Thick vs Thin Oil Qu

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All things being equal except oil, which engine will reach operating temperature faster, the one with 20 weight oil or the one with 40 weight oil? And why?
 
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Realistically the 20 will flow better long before the 40 weight. In terms of "operating temperature" the 20 weight will be at optimal flow before any 40 will be. If one actually gets 'warmer' faster then the other I wouldn't entirely know the science behind why that would be the case. At any rate a 40 stays enormously thick until its substantially hotter then a 20 weight, and thicker isn't better if its unnecessary. It just bogs down the engine.
 

FoxS

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Originally Posted By: RiceCake
Realistically the 20 will flow better long before the 40 weight. In terms of "operating temperature" the 20 weight will be at optimal flow before any 40 will be. If one actually gets 'warmer' faster then the other I wouldn't entirely know the science behind why that would be the case. At any rate a 40 stays enormously thick until its substantially hotter then a 20 weight, and thicker isn't better if its unnecessary. It just bogs down the engine.
I was asking about engine operating temperature not oil thickness / optimal flow.
 
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That was really the point of how I explained that. Will the engine reach operating temperature faster? No idea. Logically I'd have to say no, both oils would heat the same as the engine only has a certain specific heat output.
 

FoxS

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I was thinking that thinner oil would flow faster and transfer more heat into itself thus delaying how fast the engine got to full operating temperature.
 
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I would expect the higher viscosity oil to heat up faster due to its greater resistance to flow, thus creating more friction. But the 40 will never get as thin as the 20. So if fast oil temp is what you want, I suspect the 40 would be the winner.
 
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Originally Posted By: FoxS
I was thinking that thinner oil would flow faster and transfer more heat into itself thus delaying how fast the engine got to full operating temperature.
Even if it flows faster the engine itself only has a certain amount of heat to offer and the oil remains largely within the confines of the engine block, which takes considerable time to heat itself, plus all the coolant. Also if you have any sort of oil cooler, fast oil flow will make heating take longer since it will flow faster though it and into the air, which will dissipate away from the engine.
Originally Posted By: INDYMAC
I would expect the higher viscosity oil to heat up faster due to its greater resistance to flow, thus creating more friction. But the 40 will never get as thin as the 20. So if fast oil temp is what you want, I suspect the 40 would be the winner.
This is the only real practical explanation for different oil causing any measurable sort of "heatup" difference. The oil being heavier makes the engine work harder, the engine working harder to pump oil will heat it up, and battering thick oil by forcing it through an engine will cause it to heat by the property of "working" the oil itself mechanically. The difference this causes though will be absolutely minimal, if you can even measure it. Plus, it will take longer to heat up to a level of fluidity where its not robbing the engine of power considerably. It stays thick longer, so even if it heats quicker, you've lost any benefit of the fast heating. Besides, none of this accounts for multigrade oils, which completely throw variables into the mix, since they all start off at considerably thinner (0W- 5W-) viscosities, so the heating from mechanical working of the oil is gone. A 5W-30 will need less heat to match a viscosity a 5W-40 needs, but both will, essentially, follow a fairly consistent heating curve when they're cold. Thinner oils hit a point, faster, where they're no longer robbing as much engine power as a thicker oil would. That's the only difference. An engine puts out only a certain amount of heat and you're warm up times are fairly limited by them.
 
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Originally Posted By: INDYMAC
I would expect the higher viscosity oil to heat up faster due to its greater resistance to flow, thus creating more friction. But the 40 will never get as thin as the 20. So if fast oil temp is what you want, I suspect the 40 would be the winner.
I agree. Why? Thick oils run hotter on the highway - this tells me something.
 
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Very difficult to figure out as you have 3 different factors involved in heating up the oil in the sump, firstly flow rate and the thin oil flows faster so will transfer heat to the sump faster, then you have the specific heat index and thermal conductivity of the liquid involved. I think they both work in the same direction for oil, but I'm not 100% sure as it would be effected by the add pack, although I suspect all the figures for engine oil are fairly close between the two grades. That means the 20 grade should warm up the sump oil faster.
 
