Symptoms of too high or too low motor oil (hot) viscosity?

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Symptoms of too high or too low motor oil (hot) viscosity?

Postby ptuomov » Sun Jan 08, 2017 2:11 pm

Suppose that I use either too low or too high (hot) viscosity oil in an engine. What are the symptoms of the two wrong choices? What problems should I experience in either case in terms wear, failures, or oil degradation?
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Mon Jan 09, 2017 11:37 am

The reason why I'm asking this is that I'm having hard time squaring off the conventional wisdom, my own logic, and the test data and statements on 540 RAT's page: https://540ratblog.wordpress.com/2013/0 ... t-ranking/

It seems to me that oil pressure itself is there just to make sure oil gets delivered to bearings. Once it's delivered to bearings, the supply oil pressure doesn't actually do anything in those bearings, there are bigger forces at work there. What matters once the oil is in the bearings is the oil film strength, not the supply pressure. According to 540 RAT's test results, to my surprise, the (hot) viscosity and oil film strength are essentially uncorrelated with modern motor oils. That was unexpected to me. Of course, if oil has lower (hot) viscosity then more of it gets delivered at a give pressure. There will be the same pressure (at least as long as the pump has the capacity to cause the pressure relief valve to cycle) but more oil flow with low viscosity oil. This higher flow means better cooling of bearings, quicker drain back to the sump (lower viscosity), but also more oil having to drain back (more flow) and the pump sucking the sump dry quicker if there's a drain back problem (again more flow). The pump will pump the same flow against the same pressure and thus consume the same amount of power regardless of oil viscosity (as long as the relief valve cycles), but higher viscosity oil will cause more drag in the bearings.

All things considered, absent a drain back problem under say cornering g-forces, doesn't this logic prescribe using the lowest (hot) viscosity oil in the engine that allows the oil pump to build the oil pressure that the engine needs to deliver oil everywhere? Or am I off track here?

The follow-up question is what's the observable damage if one sues too low or too high oil viscosity?



The copy-past of the viscosity section fo the 540 RAT's page:
"SECTION 2 – MOTOR OIL VISCOSITY SELECTION

THE BENEFITS OF USING THINNER OIL:

• Thinner oil flows quicker at cold start-up to begin lubricating critical engine components much more quickly than thicker oil can. Most engine wear takes place during cold start-up before oil flow can reach all the components. So, quicker flowing thinner oil will help reduce start-up engine wear, which is actually reducing wear overall.

• The more free flowing thinner oil at cold start-up, is also much less likely to cause the oil filter bypass to open up, compared to thicker oil. Of course if the bypass opened up, that would allow unfiltered oil to be pumped through the engine. The colder the ambient temperature, and the more rpm used when the engine is cold, the more important this becomes.

• Thinner oil also flows more at normal operating temperatures. And oil FLOW is lubrication, but oil pressure is NOT lubrication. Oil pressure is only a measurement of resistance to flow. Running thicker oil just to up the oil pressure is the wrong thing to do, because that only reduces oil flow/lubrication. Oil pressure in and of itself, is NOT what we are after.

• The more free flowing thinner oil will also drain back to the oil pan quicker than thicker oil. So, thinner oil can help maintain a higher oil level in the oil pan during operation, which keeps the oil pump pickup from possibly sucking air during braking and cornering.

•The old rule of thumb for desired oil pressure, that we should have at least 10 psi for every 1,000 rpm, pertains to, and is highly recommended for High Performance and Racing engines. Engine bearing clearances are primarily what determines the oil viscosity required for any given engine. (NOTE: Viscosity does NOT determine an oil’s wear protection capability, like many people think. Wear protection capability is determined by an oil’s additive package, which contains the extreme pressure anti-wear components. That is why 5W30 oils can perform so much better than thicker oils in my wear protection capability testing). But, whatever the bearing clearance, for High Performance and Racing engines, it is best to run the thinnest oil we can, that will still maintain at least the old rule of thumb oil pressure, even if that means using a high volume oil pump to achieve that. A high volume oil pump/thinner oil combo is much preferred over running a standard volume oil pump/thicker oil combo. Because oil “flow” is our goal for ideal oiling, NOT simply high oil pressure. So, one of the benefits of running a high volume oil pump, is that it will allow us to enjoy all the benefits of running thinner oil, while still maintaining desirable oil pressure.

