Oil Grade Effect on Starting Cranking Speed

[RalphVa]

That's kind of how I read it,
I wish that he had included numbers for 5w-40.

Someone mentioned their using a 0w40 on the green tractor forum. I looked up M1 0w40 and calculated its cranking viscosity: lowest of any of them. Its VI is 185, very high. Most synthetics and VI additive corrected oils are about 140. Not sure whether M1 gets the 185 from the PAO polymerization and side chaining or whether they have to add a tad of VI improver. Not sure what VI improver is out there to boost the VI to 185, extremely high.

I'm just surprised that the numbers came out so consistent.

Ralph

 

[Cougsfan]

I would think this is very good info to use in proper context. Wouldn't the torque characteristics from the starter motor also be a huge factor in the cranking speed at various temperatures? I could visualize that with a higher starter motor torque rating, the starter rpms would drop off considerably less as the starter has a better ability to overcome increased friction.
Besides that it does show give a good idea of how viscosity affects friction at reduced temperatures. There are probably other factors of increased viscosity at low temperatures, particularly just after start-up. For instance, I would think that the rings might do a much better job of scraping oil off the cylinder walls until the higher viscosity oils begin to thin out, resulting in reduced cylinder wall lubrication until the oil warms up.

 

[Bavarian]

I would think this is very good info to use in proper context. Wouldn't the torque characteristics from the starter motor also be a huge factor in the cranking speed at various temperatures? I could visualize that with a higher starter motor torque rating, the starter rpms would drop off considerably less as the starter has a better ability to overcome increased friction.
Besides that it does show give a good idea of how viscosity affects friction at reduced temperatures. There are probably other factors of increased viscosity at low temperatures, particularly just after start-up. For instance, I would think that the rings might do a much better job of scraping oil off the cylinder walls until the higher viscosity oils begin to thin out, resulting in reduced cylinder wall lubrication until the oil warms up.

My opinion on the lower rpms at low temps is not caused by increased friction. It's caused by the viscosity of the oil itself where the crankshaft has to stir through. Just try it yourself. Take a spoon, a glass of water and one glass of honey. Try to stir. What's easier?
And a 15-W40 at -20 Celsius pours out from a chug like honey.
If increased friction would cause the rpms to drop, your engine wouldn't run at all.
Like I said, my opinion.

 

[RalphVa]

The cranking speed is affected by the viscosity of the oil by a square root relationship. The starting motor will just plain rotate it slower in cold weather due to this viscosity drag. All of you have experienced this. The starter will hardly rotate the engine in really cold weather.

Another thing that happens, and I have no way of calculating that, is that the oil pump will have more trouble drawing the very viscous oil into its suction. This can take several seconds are very cold temperatures with another but a 0wxx oil.

Ralph

 

[LouNY]

In a related vein, multi purpose hydraulic transmission fluid can really pull the starter down in some tractors.
Any thing that does not have a dry clutch between the engine and transmission has a harder time cranking things over in cold weather.
My Branson 8050 does not have a dry clutch just wet clutch packs for forward and reverse with an internal pump for the apply pressure.
The first winter it would not start on cold winter days when I needed it, when the dealer checked it out on warmer days or in his warm shop it was fine.
I tried multiple batteries, booster starter chargers nothing worked it would make about 2 engine revolutions and stop cranking.
Heat the transmission rear end case with a salamander and she would fire up like summer.
Ended up with a synthetic multipurpose fluid and no more issues, the wimpy starter just did not have the strength to crank every thing over it needed to.

 

[RalphVa]

Doubt a transmission will decrease pumping speed like the bearings in an engine, due to more clearance in the transmission. Less of a viscosity effect.

Transmission oils can be anywhere from 10w (ISO 32) to 30w, typically about 20w (ISO 46).