Does grade REALLY effect fuel economy???

[Clevy]

Originally Posted By: gfh77665
And I wonder if the Gov't acknowledges or accounts for the fact that slightly higher viscosity oils shear down to the lower grades anyway? Even if there is any miniscule MPG difference initially, it dissipates with use as the oil thins. Gov't numbers would only extrapolate with the very first data point, and ignore the reality of oils shearing thinner.

Unless fuel diluted all the data I've seen presented in this forum shows the SN 20 grades throws very good shear stability,so that's a non point or issue,unless as I've said fuel dilution plays a roll.
No matter what we try to convince ourselves of some other answer less viscous fluid is easier to pump,and considering the pump is driven by the engines rotating resistance so engine has to work harder to pump the oil..
Incredibly platform specific. Some engines will react different based on design.
Your foot will save you more than the oil.

 

[Shannow]

nope, not even close.

Pump power is purely volume (and most are positive displacement, so volume is all but fixed) x density of pumped fluid x pressure rise across pump.

Pump power on a PD pump is little affected by oil viscosity.

 

[gfh77665]

Originally Posted By: Clevy

Unless fuel diluted all the data I've seen presented in this forum shows the SN 20 grades throws very good shear stability,so that's a non point or issue,unless as I've said fuel dilution plays a roll.


I am not referring to the 20's they are already very thin. Many 30's end up a 20 at some point before the OCI, negating the perceived mpg difference.

 

[SteveSRT8]

Originally Posted By: Clevy
Incredibly platform specific. Some engines will react different based on design.
Your foot will save you more than the oil.


Stole my line!

Many newer engines are featuring variable displacement pumps which can compensate for some of the issues being discussed here...

 

[uart]

Originally Posted By: Shannow
nope, not even close.

Pump power is purely volume (and most are positive displacement, so volume is all but fixed) x density of pumped fluid x pressure rise across pump.


Yes, and (assuming not in bypass) the pressure increases with increasing viscosity, so Clevy's point stands.

 

[Shannow]

Originally Posted By: uart
Originally Posted By: Shannow
nope, not even close.

Pump power is purely volume (and most are positive displacement, so volume is all but fixed) x density of pumped fluid x pressure rise across pump.


Yes, and (assuming not in bypass) the pressure increases with increasing viscosity, so Clevy's point stands.


At 10gpm, and 60psi nominal, changing viscosity sufficient to make a 15psi change in oil pressure would result in 100W increase/decrease in engine parasitic loss due to the "pumpability" of the oil...you gain or lose much more by turning on the headlights...

If in bypass, both oils use the same pump power.

The drive power of the pump has virtually nothing to do with the economy effects of running thinner oils...

Clevy's claim that it was the cause isn't the case.

 

[uart]

Originally Posted By: Shannow

At 10gpm, and 60psi nominal, changing viscosity sufficient to make a 15psi change in oil pressure would result in 100W increase/decrease in engine parasitic loss due to the "pumpability" of the oil...you gain or lose much more by turning on the headlights...

Thanks for quantifying that. Yes I agree that it's only a very small amount of power, 50 to 100 Watts would be about it.

BTW. While you were typing that I was trying to quantify this myself. I used 8GPM and 10 psi (20% typical increase, for a one grade difference, at 50 psi nominal) and I got a figure around 50 Watts.