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But, for normal daily driver street engines, it is acceptable to use the old rule of thumb only as an “approximate” general guideline, not an “absolute requirement”. And for those engines, no matter what their bearing clearance is, it is best to run the thinnest oil we can, that will still maintain at least “reasonable” oil pressure, that is not too far below the old rule of thumb oil pressure.

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Using thicker oil just to achieve higher oil pressure, will simply reduce oil flow for no good reason. The oil pump relief valve determines the max oil pressure an engine can make, no matter what the oil viscosity is. And in some engines, the relief valve limits oil pressure to a max of 65 psi. But, that does not mean the engine’s redline has to be limited to exactly 6,500 rpm because of that. Oil pressure does NOT determine the engine’s redline, the mechanical design of the engine does.

Plain bearings, such as rod and main bearings, are lubricated by oil flow, not by oil pressure. Oil pressure is NOT what keeps these parts separated. Oil pressure serves only to supply the oil to the clearance between the bearings and the crankshaft journals. Those parts are kept apart by the incompressible hydrodynamic liquid oil wedge that is formed as the liquid oil is pulled in by the spinning crankshaft. As long as sufficient oil is supplied by the necessary oil pressure mentioned above, no wear can occur. And the higher flow rate of thinner oil, supplies more oil volume to the main and rod bearings, which also helps ensure that the critical incompressible hydrodynamic liquid oil wedge is always maintained.

Thinner oil will of course flow out from the bearing clearances quicker than thicker oil will. But, by making sure there is sufficient oil pressure as mentioned above, the oil supply will always stay ahead of the oil flowing out, which will maintain that critical incompressible hydrodynamic liquid oil wedge.

• Oil flow is what carries heat away from internal engine components. Those engine components are DIRECTLY oil cooled, but only INdirectly water cooled. And better flowing thinner oil will keep critical engine components cooler because it carries heat away faster than slower flowing thicker oil can. This is especially important with plain main and rod bearings, since the flow of oil through the bearings is what cools them. If you run thicker oil than needed, you will drive up engine component temps.

Here are some comparison numbers from an 830 HP road race engine on the track:

15W50 oil = 80 psi = 265* oil sump temperature

5W20 oil = 65 psi = 240* oil sump temperature

Here you can see how the thicker oil flowed more slowly through the bearings, thus getting hotter, driving up bearing temperatures and increasing sump temperatures. And the thinner oil flowed more freely and quickly through the bearings, thus cooling and lubricating them better than thicker oil, while also reducing sump temperatures.

Here’s some additional background on all this – You might be surprised by how much heat can be generated just from an oil’s internal friction, though friction may not the best term to use here. It is probably better to think of this as the heat generated due to the shearing action taking place within the oil.

It is the shearing action of the oil between the crankshaft and bearings, while the engine is under a heavy loading condition, that generates the bearing heat that we are concerned with. The oil wedge formed as the crankshaft pulls oil in and around the clearance as it spins, is liquid oil. And since liquids cannot be compressed, the oil wedge itself is what carries that heavy engine loading (oil pressure serves only to deliver oil to the crank/bearing interface) and prevents the crankshaft and bearings from coming in contact with each other, once the engine is running. Cold start up after sitting, is when the bearings and cranksaft start out in contact with each other.

The difference in flow rate, and the difference in shearing generated heat, is why the viscosity used, makes a difference in bearing and sump temperatures. Thicker oil which flows more slowly and generates more heat from shearing, it is not carrying heat away and cooling the bearings as well or as quickly as it could, so that drives up bearing temps. This in turn, causes hotter oil to be coming out of the bearings and into the sump, which is why we see higher temps on a gauge. That is the opposite of what we want.

