conventional 5W-30 has a 40C cSt of 69.74, and a 100C of 10.67, and a VI of 141.
A well known synthetic 5W-30 has a 40C of 64.8 and a 100C of 11.3, and a VI of 169.
A well known synthetic 10W-30 has a 40C of 62 and a 100C of 10, and a VI of 147.
A well known synthetic 10W-30 HM has a 40C of 78.2 and a 100C of 11.79, and a VI of 145.
The synthetic 5W-30 starts lower and goes higher than the conventional so has a better/higher VI.
In the two 10W-30's, the HM starts higher and goes higher, has a greater spread, yet a lower VI. It also has a better spread than the 5W synthetic, yet a lower VI, so you could not use the VI to compare a 5W-30 and a 10W-30. The VI only tells you the spread of a oil within a given range.
Any time you know the 40C cSt and the 100C cSt, you can calculate the VI. It depends only upon the 40C to 100C spread, at least of American sold oils. Others may/can use a different spread.
If you know the VI, you can calculate the 40C or the 100C.
Excellent answer and examples!
this was a private message response from a highly respected member of this site. Since it was a PM, I will not say who it was unless this person recognizes the response and wants to claim it.
"The VI is an indication of how the viscosity behaves over a large temperature span.
A perfect fluid would keeps its viscosity the same (such as 7.0 cSt for a DexronIII) from -50 F to 250F. A real ATF would thin to maybe 5.0 cSt at 150C and 45 cSt at lower temps.
A high VI number usually indicates that the the fluid is more stable in keeping its viscosity nearly the same over wide temperature excursions. A lower number indictaes that the viscosity varies more over wide temperature ranges.
The VI number is greatly affected by the type of base oil used and the VII, or viscosity index improver. "
"The VI is an indication of how the viscosity behaves over a large temperature span.
A perfect fluid would keeps its viscosity the same (such as 7.0 cSt for a DexronIII) from -50 F to 250F. A real ATF would thin to maybe 5.0 cSt at 150C and 45 cSt at lower temps.
A high VI number usually indicates that the the fluid is more stable in keeping its viscosity nearly the same over wide temperature excursions. A lower number indictaes that the viscosity varies more over wide temperature ranges.
The VI number is greatly affected by the type of base oil used and the VII, or viscosity index improver. "
Originally Posted By: lucerne06
so does it matter if a V.I. number is say 7 to 12 numbers apart? (like 155 to 164) for the same weight and type of oil? how much of a real difference is it between the two?
Lets create a 5W-30 and use the VI values you give. Lets start with a 40C cSt of 65 and a VI of 155. What we get is:
40C cSt...65
100C cSt..10.72
VI........155
Thins to 9.29 cSt(20 grade) at 224.42F
If we use the 164 VI, we get this:
40C cSt...65
100C cSt..11.09
VI........164
Thins to 9.29 cSt(20 grade) at 227.66F
Both start out at a 40c cSt of 65. The 155 VI thins out of grade at 224.42F. The 164 VI thins out of grade at 227.66F. The 164 VI stays in grade over a greater temperature range, but not much in this example. Does the VI difference of 155/164 really matter?
Only you can answer if a difference of 3 1/4 deg F in grade is worth it, or not. I know people that would totally reject the 155 VI. 99.999+% of people I know do not have a clue, are not even suspicious there is such a thing as a VI.
If the VI were greater, say a 139 for a conventional 5W-30 compared to a 164 for a synthetic 5W-30, it wouldn't be much of a contest.
so does it matter if a V.I. number is say 7 to 12 numbers apart? (like 155 to 164) for the same weight and type of oil? how much of a real difference is it between the two?
Lets create a 5W-30 and use the VI values you give. Lets start with a 40C cSt of 65 and a VI of 155. What we get is:
40C cSt...65
100C cSt..10.72
VI........155
Thins to 9.29 cSt(20 grade) at 224.42F
If we use the 164 VI, we get this:
40C cSt...65
100C cSt..11.09
VI........164
Thins to 9.29 cSt(20 grade) at 227.66F
Both start out at a 40c cSt of 65. The 155 VI thins out of grade at 224.42F. The 164 VI thins out of grade at 227.66F. The 164 VI stays in grade over a greater temperature range, but not much in this example. Does the VI difference of 155/164 really matter?
If the VI were greater, say a 139 for a conventional 5W-30 compared to a 164 for a synthetic 5W-30, it wouldn't be much of a contest.
