Viscosity Index. When is the VI considered high.

[alcyon]

Another question related to NOACK.
Lets say I have a Pennzoil conventional SAE30 which is a group II no VII oil, and on the other hand a SHell Group3 5W-40 oil with some VII.
Would it ba safe to say that the Pennzoil conventional 30 should have lower NOACK values?

 

[SonofJoe]

Here's my two pennethworth...

Higher VI is a good thing. Higher VII is generally a bad thing, not just because of shearing but because directionally pistons get dirtier, the more VII you have.

VI, whether you get it from better base oil or more VII polymer, increases the wider the viscosity grade (ie VI increases as you move from 5W20 -> 5W30 -> 5W40 etc)

For any given viscosity grade, the higher the VI of the base oils you use, the lower will be the Noack of the finished oil. Conversely, for a given viscosity grade, the more VII you include, the higher the Noack will be.

It's not exactly true (because of the variations in J-300) but all things being equal, oils with a similar cross-grade spread will have similar VIs (ie a 5W20 will be similar to a 10W30 which will be similar to a 15W40)

In oil design, VI is a floating parameter. You get what you get because other parameters a fixed. VI is calculated from KV40 and KV100 but oils are generally defined by their high temperature viscosity (either KV100 or HTHS) and low temperature viscosity (CCS).

VI tends not to be massively impacted by the type of VII polymer you use (high SSI or low SSI or by OCP or Hydrogenated Styrene-Butadiene/Iso-prene). The one exception to this rule is PMA which, for a given KV100 & CCS, will always give you a lower KV40, and consequently a higher VI, than other VII types.

 

[userfriendly]

One would think then, that mono-grades would be less expensive than multi-grades, but that does not seem to be the case.
My grandmother, born in Dumfries called expensive things "dear", where as Americans call them "spendy".
Class, please make a sentence using the words dear and spendy.
My dear mother must have been bi-polar, as on a shopping trip, she was spendy enough for ten women.

 

[SonofJoe]

Originally Posted By: userfriendly
One would think then, that mono-grades would be less expensive than multi-grades, but that does not seem to be the case.
My grandmother, born in Dumfries called expensive things "dear", where as Americans call them "spendy".
Class, please make a sentence using the words dear and spendy.
My dear mother must have been bi-polar, as on a shopping trip, she was spendy enough for ten women.


In terms of costs of manufacture, monogrades are indeed cheaper to make than the equivalent multigrade. Things go up and down in price but typically you're looking at base oils costing hundreds of $US/MT and additives costing thousands of $US/MT. Not only do monogrades not contain VII polymer, generally speaking you need less DI additive to mitigate the problems caused by VII. However it's a classic feature of the engine oil market that the relationship between what stuff costs to make and what it retails for borders on the non-existant.

 

[BrocLuno]

For lots of motors in mild climate scenarios, I'll take a mono-grade any day. especially in boat engines where there is no real "off load" time. As the RPM goes up in most marine engines, the load goes up too and mono's are very nice (assuming quality oil).

My journey was much like Shannow's and Jetronic's. I'm now down to a spread of 15 as in Delo SD 15w30 as about the best for serious engines with some real cam and spring pressures in them). Either narrow spread or mono-grade. No wopping big spreads except for mild OEM motors

 

[Ken_K]

This viscosity index discussion from 2016 is one of the most educational pieces I've read. Answered several questions for me. Just bumping in hopes it helps other too.