On the other hand, quicker flowing thinner oil, not only generates less heat from shearing, but it also carries heat away much quicker, keeping bearing temps down. And this means the oil coming out from the bearings, and going into the sump, is also cooler. And that is why we see the cooler sump temps. This is precisely what we saw with the road race engine example above.

If an engine is running hot, use a thinner oil to increase flow, increase internal component cooling, and help keep sump temperatures down. Keeping oil temps down is important to help keep oil below the threshold of thermal breakdown.

• Thinner oil will typically increase HP because of less viscous drag and reduced pumping losses, compared to thicker oils. That is why very serious Race efforts will generally use watery thin oils in their engines. But, an exception to this increase in HP would be in high rpm hydraulic lifter pushrod engines, where thinner oil can allow the lifters to malfunction at very high rpm. In everyday street vehicles, where fuel consumption is a consideration, thinner oils will also typically increase fuel economy. The majority of new cars sold in the U.S. now call for 5W20 specifically for increased fuel economy. And now Diesel trucks are increasingly calling for 5W30, also for fuel economy improvement.

• Relatively few engines are built with loose enough bearing clearances, to ever need to run oil thicker than a multi-viscosity 30 weight (though some may need a high volume oil pump). The lower the first number cold viscosity rating, the better the cold flow. For example, 0W30 flows WAY better cold than 20W50. And 0W30 flows WAY better cold than straight 30wt, which is horrible for cold start-up flow and should be avoided at all cost. And the lower the second number hot viscosity rating, the better the hot flow. For example, hot 0W30 flows WAY better hot than 20W50.

* The churning action of rotating and reciprocating internal engine components, along with oil spraying out from between pressurized components, traditional oil pumps with their old-tech spur gear design, old tech oil pressure relief valves, and overall windage, all contribute in varying degrees, to causing the engine oil to become aerated, which is exhibited by air bubbles/foam in the oil. Air bubble-filled foamy oil, is what typically causes engines running on a dyno to experience oil pressure drops, assuming they have acceptable oil drain-back from the top end, and are keeping the oil pump pickup submerged. Also, air bubble-filled foamy oil, is what typically causes engines being run hard in cars, to experience drops in oil pressure, assuming the oil pump pickup is still submerged in oil. And if that isn’t bad enough, air bubble-filled, foamy oil cannot lubricate critical internal components properly. For proper lubrication of critical components, you need incompressible “liquid” oil, NOT compressible air bubble-filled foamy oil.

This is an issue to take very seriously, if you want to provide your engine with the best possible lubrication protection. If this aerated oil issue is bad enough, it can cause wear, damage or outright engine failure. And it can be extremely difficult to diagnose, in the event of an outright engine failure. Because when you take the engine apart for examination, you typically can’t find anything wrong at all, other than say the rod and/or main bearings that failed. That’s because the air bubbles/foam are long gone by then.

You can’t do much about the churning action of rotating and reciprocating internal engine components, nor can you do much about the oil spraying out from between pressurized components. Though you can try to reduce windage problems by selecting the best oil pan designs. You can also select a superior smoother flowing gerotor oil pump design with its internal bypass relief valve. But, the one thing that is the easiest to change to reduce engine oil aeration concerns, is to choose the proper engine oil viscosity.

Heavy thick oils such as 5W50 and 20W50, that are of course 50 weight oils at normal operating temperature, are slower to release and eliminate air bubbles/foam, than thinner oils such as 5W30 and 10W30 that are 30 weight oils at normal operating temperature. Motor oils do of course contain anti-foaming agents to help control (though not altogether eliminate) air bubbles/foam. But, the air bubbles that will still be present in the oil anyway, have to travel through the oil to be released. And thicker heavier oils slow down that process, leaving compromised lubrication. Adding aftermarket oil treatments that thicken the oil more, makes aeration issues even worse, by causing further slowing of air bubble release. Data on this is not widely published, so I have future testing planned that will provide much needed test data on this subject. But in the meantime, keep in mind that thinner oils such as 5W30 and 10W30, allow air bubbles to travel through the oil and be released quicker, making them a better viscosity choice to fight motor oil aeration issues, and provide the best possible lubrication protection for your engine.