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With this method, Oz oils seem to work as well. I always thought the oil was designed in a sense quality wise to achieve this, some better than others. BUt now seeing this, it doesnt mean much in the sense that a certain brand of oil may be designed to be slightly thicker at operating compared to another brand and its VI will be lower, which people may judge on a quality basis.
How did you find out the thinning rate at higher temps ?
How did you find out the thinning rate at higher temps ?
Last edited:
Originally Posted By: vxcalais
With this method, Oz oils seem to work as well. I always thought the oil was designed in a sense quality wise to achieve this, some better than others. BUt now seeing this, it doesnt mean much in the sense that a certain brand of oil may be designed to be slightly thicker at operating compared to another brand and its VI will be lower, which people may judge on a quality basis.
How did you find out the thinning rate at higher temps ?
I use the Widman graph.
http://www.widman.biz/Seleccion/Viscosidad/Conversiones/Graph/graph.html
When a blender designs an oil, he probably has a viscosity goal in mind. He may want to favor one end of the temperature range. I find a lot of PCMO in the 30 grade that have a 100C cSt from about 9.7 to 11.7. If he wants an 11.7 100C cSt, and also wants the pump/flow of a 5W, he has to select a base oil that gives the 5W characteristics. If it is going to be a high quality oil, he will also use the base oil that will meet the 100C cSt goal with the fewest VII.
The lower the 40C, and the higher the 100C, the greater the quality of the base stock must be to use the fewer VII. One company that I am familiar with has both a 5W-30 and a 10W-30 with the same 100C cSt. To accomplish this, they used a heavier base stock in the 10W-30(evidenced by the higher HTHS number) The 5W-30 could not use as heavy a base stock so it must have more VII. It would be more prone to the effects of shear.
I do not know very much about the overall effects of VII in PCMO. When the oil shears, what are the effects caused by the VII? Do they turn into sludge, deposits, or what. I do know that a 5W-30 can be made that has a better cold flow and that also thins less at temperature than a 10W-30 with the same 100C cSt, BEFORE ANY SHEAR EFFECTS. I do know from an extensive analysis of UOA's that 5W-30 does shear more than 10W-30 so the 5W-30 advantage would be shorter lived, if both started at the same 100C cSt. I also see that a lot of 5W-30 start out with a higher 100C cSt, I assume to compensate, somewhat, for this.
With this method, Oz oils seem to work as well. I always thought the oil was designed in a sense quality wise to achieve this, some better than others. BUt now seeing this, it doesnt mean much in the sense that a certain brand of oil may be designed to be slightly thicker at operating compared to another brand and its VI will be lower, which people may judge on a quality basis.
How did you find out the thinning rate at higher temps ?
I use the Widman graph.
http://www.widman.biz/Seleccion/Viscosidad/Conversiones/Graph/graph.html
When a blender designs an oil, he probably has a viscosity goal in mind. He may want to favor one end of the temperature range. I find a lot of PCMO in the 30 grade that have a 100C cSt from about 9.7 to 11.7. If he wants an 11.7 100C cSt, and also wants the pump/flow of a 5W, he has to select a base oil that gives the 5W characteristics. If it is going to be a high quality oil, he will also use the base oil that will meet the 100C cSt goal with the fewest VII.
The lower the 40C, and the higher the 100C, the greater the quality of the base stock must be to use the fewer VII. One company that I am familiar with has both a 5W-30 and a 10W-30 with the same 100C cSt. To accomplish this, they used a heavier base stock in the 10W-30(evidenced by the higher HTHS number) The 5W-30 could not use as heavy a base stock so it must have more VII. It would be more prone to the effects of shear.
I do not know very much about the overall effects of VII in PCMO. When the oil shears, what are the effects caused by the VII? Do they turn into sludge, deposits, or what. I do know that a 5W-30 can be made that has a better cold flow and that also thins less at temperature than a 10W-30 with the same 100C cSt, BEFORE ANY SHEAR EFFECTS. I do know from an extensive analysis of UOA's that 5W-30 does shear more than 10W-30 so the 5W-30 advantage would be shorter lived, if both started at the same 100C cSt. I also see that a lot of 5W-30 start out with a higher 100C cSt, I assume to compensate, somewhat, for this.
VI is a measure of an oil's resistance to viscosity change with temperature change. As has been noted, it's based on a formula using the vis at 40C and 100C. A "perfect" oil would have the same vis at 40C and 100C and a VI of 1000. It would just as thin at -40C as it is at 100C.
