Relationship between VI and VII

[OilUzer]

What is the relationship between Viscosity Index (VI) and Viscocity Index Improvers (VII) if any at all?

-1-
VI is change of viscosity with variations in temperature ...
The higher the VI, the smaller the relative change in viscosity with temperature and high VI is good.

-2-
Based on some datasheets that i have looked at, it seems like that 5w30's have higher VI than 10w30's.
Q: is this true in general and for all oil brands?

-3-
10W-30's have less VII (not relevant here but also less shearing) compared to 5w30's.
Q: is this true for all oil brands?

so my original thinking was that 10w30 should also have a better VI than 5w30 but that does not seem to be the case!
there is disconnect in regards to VI and VII relationship that i don't understand. maybe there is no connection at all!

Can one of the experts please explain?
I tried to google it but didn't find anything quickly!

Thank you.

 

[OilUzer]

Please don't let the word "expert" scare you! that has never stopped me ...

 

[ZeeOSix]

http://www.machinerylubrication.com/Read/1327/viscosity-index-improvers

 

[OilUzer]

Thanks for the link re VII. my main question is:
if 10w30's have less VII than 5w30's ... why is the VI of 5w30's (which have more VII) are typically higher than VI of 10w30 of the same oil? e.g. IIRC, Pennzoil Platinum PurePlus 5w30 has VI of 170 and Pennzoil Platinum PurePlus 10w30 VI is 150. I have seen this with some other brands as well ...
like I said maybe there is no connection between VI and VII !!??

 

[Shannow]

Viscosity Index "Improvers" IMPROVE the Viscosity Index, i.e. make it higher.

Dino base oils will have natural VIs around the 100-110.
Synthetics 130-140...anything more than that is probably enhanced with VIIs (and lower basestock viscosity).

a 10W30 will have (typically) less than a 5W30.

 

[OilUzer]

I see, it sounds very simple the way you explained it ... I have a hang up somewhere and need to think about it.

I was going based on this definition:
The higher the VI, the smaller the relative change in viscosity with temperature ... or
The lower the VI, the more the viscosity is affected by changes in temperature ...


and assumed that 10w30 DOES have a "smaller relative change in viscosity with temperature" as oppose to 5w30 and therefore should have a higher VI than 5w30. I think this assumption is incorrect. Need to think about it. Thanks

 

[Shannow]

Ahhh....

Give me 5 mins...

 

[Shannow]

Thanks given to Widman's charting tool.



See how the SAE 30 (monograde) has a much higher viscosity at 40C than the others, then the 10W30 and the 5W30 ?

The "flatter" the curve the higher the "Viscosity Index". The monograde has a VI of 110, but meets the SAE30 grade at 100C. The 10W30 has a VI of 134, and meets the SAE30 grade at 100C. The 5W30 has a VI of 151, and meets the SAE30 grade at 100C.

I think that you are getting the "W" reconfused again.

The second number is the number that's "operating temperature", so a 30 is a 30 is a 30 (will not go into HTHS).

Higher viscosity index means that the viscosity at 40 C is lower, i.e. it thickens less from oeprting viscosity.

"W" grades are the limits of pumpability at low temperatures.

 

[Virtus_Probi]

I think Shannow's chart is very useful and I hate to be the quibbler, but I'm thinking the Citgo 10W30 (not the Dino) must be mislabeled with a viscosity of 6.7 cSt at 100C? Isn't that an XW20?

 

[Shannow]

Originally Posted By: Virtus_Probi
I think Shannow's chart is very useful and I hate to be the quibbler, but I'm thinking the Citgo 10W30 (not the Dino) must be mislabeled with a viscosity of 6.7 cSt at 100C? Isn't that an XW20?


:blech: good pick-up.

Was typing "10W" and the fingers added the 30, as that was the next train of though coming on the split screen.

 

[OilUzer]

Shannow,
Thanks for taking time to explain this. I really aPennzoil Platinumreciate it!

I had to read your post twice (its getting kind of late for me) but it makes sense now.

Originally Posted By: Shannow
I think that you are getting the "W" reconfused again.

You are right but it is now clear ... btw, you have a very good memory. lol

unrelated to my original question:
I suPennzoil Platinumose in an ideal world, the curve would have a slope of 0 (flat line) and VI = infinity.
High VI is still good but after reading this, for example vi diff of 20 (150 vs. 170 - Pennzoil Platinum 10w30 vs. Pennzoil Platinum 5w30) is no longer as significant as I used to think ...

 

[Shannow]

Viscosity keeps parts separated in hydrodynamic lubrication.
Additives control wear when the parts get closer.
Additives typically work better at temperatures over around 70-75C.

So an oil that is the ideal operating viscosity and has no change with temperature would have to be coupled with an additive pack that worked well at (say) -40C...and they aren't around.

Viscosity helps during this warmup phase/

Note to detractors that's NOT saying to run SAE40 for cold starts.

 

[JAG]

I agree with Shannow, that an oil with a constant viscosity as a function of temperature is not ideal at preventing wear during the starting and warm-up phase, assuming a reasonable ambient temperature. It would help gas mileage though.

 

[OilUzer]

Originally Posted By: Shannow
... Viscosity helps during this warmup phase/

Note to detractors that's NOT saying to run SAE40 for cold starts.


i see what you are getting at. there is always a trade-off!

If the viscosity graph is a straight line (slope=0 and VI=infinity) then at very low temps, the oil may be "too thin" for its good.
i.e. (if I understand you correctly) the benefits of engine warming up quicker outweighs the benefit of having a very low cSt at very cold temperatures.

maybe in Alaska they should run SAE40 to warm up quicker! j/k

 

[Shannow]

It most definitely WOULD warm up quicker...more drag...assuming that it pumped first.

 

[fdcg27]

This has long been my understanding.
Until the oil gets warm enough to bring the add pack into play, viscosity is all you have.
That being the case, the relative thickness of the oil at start-up and through warm-up is a desirable quality.
One might thus question the value of high VI motor oils, which must necessarily contain a higher VII dose as well.
For a car routinely driven enough miles on a cold start for the engine oil to reach a normal operating temperature, as our cars typically are, a high VI oil would offer no advantage in fuel economy either.

 

[MolaKule]

Quote:
I was going based on this definition:
The higher the VI, the smaller the relative change in viscosity with temperature ... or
The lower the VI, the more the viscosity is affected by changes in temperature ...


In this statement they are speaking to the base oil only.

In a finished lubricant, the base oil, PI package, and VII ALL affect the final VI.