• Thicker oils DO NOT automatically provide better wear protection than thinner oils, as some people mistakenly believe. Extensive “dynamic wear testing under load” of approximately 200 motor oils, has shown that the base oil and its additive package “as a whole”, with the primary emphasis on the additive package, which is what contains the extreme pressure anti-wear components, is what determines an oil’s wear protection capability, NOT its viscosity. In fact, the test data has shown that 5W20 oils can provide INCREDIBLE wear protection with over 120,000 psi load carrying capability/film strength/shear resistance, while 15W50 oils can sometimes only provide UNDESIRABLE wear protection with less than 60,000 psi. So, DO NOT use thicker oil under the assumption that it can provide better wear protection for our engines, because that is simply NOT TRUE.

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BOTTOM LINE: Thinner oils are better for most engine lubrication needs."
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Newold1 » Mon Jan 09, 2017 11:57 am

If that viscosity of oil hot at the operating oil pressure can maintain the hydrodynamic wedge of oil between the bearing and the journal surface with the given clearance of that interface.

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Kevin Johnson » Mon Jan 09, 2017 12:05 pm

I would think you need access to other standard test results for motor oils as well as the results of controlled tests in actual engines (like at SwRI). A criticism of Rat's test was that it was not precisely delineated so that others could duplicate it.

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Mon Jan 09, 2017 12:27 pm

Newold1 wrote:If that viscosity of oil hot at the operating oil pressure can maintain the hydrodynamic wedge of oil between the bearing and the journal surface with the given clearance of that interface.


I don't think that the supply pressure has anything to do with maintaining the wedge other than supplying the oil. Supplying the oil is important. But once it's in, then I think that the oil is on its own. What determines the performance at that situation?
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Mon Jan 09, 2017 12:29 pm

Kevin Johnson wrote:I would think you need access to other standard test results for motor oils as well as the results of controlled tests in actual engines (like at SwRI). A criticism of Rat's test was that it was not precisely delineated so that others could duplicate it.


Is there any other test evidence that suggests that load holding ability in bearing is positively correlated with SAE viscosity grade? That's not a statement, it's an honest question.
Last edited by ptuomov on Mon Jan 09, 2017 12:50 pm, edited 1 time in total.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby statsystems » Mon Jan 09, 2017 12:39 pm

ptuomov wrote:
Kevin Johnson wrote:I would think you need access to other standard test results for motor oils as well as the results of controlled tests in actual engines (like at SwRI). A criticism of Rat's test was that it was not precisely delineated so that others could duplicate it.


Is there any other test evidence that suggests that load holding ability in bearing is positively correlated with SAE viscosity grade?




Now you need to spend the time unlearning everything 540rat wrote.

If you want to learn, call someone like LAT, Torco etc and talk to them. Call Ishahara-Johnson and grab a pencil and paper to take notes. Or, you can ask Kevin to type it all out on here!!!!!!!!!


That's the best way to get a handle on oil. Not some blogger dude with hatchet job test methods.

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Mon Jan 09, 2017 12:46 pm

statsystems wrote:
ptuomov wrote:
Kevin Johnson wrote:I would think you need access to other standard test results for motor oils as well as the results of controlled tests in actual engines (like at SwRI). A criticism of Rat's test was that it was not precisely delineated so that others could duplicate it.


Is there any other test evidence that suggests that load holding ability in bearing is positively correlated with SAE viscosity grade?


Now you need to spend the time unlearning everything 540rat wrote.

If you want to learn, call someone like LAT, Torco etc and talk to them. Call Ishahara-Johnson and grab a pencil and paper to take notes. Or, you can ask Kevin to type it all out on here!!!!!!!!!

That's the best way to get a handle on oil. Not some blogger dude with hatchet job test methods.


