VII shearing - what then?

[NoGoal]

Viscosity Index Improvers shear under stress. What happens to them? Do they just float around in the oil, getting in the way? Do they have friction reducing properties, or anti-wear properties?
A related question: is a 5W40 with a high % of VII, that shears to a 5W30 somehow better/worse than a shear stable 5W30 that has a lower % of VIIs and stays in grade (assuming they both have a simlar initial HTHS, say 3.5)?
Just curious.

Curiosity killed the cat. I am Schrodinger, and can be thought of as both flamed and unflamed for the moment.

 

[frankbee3]

I think that is a great question. My understanding is that the VII polymers can produce deposits in the combustion chambers. Some VII’s are better than others. I am still a fan of mono grade oils for that reason. That said, I am using multi grade oils, especially during winter.

I am interested in the answers that show up in this thread.

 

[JAG]

When a VII breaks apart, its multiple pieces have less thickening ability than prior to the breakage. They do just float around, partly adding to the viscosity. Importantly, in an oil that has undergone permanent shearing, in terms of percentage loss, the HTHS suffers less than the kinematic viscosity. With common polymeric VIIs, but not all polymeric VIIs, the percent drop in HTHS is roughly 1/2 of the percent drop in KV100.

Friction reduction properties of broken VIIs: overall in an engine, due to being broken, that tends to reduce friction.
Antiwear properties of broken VIIs: only in the sense that relative to them not being there at all, yes. Relative to prior to them being broken, no.

A related question: is a 5W40 with a high % of VII, that shears to a 5W30 somehow better/worse than a shear stable 5W30 that has a lower % of VIIs and stays in grade (assuming they both have a simlar initial HTHS, say 3.5)?

I recently read a study on that. I think there is a thread about it here. They subjected the oils to a wide range of shear rates that an engine does that is run through low and high loads and rpms. Shear rates went lower and higher than the shear rate of the HTHS test. The shear stable 5W-30 oil behaved as overall more viscous than the xW-40 with similar HTHS viscosity due to the latter having more permanent and temporary shear. That is, the fuel economy was worse with the shear stable 5W-30. At very high shear rates, the effect of the VIIs on viscosity gets very small. Some polymeric VIIs can increase deposits in various locations/conditions. Gokhan said the PMA type are especially prone to that. Complicating matters is some dispersants are also VIIs, so do they increase or decrease deposits? I’ve seen sales literature from manufacturers of them that claim they decrease deposits, but I don’t take their word for it given their financial investment in the matter.