Pumping viscosity test temps: _W

[vinu_neuro]

Why are pumping viscosities for various _W grades not all tested at the same temps? For example the 0W pumping visc is given at -40C, 5W at -35C, 10W at -30C. This makes it impossible to compare cold weather start-up performance between an oil such as Pennzoil 5W-40 vs. say Mobil 1 0W-40.

 

[d00df00d]

Correct. Instead, it lets you compare the lowest temp at which different oils meet certain limits of pumpability. You can still compare pumpability numbers between oils with the same W rating.

Not the ideal state of affairs, but such is life when you're working with products that have to be sold to people who know nothing about them.

 

[addyguy]

You can use a viscosity calculator (Widman) to calculate these amounts, I think.

I think the 'rule of thumb' is that for every 5C you go colder than the listed viscosity, you multiply the value by 2.0 for conventional oils, and 1.7 for synthetics.

Eg., a conventional 5W oil 13,000 MRV at -35C. At -40B, it is 26,000 MRV.....I think.....

 

[vinu_neuro]

The viscosity calculators out there are for kinematic viscosities, cst. Pumping viscosity cP which afaik is absolute/dynamic viscosity. I don't understand how it's possible for some oils to have a lower kinematic viscosity than others at low temps, but at the same time have a higher pumping visc. If someone would like to take a stab that too, I'd appreciate it.

Also what exactly is pumping viscosity related to in terms of protection, ie. is it only relevant until the point an engine reaches consistent oil pressure (start-up) or beyond that too?

 

[CATERHAM]

Originally Posted By: vinu_neuro
The viscosity calculators out there are for kinematic viscosities, cst. Pumping viscosity cP which afaik is absolute/dynamic viscosity. I don't understand how it's possible for some oils to have a lower kinematic viscosity than others at low temps, but at the same time have a higher pumping visc. If someone would like to take a stab that too, I'd appreciate it.

Also what exactly is pumping viscosity related to in terms of protection, ie. is it only relevant until the point an engine reaches consistent oil pressure (start-up) or beyond that too?


You're right, kinematic viscosity graphs are only half way reliable down to about -15C for 0W-XX oils ans less so for heavier grades.

A 0W-XX must have a pumping viscosity no greater than 60,000 cP
at -40 degrees. That's mighty thick oil and not surprisingly why MRV is called borderline pumping. Obviously you want the oil to be as light as possible as even the lightest 0W-XX oil you can buy, Mobil 1 0W-20, is still mighty thick with a MRV of 10,400cP.
How quickly the oil pressure comes up I would imagine is an engine specific question, but in theory if it's a 0W-XX oil in the sump, it should safely pump at -40.

 

[vinu_neuro]

Is there any way to compare 0W and 5W (40wt) oils at around -20C?

 

[A_Harman]

Originally Posted By: vinu_neuro
Why are pumping viscosities for various _W grades not all tested at the same temps? For example the 0W pumping visc is given at -40C, 5W at -35C, 10W at -30C. This makes it impossible to compare cold weather start-up performance between an oil such as Pennzoil 5W-40 vs. say Mobil 1 0W-40.


The different test temperatures are required in the SAE J300 specification. You can be sure that if you are comparing a 0w and 5w oil, the 0w will definitely have lower viscosity at the same cold cranking temperature than the 5w.

A case I have found to illustrate is Petro Canada Duron Synthetic 0w30:
CCS @ -35C = 5651
CCS @ -30C = 3192
Here the CCS increased by 77% for a 5C decrease in temperature.

Or for MRV on Petro Canada Duron 0w40:
MRV @ -40C = 47864
MRV @ -35C = 23320
Here the MRV increased by 105% for a 5C decrease.

 

[CATERHAM]

Originally Posted By: vinu_neuro
Is there any way to compare 0W and 5W (40wt) oils at around -20C?

Not really.
But -20C is pretty cold and you can safely assume the heaviest 0W-40 in terms of the highest MRV or CCS, will still be lighter than the lightest 5W-40 at that temperature.