I'm a complete neophyte, so I don't know. That's why I am asking. If you can point me to something, I'd love to read of test data or something that teaches me how higher SAE grade viscosity oil can hold more bearing load. I mean... it makes intuitive sense that thicker liquid isn't going to burst out of there as easily as less thick liquid, but then everything is happening so quickly in there that I don't know that I can rely on my intuition. (I'm more confident that higher bearing speed requires lower viscosity and higher temperature higher viscosity.)
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Zmechanic » Mon Jan 09, 2017 1:21 pm

ptuomov wrote:I'm a complete neophyte, so I don't know. That's why I am asking. If you can point me to something, I'd love to read of test data or something that teaches me how higher SAE grade viscosity oil can hold more bearing load. I mean... it makes intuitive sense that thicker liquid isn't going to burst out of there as easily as less thick liquid, but then everything is happening so quickly in there that I don't know that I can rely on my intuition. (I'm more confident that higher bearing speed requires lower viscosity and higher temperature higher viscosity.)


There's a bit of a counter-effect there though. There is self-heating of the oil due to shear stress being placed on it by the rotating/moving parts. If the oil stays in that area too long, it will over heat and lose some of its film and lubricating properties.

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby David Redszus » Mon Jan 09, 2017 2:41 pm

The Sommerfeld Number correlates closely to the coefficient of friction in the
bearing. As oil viscosity, speed, bearing diameter and length increase, so does
the S value. As bearing clearance, temperature and load increase, the S value
decreases. A lower S value will produce a lower coefficient of friction.

Up to a point. That point is a S value of approximately .005 at which friction is
minimized. Below that value we leave the fluid film lubrication regime and enter
the very dangerous boundary lubrication zone. This is where metal to metal
contact is ever present and the bearing will quickly self-destruct.

By entering the viscosity properties of candidate oils, temperature and operating
conditions, it is possible to determine which oil viscosity will provide the required
viscosity necessary to produce maximum power and yet adequately protect
the engine from destruction.

The bearing flow area is the bearing clearance window through which the oil
exits the bearing. A tight bearing may raise oil pressure but will reduce the area
through which the oil can flow.

If we know the oil pressure, specific gravity, and hot viscosity, we can calculate the
bearing flow rate.

If we know the specific heat of the oil, we can calculate the heat transfer for each bearing.

The science of bearing lubrication consists primarily of keeping metal parts from touching
and to keep them cool enough to prevent lubricant destruction. Anti-wear additives are
a secondary consideration which is needed when metal parts cannot be prevented from
touching each other.

The Sommerfield equation provides such valuable insight to lubrication issues that it
should become part of every racer's intellectual tool box.

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Mon Jan 09, 2017 5:30 pm

So the Sommerfeld number S is (oil viscosity times bearing speed divided by pressure on the shaft) times (shaft diameter divided by double-sided bearing clearance)^2. The friction coefficient appears to be a function of the first part (oil viscosity times bearing speed divided by pressure on the shaft), is that Hersey's number, and that's the only way the viscosity, speed, and the pressure enter the friction coefficient formula. The minimum friction coefficient is produced at some Hersey's number: That's where the sum of metal-rubbing friction and oil-shearing friction is minimized. But since from the wear perspective we really dislike the metal-rubbing friction, we're going for much higher, 3-15x, Hersey's number and move far to the right on the graph. And then we're in the hydrodynamic lubrication regime and there's very little metal-to-metal wear, even though friction is not minimized.

Does the above provide any practical insights for a neophyte amateur hot rodder? If I understood that correctly, does it mean that if I start with an engine that has everything correctly designed and if I increase the pressure on the shaft by say 50% (say, I'm turbocharging the engine and peak compression load goes up by that much) while the engine speed stays the same, then I should increase the viscosity (not SAE grade) of the oil also by 50%? If I increase the viscosity of the oil by that 50%, then isn't the engine going to flow a lot less oil and bearings run a lot hotter?

David Redszus wrote:The Sommerfeld Number correlates closely to the coefficient of friction in the bearing. As oil viscosity, speed, bearing diameter and length increase, so does the S value. As bearing clearance, temperature and load increase, the S value decreases. A lower S value will produce a lower coefficient of friction.

Up to a point. That point is a S value of approximately .005 at which friction is minimized. Below that value we leave the fluid film lubrication regime and enter the very dangerous boundary lubrication zone. This is where metal to metal contact is ever present and the bearing will quickly self-destruct.