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Originally Posted By: RiceCake
Originally Posted By: FoxS
I was thinking that thinner oil would flow faster and transfer more heat into itself thus delaying how fast the engine got to full operating temperature.
Even if it flows faster the engine itself only has a certain amount of heat to offer and the oil remains largely within the confines of the engine block, which takes considerable time to heat itself, plus all the coolant. Also if you have any sort of oil cooler, fast oil flow will make heating take longer since it will flow faster though it and into the air, which will dissipate away from the engine.
Originally Posted By: INDYMAC
I would expect the higher viscosity oil to heat up faster due to its greater resistance to flow, thus creating more friction. But the 40 will never get as thin as the 20. So if fast oil temp is what you want, I suspect the 40 would be the winner.
This is the only real practical explanation for different oil causing any measurable sort of "heatup" difference. The oil being heavier makes the engine work harder, the engine working harder to pump oil will heat it up, and battering thick oil by forcing it through an engine will cause it to heat by the property of "working" the oil itself mechanically. The difference this causes though will be absolutely minimal, if you can even measure it. Plus, it will take longer to heat up to a level of fluidity where its not robbing the engine of power considerably. It stays thick longer, so even if it heats quicker, you've lost any benefit of the fast heating. Besides, none of this accounts for multigrade oils, which completely throw variables into the mix, since they all start off at considerably thinner (0W- 5W-) viscosities, so the heating from mechanical working of the oil is gone. A 5W-30 will need less heat to match a viscosity a 5W-40 needs, but both will, essentially, follow a fairly consistent heating curve when they're cold. Thinner oils hit a point, faster, where they're no longer robbing as much engine power as a thicker oil would. That's the only difference. An engine puts out only a certain amount of heat and you're warm up times are fairly limited by them.
+1 on multigrade. The issue is moot.
 
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I thought the question was about SAE 20 and SAE 40 grade oils. Where do you get multi-viscosity from in the discussion?
 

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Energy is always directly proportional to Temperature as in E = kT. It take more energy to move and pressurize a thicker oil, so therefore, the temperature of a thicker oil should increase faster than a thinner oil. Where does the energy come from? It has to come from the engine during the conversion of chemical enery to mechanical energy, so the engine should warm up faster with a thicker oil. All other things being equal, the thinner oil should flow better and lubricate faster in cold temps because of it's lower viscosity.
 
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Originally Posted By: MolaKule
Energy is always directly proportional to Temperature as in E = kT. It take more energy to move and pressurize a thicker oil, so therefore, the temperature of a thicker oil should increase faster than a thinner oil. Where does the energy come from? It has to come from the engine during the conversion of chemical enery to mechanical energy, so the engine should warm up faster with a thicker oil. All other things being equal, the thinner oil should flow better and lubricate faster in cold temps because of it's lower viscosity.
Science has spoken on the oil. However:
Originally Posted By: FoxS
All things being equal except oil, which engine will reach operating temperature faster, the one with 20 weight oil or the one with 40 weight oil? And why?
OP asked about engine operating temperature, but did not say what parameters defined operating temperature. Ambient temperature and oil flow rates at that temperature, to name a couple of factors involved, would all affect the time required. Perhaps a real world test would answer this question best. Carry on. popcorn2
 

MolaKule

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Quote:
which engine will reach operating temperature faster, the one with 20 weight oil or the one with 40 weight oil?
The rate of increase in temprature will be higher for a 40 weight than a 20 weight, and by an experiment I did years earlier, a 40 weight will actually run hotter compared to a 20 weight.
 
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Originally Posted By: MolaKule
Quote:
which engine will reach operating temperature faster, the one with 20 weight oil or the one with 40 weight oil?
The rate of increase in temprature will be higher for a 40 weight than a 20 weight, and by an experiment I did years earlier, a 40 weight will actually run hotter compared to a 20 weight.
Enlightening. Thank you
 
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Originally Posted By: MolaKule
Quote:
which engine will reach operating temperature faster, the one with 20 weight oil or the one with 40 weight oil?
The rate of increase in temprature will be higher for a 40 weight than a 20 weight, and by an experiment I did years earlier, a 40 weight will actually run hotter compared to a 20 weight.
How much hotter?
 

MolaKule

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It's on here somewhere but I did IR readings on an air cooled single cylinder engine during a hot summer day in Kansas. As I recall, the thicker oil caused a head temperature of about 50F hotter than the thinner oil.
 
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