(In a Newtonian fluid, viscosity change with temp is totally linear. In a non-Newtonian fluid (which all multi-grade oils are), viscosity change is not linear because the viscosity of the oil also changes because of shearing.)
(In a Newtonian fluid, viscosity change with temp is totally linear. In a non-Newtonian fluid (which all multi-grade oils are), viscosity change is not linear because the viscosity of the oil also changes because of shearing.)
Originally Posted By: FrankN4
Originally Posted By: vxcalais
With this method, Oz oils seem to work as well. I always thought the oil was designed in a sense quality wise to achieve this, some better than others. BUt now seeing this, it doesnt mean much in the sense that a certain brand of oil may be designed to be slightly thicker at operating compared to another brand and its VI will be lower, which people may judge on a quality basis.
How did you find out the thinning rate at higher temps ?
I use the Widman graph.
http://www.widman.biz/Seleccion/Viscosidad/Conversiones/Graph/graph.html
When a blender designs an oil, he probably has a viscosity goal in mind. He may want to favor one end of the temperature range. I find a lot of PCMO in the 30 grade that have a 100C cSt from about 9.7 to 11.7. If he wants an 11.7 100C cSt, and also wants the pump/flow of a 5W, he has to select a base oil that gives the 5W characteristics. If it is going to be a high quality oil, he will also use the base oil that will meet the 100C cSt goal with the fewest VII.
The lower the 40C, and the higher the 100C, the greater the quality of the base stock must be to use the fewer VII. One company that I am familiar with has both a 5W-30 and a 10W-30 with the same 100C cSt. To accomplish this, they used a heavier base stock in the 10W-30(evidenced by the higher HTHS number) The 5W-30 could not use as heavy a base stock so it must have more VII. It would be more prone to the effects of shear.
I do not know very much about the overall effects of VII in PCMO. When the oil shears, what are the effects caused by the VII? Do they turn into sludge, deposits, or what. I do know that a 5W-30 can be made that has a better cold flow and that also thins less at temperature than a 10W-30 with the same 100C cSt, BEFORE ANY SHEAR EFFECTS. I do know from an extensive analysis of UOA's that 5W-30 does shear more than 10W-30 so the 5W-30 advantage would be shorter lived, if both started at the same 100C cSt. I also see that a lot of 5W-30 start out with a higher 100C cSt, I assume to compensate, somewhat, for this.
Nice response Frankn4
It is interesting when shear comes into play.
Originally Posted By: vxcalais
With this method, Oz oils seem to work as well. I always thought the oil was designed in a sense quality wise to achieve this, some better than others. BUt now seeing this, it doesnt mean much in the sense that a certain brand of oil may be designed to be slightly thicker at operating compared to another brand and its VI will be lower, which people may judge on a quality basis.
How did you find out the thinning rate at higher temps ?
I use the Widman graph.
http://www.widman.biz/Seleccion/Viscosidad/Conversiones/Graph/graph.html
When a blender designs an oil, he probably has a viscosity goal in mind. He may want to favor one end of the temperature range. I find a lot of PCMO in the 30 grade that have a 100C cSt from about 9.7 to 11.7. If he wants an 11.7 100C cSt, and also wants the pump/flow of a 5W, he has to select a base oil that gives the 5W characteristics. If it is going to be a high quality oil, he will also use the base oil that will meet the 100C cSt goal with the fewest VII.
The lower the 40C, and the higher the 100C, the greater the quality of the base stock must be to use the fewer VII. One company that I am familiar with has both a 5W-30 and a 10W-30 with the same 100C cSt. To accomplish this, they used a heavier base stock in the 10W-30(evidenced by the higher HTHS number) The 5W-30 could not use as heavy a base stock so it must have more VII. It would be more prone to the effects of shear.
I do not know very much about the overall effects of VII in PCMO. When the oil shears, what are the effects caused by the VII? Do they turn into sludge, deposits, or what. I do know that a 5W-30 can be made that has a better cold flow and that also thins less at temperature than a 10W-30 with the same 100C cSt, BEFORE ANY SHEAR EFFECTS. I do know from an extensive analysis of UOA's that 5W-30 does shear more than 10W-30 so the 5W-30 advantage would be shorter lived, if both started at the same 100C cSt. I also see that a lot of 5W-30 start out with a higher 100C cSt, I assume to compensate, somewhat, for this.
Nice response Frankn4
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