Synthetic oil worse for cooling?

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Taken from here: https://www.vwheritage.com/lowdown-oil
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Synthetic oils typically have quite different heat transfer characteristics to mineral oils. On modern water-cooled units this is not a problem, but on the air-cooled unit where a significant part of the overall engine cooling comes through oil cooling, the use of synthetic oils can cause problems. As the amount of heat dissipated by the oil is reduced, the engine runs hotter.
I thought it was the opposite and synthetic oil is better for cooler running due to reduced friction?
 
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Originally Posted by slybunda
Taken from here: https://www.vwheritage.com/lowdown-oil
Quote
Synthetic oils typically have quite different heat transfer characteristics to mineral oils. On modern water-cooled units this is not a problem, but on the air-cooled unit where a significant part of the overall engine cooling comes through oil cooling, the use of synthetic oils can cause problems. As the amount of heat dissipated by the oil is reduced, the engine runs hotter.
I thought it was the opposite and synthetic oil is better for cooler running due to reduced friction?
I think you have two effects there--reduced friction would lead to cooler engine & oil. But that is separate from the heat transfer capability of the oil, or in other words, the ability of the oil to absorb (and then release) heat. Two different things. I have not heard of this before, heat transfer being different. I wouldn't think it would be much different though.
 
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I first (and maybe never since then until now) heard this from a Conklin distributor 25-35 years ago. I think the rationale was small, uniform molecule size in synthetics vs. varied, larger size molecules in dino oil. Still don't know if it has any functional significance or how you'd determine than, but I suppose there is a way to quantify heat transfer through a material and its interaction at material interfaces. As an aside, how come no one talks about UOAs using Conklin products - looks like the they're still in business, if the existence of a website is an indication.
 
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I have a small Honda CB Seven Fifty for around town (not the stone old CB750), it is air cooled 74 HP with a small oil cooler and a mechanical screw in oil temp gauge. No noticeable difference between Dino and synthetics. I am not buying it.
 
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This is way outside my wheelhouse but what the heck, I'll take a stab at this...Wouldn't density be a factor in determining thermal capacity? And isn't mineral typically denser than a synthetic (look at a PDS), so it stands to reason mineral would be a "better" heat transfer medium, no? But it's got downsides right? Like not very thermally stable at high temps, which is where synthetics excel. I know minerals are used a lot in air cooled systems like transformers and such..
 
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Several years ago Briggs and Stratton demonstrated that there was a 10 degree reduction in temperature when dino was drained out and run with Synthetic. I can personally attest to the cooling effect of syn. I bought an air cooled John Deere riding mower with a 25 horsepower Kohler engine. The thing ran so hot that when it was shut down it would knock and run on for a good bit. To get it to keep from running on I had to cool it down for at least three minutes before shutting it off. And the thing radiated heat from the engine like a stove. I drained out the factory fill and filled with Mobil 1 5-30. The thing no longer ran on and I could shut it down a few seconds after mowing without run on. There was also noticeably less radiant heat coming off the engine. I told the story to an old mechanic and he said this was BS. But I saw and experienced it and have run nothing but syn in everything ever since.
 
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Same way people try to say that you can't use synthetic oil in a Rotary (RX-7, RX-8) engine, then consider themselves to have been knowledgeable...
 
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Less friction = less heat but I think most of what will heat your engine is the firing of the cylinders not friction no matter what oil you use. If there was so much friction to cause the engine to heat to 190 deg within a few minutes our engines wouldn't last. J.M.O.
 
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Less friction = less heat but I think most of what will heat your engine is the firing of the cylinders not friction no matter what oil you use. If there was so much friction to cause the engine to heat to 190 deg within a few minutes our engines wouldn't last. J.M.O.
 
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Originally Posted by Mad_Hatter
This is way outside my wheelhouse but what the heck, I'll take a stab at this...Wouldn't density be a factor in determining thermal capacity? And isn't mineral typically denser than a synthetic (look at a PDS),..
Partly right. Ability of a fluid to transfer heat depends on its viscosity, density, specific heat, and thermal conductivity.
 

MolaKule

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You have to split off temperature rise due to friction and the transfer of thermal energy due to the fluids alone.
Originally Posted by MolaKule
Here is some comparative information I derived using the Heat Convection and Conduction formulas: NOTE: This was for pure mineral oil verses pure PAO synthetic both 10.5 [email protected] Convective Calc: H = F.rho.c(To-Ti), where H is heat energy in Joules, F is volume flow in cubic meters/s, c is heat capacity in Joules/kg.C, and temps in C. The c for synthetic oil is 2000 J/kg.C and c for dino is 1780 J/kg.C. I assumed a flow rate was 1/4 liter per second, To is temp out of a journal bearing = 100 C, and Ti was oil temp into bearing = 80C, representing a temp rise of 20C, which is a rule of thumb. Hs = convective energy transfer in Joules for synthetic = 10.65 J Hd = convective energy transfer in Joules for dino (mineral oil) = 9.3 J. Therefore, pure synthetic oil is 13% more efficient at convective heat transfer. The "rho" factor (oil convection heat transfer) is the oil's density, measured in kg/cubic meters, and since both oils were so close in density, I used 1.065 kg/cubic meter. Using the Heat Conduction formula: H = kA(To-Ti/L), where H is heat Power in W.m, k is heat conduction coefficient in W/meter-squared/C, and temps in C. The k for synthetic oil is 0.16 and k for dino is 0.128, To is temp out of a journal bearing = 100 C, and Ti was oil temp into bearing = 80C, representing a temp rise of 20C, which is a rule of thumb. L is the thickness of the oil film which is on the order of 1um at high loads. A is area of film assumed to be a patch of area of 1 mm squared. Hs = conductive heat transfer of synthetic oil in W = 3200 W, Hd = conductive heat transfer of dino (mineral oil) in W = 2560 W. Therefore, pure synthetic oil is 20% more efficient at conductive heat transfer than mineral oil. The same film thickness for both dino's and synth's were used for the calculations. These are heat transfer formulas from thermodynamics and the constants I used for "c" and "k" were from Michael J. Neal's, The Handbook of Tribiology. (From BITOG 07/02)
 
