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#4664787 - 02/13/18 12:53 PM Thicker oil better for engine start up?
DriveHard Offline


Registered: 11/24/03
Posts: 1072
Loc: Middle of Iowa
I see and read lots of people talk about how thinner oil is better for engine start up because it "gets to the bearings faster". I have not really put any thought effort into it, and generally considered it an acceptable argument. After some thought, I am not so certain this is a valid argument.

Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for

Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases

so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.

If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.

Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.

Looking forward to the discussion.
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#4664807 - 02/13/18 01:30 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
demarpaint Offline


Registered: 07/03/05
Posts: 29809
Loc: NY
Interesting topic. As Trav has stated earlier today, and several others have stated over the years, bearings already have enough residual oil to protect them from a cold start or from sitting for a long period of time. If I were inclined to worry, I'd worry more about cylinder walls than bearings during a cold start.
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#4664811 - 02/13/18 01:34 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
andyd Offline


Registered: 09/25/04
Posts: 7168
Loc: Marshfield , MA
During a cold start, thin oil is good for about 30 seconds. After that it is a liability. 30 seconds is about how long it took for the thrust bearing in BIL's Datsun 510 to quiet down in a cold start. grin2
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#4664812 - 02/13/18 01:35 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
KrisZ Offline


Registered: 12/23/06
Posts: 7476
Loc: Toronto, Canada
This has been pointed out numerous times and there is a thread or two that explains the difference between fixed volume/variable pressure and fixed pressure/variable volume pumps. However the most enthusiastic thin oils fans flatly ignore all these inconvenient facts and keep on harping flow=lubricating arguments.


Edited by KrisZ (02/13/18 01:36 PM)
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#4664814 - 02/13/18 01:37 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
fsskier Offline


Registered: 07/22/08
Posts: 824
Loc: Iowa
Just a few quick thoughts: The oil pump is a positive flow pump...but the oil pressure relief valve dumps it all back into the crankcase. The multifaceted report....funded by GM and several major oil companies and published years ago in the SAE journal (sadly, you need to be a member to read) noted that:

The pistons/cylinders are only lubricated by oil that flys offs the rod bearings and the tiny squirter hole on the rods. The crankshaft oil is fed from the mains, though the crank, then into the rod bearings....by then under almost no pressure and no flow when thick and cold....the end result is only one drop every few seconds flying up to the cylinder/piston interface. Later, when the oil is hot and thin this is a continuous shower of oil. The conclusion of this study was related to the switch from 10W30 to 5W30.....and that many typical driving profiles that this alone would increase engine life by 30 percent. Data was reinforced by multiple testing procedures, including pistons, rings and sleeves using various radioactive isotopes and running instant real time wear measurements. The first few minutes of operation cause more wear then the next hour!

Wish I could provide a link to this published study (likely 30 years ago) but years ago while using 10W40 and 20W50 I often rebuilt engines after 100K miles. Now using 0W20 in sometimes identical engines and with no substantial design changes for 300K miles and seeming no wear from a users viewpoint.

Yeah, no reason to doubt that thinner oil at startup is important for those of us in the north.

fsskier


Edited by fsskier (02/13/18 01:38 PM)

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#4664820 - 02/13/18 01:49 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
Snagglefoot Offline


Registered: 12/31/17
Posts: 1777
Loc: Alberta
Originally Posted By: DriveHard
I see and read lots of people talk about how thinner oil is better for engine start up because it "gets to the bearings faster". I have not really put any thought effort into it, and generally considered it an acceptable argument. After some thought, I am not so certain this is a valid argument.

Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for

Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases

so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.

If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.

Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.

Looking forward to the discussion.


Good discussion. I suspect if the oil was very cold and thick the pump might cavitate because it is simply atmospheric pressure (approx. 14.7 psi) that feeds the inlet to the pump. The pump is bolted to the block but there is a pickup tube extending down into the sump. That would be one issue.

Sf

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#4664824 - 02/13/18 01:52 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
CT8 Offline


Registered: 10/09/14
Posts: 11025
Loc: Idaho
What most miss is depending on the temps at start up is how long the oil takes to go from the oil pan to the oil pump. At room temps a straight 30 or 40 will get to the pump plenty fast. At -5*f it may never reach the pump
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#4664832 - 02/13/18 01:59 PM Re: Thicker oil better for engine start up? [Re: fsskier]
OVERKILL Online   content


Registered: 04/28/08
Posts: 36480
Loc: Ontario, Canada
Originally Posted By: fsskier
Just a few quick thoughts: The oil pump is a positive flow pump...but the oil pressure relief valve dumps it all back into the crankcase.