By entering the viscosity properties of candidate oils, temperature and operating conditions, it is possible to determine which oil viscosity will provide the required viscosity necessary to produce maximum power and yet adequately protect the engine from destruction.

The bearing flow area is the bearing clearance window through which the oil exits the bearing. A tight bearing may raise oil pressure but will reduce the area through which the oil can flow.

If we know the oil pressure, specific gravity, and hot viscosity, we can calculate the bearing flow rate.

If we know the specific heat of the oil, we can calculate the heat transfer for each bearing.

The science of bearing lubrication consists primarily of keeping metal parts from touching and to keep them cool enough to prevent lubricant destruction. Anti-wear additives are a secondary consideration which is needed when metal parts cannot be prevented from touching each other.

The Sommerfield equation provides such valuable insight to lubrication issues that it should become part of every racer's intellectual tool box.
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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Geoff2 » Tue Jan 10, 2017 3:36 am

I often see claims that the load bearing capacity of the oil is not related to it's viscosity. The claim is that the lower viscosity oil flows more easily...........which begs the question: why isn't 10w-30 used in rear axles if it's load bearing capacity is the same as 75/140.

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby ptuomov » Tue Jan 10, 2017 8:31 am

Geoff2 wrote:I often see claims that the load bearing capacity of the oil is not related to it's viscosity. The claim is that the lower viscosity oil flows more easily...........which begs the question: why isn't 10w-30 used in rear axles if it's load bearing capacity is the same as 75/140.


To be clear, I'm asking and not telling!

It seems to me that from the formulas posted above, higher bearing load requires higher oil viscosity to get to the hydrodynamic lubrication regime at the same bearing speed. I can kind of understand that.

However, once we're in the hydronynamic lubrication regime, does viscosity make any difference to the wear? My intuition says no, but I'm not sure.

If we fall out of the hydrodynamic (thick film) lubrication regime into the mixed (thin film) lubrication regime, which is presumably more likely with low-viscosity oil, then does viscosity matter again? Or is it just then the film strength that is analogous to what 540 RAT has been testing?

And is there any other downsides from too high viscosity other than higher friction causing power loss, higher friction causing more heat, and and lower oil flow causing poorer bearing cooling?

Questions, questions -- don't interpret my questions as answers!

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby David Redszus » Tue Jan 10, 2017 2:48 pm

The determination of correct oil viscosity involves the consideration of the the following variables.

Journal diameter/in
Bearing clearance/in
Bearing length/in
Load/lbs
rpm
Oil temp/F
Oil viscosity @40c/cSt
Oil viscosity @ 100c/cSt
Specific gravity
Oil pressure/psi

From these variables, we can construct a 10 x 10 matrix, resulting in 100 possible combination of factors that will
influence oil functional performance. It was much easier to construct an excel worksheet containing those
variables and then select specific variables as they apply to a given engine.

With regard to selection of hot oil viscosity, the minimun specified by many engine mfgs is 3.5cSt. Below this value the oil film thickness is too thin to keep metals from touching; anti-wear additives must be used to prevent destruction.

If a more viscous oil is used, metals may not touch but more energy is needed to move the oil through the
engine. In addition, thicker than necessary oil produces an abrasion wear component so that wear is increased.

The Sommerfeld curve looks like a V shape. The left side is boundary layer while the right side is hydrodynamic lubrication. The ideal S value is at the tip of the V. Either side of the vortex increases wear; the left through metal to metal contact, the right through abrasive wear.

If we add 20% to the bearing load, the Sommerfeld S number goes down by 19%. To regain the previous S value, we can increase the oil viscosity or we can increase the bearing speed by 20%.

We still have not considered shear properties nor cavitation issues.

If one had the ambition to send me specifications for a real engine using the above posted variables and units, I would be willing to calculate the S values and necessary oil viscosity.

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Re: Symptoms of too high or too low motor oil (hot) viscosit

Postby Geoff2 » Wed Jan 11, 2017 4:44 am

I was asking too!
Because to me, some of the claims made about lighter viscosity oils do not add up.


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