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MolaKule

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Originally Posted by andyd
Yes Thank you. When I read thread. I felt that synth would conduct heat better because its molecules are identical.. any basis in fact? TIA grin2
I think it has more to do more with the way the molecules link up, i.e, the molecular structure. Since in reality finished oils are a mix of mineral and synthetic base oils, the article limked above really has no scientific support.
 
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Originally Posted by MolaKule
You have to split off temperature rise due to friction and the transfer of thermal energy due to the fluids alone.
Originally Posted by MolaKule
Here is some comparative information I derived using the Heat Convection and Conduction formulas: NOTE: This was for pure mineral oil verses pure PAO synthetic both 10.5 [email protected] Convective Calc: H = F.rho.c(To-Ti), where H is heat energy in Joules, F is volume flow in cubic meters/s, c is heat capacity in Joules/kg.C, and temps in C. The c for synthetic oil is 2000 J/kg.C and c for dino is 1780 J/kg.C. I assumed a flow rate was 1/4 liter per second, To is temp out of a journal bearing = 100 C, and Ti was oil temp into bearing = 80C, representing a temp rise of 20C, which is a rule of thumb. Hs = convective energy transfer in Joules for synthetic = 10.65 J Hd = convective energy transfer in Joules for dino (mineral oil) = 9.3 J. Therefore, pure synthetic oil is 13% more efficient at convective heat transfer. The "rho" factor (oil convection heat transfer) is the oil's density, measured in kg/cubic meters, and since both oils were so close in density, I used 1.065 kg/cubic meter. Using the Heat Conduction formula: H = kA(To-Ti/L), where H is heat Power in W.m, k is heat conduction coefficient in W/meter-squared/C, and temps in C. The k for synthetic oil is 0.16 and k for dino is 0.128, To is temp out of a journal bearing = 100 C, and Ti was oil temp into bearing = 80C, representing a temp rise of 20C, which is a rule of thumb. L is the thickness of the oil film which is on the order of 1um at high loads. A is area of film assumed to be a patch of area of 1 mm squared. Hs = conductive heat transfer of synthetic oil in W = 3200 W, Hd = conductive heat transfer of dino (mineral oil) in W = 2560 W. Therefore, pure synthetic oil is 20% more efficient at conductive heat transfer than mineral oil. The same film thickness for both dino's and synth's were used for the calculations. These are heat transfer formulas from thermodynamics and the constants I used for "c" and "k" were from Michael J. Neal's, The Handbook of Tribiology. (From BITOG 07/02)
So how to explain the statement in the supplied article? Are we/I (mis) understanding what is being said, or is it just patently false?...(or like is the efficiency of synthetic somehow reduced with respect to the cooling method, air v. liquid cooling?..or is synthetic a good conductor of heat but doesn't as readily transfer the heat (let go of) as well as a dino?.. just thinking out loud and trying to square the statement in the article with what you just wrote.. can both what you said and what's the article purports be true?)
 
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MolaKule

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Originally Posted by Mad_Hatter
...So how to explain the statement in the supplied article?
See posts #5313413 and #5313477 above.
 
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Hey MolaKule, Another one here admitting that I am way out of my depth on this subject, but wouldn't the add pack have more to do about thermal conductance than the oil itself? In my limited understanding, it seems that the additive pack would make an oil significantly more conductive than straight oil. A lot like how highly polished water, with all the minerals removed, is non-conductive. Is motor oil similar to this at all?
 

MolaKule

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Originally Posted by BHopkins
Hey MolaKule, Another one here admitting that I am way out of my depth on this subject, but wouldn't the add pack have more to do about thermal conductance than the oil itself? In my limited understanding, it seems that the additive pack would make an oil significantly more conductive than straight oil. A lot like how highly polished water, with all the minerals removed, is non-conductive. Is motor oil similar to this at all?
Absolutely as the additive package is part of the finished oil. How much more the additive package would contribute to the convectivity or conductivity would only add about 5% or so would be my guess. As I stated above, "Since in reality finished oils are a mix of mineral and synthetic base oils, the article linked above really has no scientific support." Since the oil is continually moving through the engine, it should be that its the convective thermal energy transfer that is greatest.
 
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