It definitely does not dump it all back into the crankcase. This is easily demonstrated with a valve cover off and watching the oil come out the pushrods. One can also overwhelm the bypass, which is observed by indicated oil pressure exceeding the relief pressure. I've seen both on a cold start with heavy oil in an SBC.
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#4664833 - 02/13/18 02:00 PM Re: Thicker oil better for engine start up? [Re: Snagglefoot]
OVERKILL Online   content


Registered: 04/28/08
Posts: 36480
Loc: Ontario, Canada
Originally Posted By: Snagglefoot
Originally Posted By: DriveHard
I see and read lots of people talk about how thinner oil is better for engine start up because it "gets to the bearings faster". I have not really put any thought effort into it, and generally considered it an acceptable argument. After some thought, I am not so certain this is a valid argument.

Assumptions:
the viscosity of the oil is not so high that it would cause oil pump cavitation
oil follows the same path for

Some quick points:
in general, Volumetric efficiency of a fixed displacement pump INCREASES as fluid viscosity increases

so, when an engine is first started, the pump starts pumping the exact same theoretical volume of oil (displacement * speed), but the volume of oil being pumped is actually a function of its volumetric efficiency (displacement * speed * Ve), where the efficiency decreases the volume of oil being pumped. If the volumetric efficiency for two identical pumps is the same, and the only difference is the viscosity of the fluid, then the pump pumping the fluid with the higher viscosity will be pumping more volume.

If the pump pumping MORE fluid due to the higher viscosity has the same oil passages to fill to get to all the parts of the engine, then how does it take longer for the oil to get to all the components when the oil has a higher viscosity? Not to mention that the volumetric losses through bearings/etc. are lower due to the higher viscosity, further increasing the volume of oil available to fill the oil passages, and get to the extents of the oil system even more quickly.

Another question...does bulk modulus also increase with increased viscosity? If so, that would also help this argument.

Looking forward to the discussion.


Good discussion. I suspect if the oil was very cold and thick the pump might cavitate because it is simply atmospheric pressure (approx. 14.7 psi) that feeds the inlet to the pump. The pump is bolted to the block but there is a pickup tube extending down into the sump. That would be one issue.

Sf


That's what the MRV test is designed to replicate, the ability for the oil to be drawn into the pump.
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#4664835 - 02/13/18 02:01 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
KrisZ Offline


Registered: 12/23/06
Posts: 7476
Loc: Toronto, Canada
That is why winter rating was developed. The MRV test ensures no cavitation will happen as long as the W rating temperaturę is not exceeded. Why is all of this news to people? I thought this was an oil forum.
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#4664861 - 02/13/18 02:32 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
NGRhodes Offline


Registered: 01/06/13
Posts: 329
Loc: West Yorkshire, UK
http://www.emeraldinsight.com/doi/pdfplus/10.1108/00368790010352691

"This study has demonstrated a correlation between relative average cylinder liner wear rates at low engine start-up temperatures, base oil composition and oil viscosity"

Emphasis mine.

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#4665130 - 02/13/18 07:17 PM Re: Thicker oil better for engine start up? [Re: DriveHard]
Garak Offline


Registered: 12/05/09
Posts: 24620
Loc: Regina, Saskatchewan, Canada
Shannow has posted extensively about timed oil flow. If he stumbles across the thread, he might be able to post his charts again.
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#4665136 - 02/13/18 07:26 PM Re: Thicker oil better for engine start up? [Re: Garak]
4WD Online   confused


Registered: 09/21/10
Posts: 6439
Loc: Texas
Originally Posted By: Garak
Shannow has posted extensively about timed oil flow. If he stumbles across the thread, he might be able to post his charts again.


This chart would be interesting with very cold oil:


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#4665198 - 02/13/18 08:18 PM Re: Thicker oil better for engine start up? [Re: Garak]
Ducked Offline


Registered: 10/25/12
Posts: 4395
Loc: Taiwan
Originally Posted By: Garak
Shannow has posted extensively about timed oil flow. If he stumbles across the thread, he might be able to post his charts again.


This is the usual counter-post, though its from a rather old study and you'd think there'd be a bit more data on this rather basic question. IIRC rocker oiling time was judged on the basis of sound.


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#4665208 - 02/13/18 08:26 PM Re: Thicker oil better for engine start up? [Re: NGRhodes]
Ducked Offline


Registered: 10/25/12
Posts: 4395
Loc: Taiwan
Originally Posted By: NGRhodes
http://www.emeraldinsight.com/doi/pdfplus/10.1108/00368790010352691

"This study has demonstrated a correlation between relative average cylinder liner wear rates at low engine start-up temperatures, base oil composition and oil viscosity"

Emphasis mine.


So I have to pay 32 USD to read that and then if I'm convinced I have to buy "full synthetic SAE 5W40 grade oils based on Polyalphaolefin"", and a diesel truck.

Ooer! Just as well cold starts mean 10C here.

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