Viscosity at 85°C vs 100°C

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Assume Pennzoil Platinum dexos1 Gen 2 0W-20 with typical cSt @ 40°C of 43.4 and typical cSt @ 100° of 8.6 per the TDS. If these viscosity numbers are put in to the NiMAC calculator (Andrade correlation) and 85°C is used as the operating temperature, the calculated viscosity is 11.78. (Realizing that various factors may cause the result to to slightly more or less, let's assume the 11.78 is accurate.) If the oil temperature in my car runs at 85°C (which it averaged on a recent 200 mile road trip), is the 11.78 cSt the equivalent of having an SAE 30 grade oil in the engine? Put another way, will an 11.78 Cst @ 85°C oil have the same functionality/characteristics/properties as an 11.78 cSt @ 100°C oil?
 
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I believe that shorter chains behave differently in an operating engine in at most operating temps than long chains. Viscosity is just one measurable characteristic.
 
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Maybe this is another reason why manufacturers of fuel-diluting engines spec a 'thin' oil and then run it on the 'cool' side (strictly my theory) ... keeps viscosity acceptable under typical, non-stressed conditions. Perhaps the result is the acceptable UOAs we see on fuel-diluters. And maybe running oil on the cool side is why fuel that gets into the oil doesn't vaporize to the extent it otherwise would at a higher temperature. Kind of makes the SAE specs a bit irrelevant and mucks up trying to choose the 'correct' oil, especially for mixed-use applications (think commuting today, heavy towing tomorrow).
 
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Originally Posted by doyall
If the oil temperature in my car runs at 85°C (which it averaged on a recent 200 mile road trip), is the 11.78 cSt the equivalent of having an SAE 30 grade oil in the engine? Put another way, will an 11.78 Cst @ 85°C oil have the same functionality/characteristics/properties as an 11.78 cSt @ 100°C oil?
No. Your SAE 0W-20 oil is always a SAE 0W-20 oil and behaves like a SAE 0W-20 oil. If you were to rephrase the question like this: "is the 11.78 cSt the equivalent of having an SAE 30 grade oil in the engine if the engine were at 100°C" Then the answer would be "yes".... sort of. The temperature of the oil also impacts how various chemical reactions take place, including how various molecules bind with the surface and how the viscosity modifier is behaving. So from a purely physical point of view then yes, a 0W-20 oil with 11.78 cSt @85°C will behave like a xW-30 oil with 11.78 cSt @100°C. However at a chemical level there will be some slight differences.
 
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Originally Posted by weasley
... The temperature of the oil also impacts how various chemical reactions take place, including how various molecules bind with the surface and how the viscosity modifier is behaving. ...
Then by the engine manufacturer deciding to set their engine operating temperature at something less than the oil manufacturer's (and SAE's I suppose) specs, the engine manufacturer is possibly not allowing the oil to properly perform it's intended objective/function. That would not just muck things up, it would make it virtually impossible for even the enthusiast (much less typical) consumer to make the 'best' choice since only the manufacturers themselves would have access to all the proprietary data.
 
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Let's put it this way, a 20 grade oil that's 11.78 cSt @ 85*C and a 30 grade oil that's 11.78 cSt @ 100*C with the same add pack. The 30 grade at 100*C would have better wear protection, lower friction, and better cleaning ability since the anti-wear, friction modifier, and detergent additives would be more reactive at the higher temperature. Now this is a minor point on wear and friction since most of the iron wear and friction in an engine comes from the rings and cylinder walls where there's plenty of combustion heat to get things activated. The same could be said about detergents. It's not so much about sump temperature as it is about localized temperature in that regard. Typical engine oils are ~80% base oil and ~20% add pack, but it's flipped the other way around in terms of importance.
 
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Originally Posted by doyall
Put another way, will an 11.78 Cst @ 85°C oil have the same functionality/characteristics/properties as an 11.78 cSt @ 100°C oil?
I know where you are coming from on this because I do something similar on my motorcycle which runs very cool. As has already been pointed out the Kinematic viscosity comparison may be valid but the HTHS is likely to be different. HTHS viscosity is what determines the operating conditions in the bearing areas that count and the oil temperature will be higher than 85 deg C in these areas. If you are going to use a thinner oil because the engine runs cool I would check the HTHS to make sure it's not too much lower than the manufacturers recommended oil.
 
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The oil might be 85c on your gauge, but in the main bearings and cylinder heads it might be much higher, thats where HTHS is important and the KV is less.
 
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Originally Posted by jbutch
The oil might be 85c on your gauge, but in the main bearings and cylinder heads it might be much higher, thats where HTHS is important and the KV is less.
Yes, but if the oil is 15 degrees less hot when the oil pump picks it up, is it still cooler by roughly the same margin after it enters the bearings? I'm guessing that's what doyall was thinking about. The temperature at which HTHS is defined is meant to be representative, but doesn't mean the actual temperature in the bearings of every engine will be the same, or that it isn't influenced by sump temperature.
 
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With an ideal viscosity for the bearing clearance, you can usually expect a 40-75*F oil temperature delta through the bearings. Part throttle, low load will be on the lower end of that range and wide open throttle will be on the higher end. Oil temperature starting at 200*F will be 240-275*F exiting the bearings depending on load. When you run a higher viscosity than the clearance is intended for, that delta increases. This delta applies regardless of the starting temperature. If you crank the engine up on a cold day and start driving with say 0*F sump temps and several hundred centistokes viscosity, it's not unreasonable for bearing temp delta to be 100+*F due to the high viscosity. This also helps the oil heat up faster than just idling. This also applies for those using a higher grade oil during the summer. The higher viscosity creates a higher delta which increases oil temps overall. This is for typical street engines. Racing engines can exceed that figure by quite a bit. NASCAR Cup engines, for example, regularly top 75*F delta with 0w-20 oil, sump temp of 280*F, and bearing temp delta of 355+*F.
 
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