Are thinner oils about fuel economy or tighter engines?
- Thread starter ZZman
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Yes it’s “molecules spacing” for sure.
What does GD&T have to do with this discussion?
Internal clearances have not changed since the 40's. There is a limit to how tight clearances can be.
I'm the one who always messes that up between tolerances and clearances LOL. I am grateful that we have the great mind here to help me remember that there is a difference.
MolaKule
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This gets into my current field of study, Physical Chemistry.
Paraffinic molecules (Group II, III, PAO, etc) average about 5 nanometers in size, or ~ 0.0000002 inches, or 0.2 micro inches or 0.2 millionths of an inch.
Additives, such as ZDDP molecules, may range up to 10 nanometers in size or ~ 0.0000004 inches, , or 0.4 micro inches.
In summary, engine oil molecules, with their nanometer sizes, will pass freely through bearing clearances of say, 0.0019 inches, which is the middle spec for my Nissan 4.0L engine's main bearing Clearances. Those bearing clearances are therefor ~ 4,750 times larger than the average engine oil molecule.
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Reduction in costs and increases in performance for computer memory is a far different thing. Moore's law ruled for so long - that on average transistors per unit area would double about every two years and the performance would also increase.
But what's really happened with computer memory is booms and busts with regards to over and underproduction or strong or weak demands.
If the journal bearing has a clearance of 0.0019 inch, and if the journal was centered (the best it can do while turning), then the running clearance would be a max of 0.0019 inch/2 = 0.00095 inch. A running journal bearing can have a pretty small MOFT, like around 5-20 microns. The MOFT is minimized where the max loads are going through the rod due to combustion and inertial directional changes. Anyway, if the additives molecule size is around 0.4 microns, then it's still about 5μ/0.4μ = 12.5 times smaller than the minimum clearance if the MOFT gets down to 5μ. Your 4.750 times larger clearance than the molecules seems off.
so..... anchors 
away use a 0w30 or similar where 0w20 recommended no big deal????
away use a 0w30 or similar where 0w20 recommended no big deal????
MolaKule
Staff member
Maybe it's late but 0.0019 inches bearing clearance/0.4X10-6 inches molecular size = a ratio of 4,750.
Your original post said 4.750, not 4,750.
Plus, the bearing clearance called out in service manuals is the total clearance (delta between journal OD and bearing ID), not the running clearance with the journal centered in the bearing, as explained earlier.
MolaKule
Staff member
Well, I thought it said it was a ratio of 4,750, with a comma.
I am not not sure what we are debating here. When you measure the bearing clearance with say a Plastigage, you are measuring the distance between the journal surface and the bearing surface (as per the manual for general over hall procedures). That distance was compared to the size of a formulated engine oil molecule, nothing more, nothing less, in order to show the magnitude of the difference.
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When people see 0W, the first thing that comes to mind is that I'm getting a good oil ... and not that it meets or exceeds my winter startup temperature requirements.
Not saying that a 0W-XY is not a "good" oil but this designation seems to be losing it's "intended" meaning. Kind of a paradigm shift. No?
Not saying that a 0W-XY is not a "good" oil but this designation seems to be losing it's "intended" meaning. Kind of a paradigm shift. No?
I'm just pointing out that the more realistic comparison of molecule size to bearing clearances is to compare the molecule size to the actual running MOFT clearance in the bearing. Bearings don't exhibit the full shop manual clearance (journal OD vs bearing ID) when they are operating. The actual running clearance can be in the 5 to 20 micron range, which would be 0.000197 to 0.000787 inch. So if the molecule was 0.0000004 inch, the size ratio in an operating journal bearing could actually be as low as 0.000197/0.0000004 = 493, which is much less than 4,750. But even with a minimum MOFT clearace, the molecule is still way smaller than the running MOFT.
I have a 44 year old BMW Airhead motorcycle and the minimum main bearing clearances are 0.0007" and big ends 0.0009"
So it's not that clearance specifications have changed but it might be about how they are achieved. To get those tight clearances there is a quite a choice of coded grades of bearings and shells. Today they might be able to machine accurately enough to avoid so much selective assembly.
The real irony is that in spite of the fine tolerances these engines are built to, the general mantra on airhead forums is that 20W50 must be used. I am not of that persuasion at all and have use 5W40 for many years and wonder if I shouldn't be trying a 5W30 as my engine runs the oil quite cool.
So it's not that clearance specifications have changed but it might be about how they are achieved. To get those tight clearances there is a quite a choice of coded grades of bearings and shells. Today they might be able to machine accurately enough to avoid so much selective assembly.
The real irony is that in spite of the fine tolerances these engines are built to, the general mantra on airhead forums is that 20W50 must be used. I am not of that persuasion at all and have use 5W40 for many years and wonder if I shouldn't be trying a 5W30 as my engine runs the oil quite cool.
The discussion is about tolerances and clearances isn't it?
MolaKule
Staff member
Here is some info that may provide some Background:
GD&T: The Basics of Geometric Dimensioning and Tolerancing
This guide explains GD&T symbols, principles of dimensioning and tolerancing, and provides guidelines for both traditional and digital manufacturing.
formlabs.com
My response to the needing thinner oils was not so much for bearing clearances, but because of the variable valve timing phasers or actuators that use very fine mesh screens to keep debris out of the works. And how much easier it was for a thinner fluid to flow through them, especially when the oil is very cold. GD&T was my life for many years.
Its proof of concept of flow through a screen. Isn't gravity one of the methods to test viscosity, you know the ball drop in the tube?Gravity is not a positive displacement oil pump.
And besides many engines now use a variable displacement pump.
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