That old oil pressure myth again

[C M Smith <cmsmith@domain.elided>]

There is no oil pump made that will fit a car engine and develop enough 
pressure to keep the bearings apart from the journals at TDC if 
the  journal were not rotating in the bearing shell. It is the rotation of 
the journal that protects the bearing. The pressure required to keep the 
surfaces apart is generated by the shear in the oil. Consider that the oil 
stuck* to the bearing shell is "stationary" while the oil stuck to the 
journal surface is also stationary, relative to the surfaces. the two 
surfaces are clearly moving relative to each other. Sheer forces in the 
film of oil between the bearing and the journal develop a wedge of oil 
which forces the surfaces apart. The bigger the speed differential the 
higher the force. * This is due to a boundary layer effect as well as the 
material of the bearing shell.

The oil pump ensures that an oil film is there when the engine first fires, 
and thereafter ensures that a fresh supply of cooler oil is fed to the 
bearing in sufficient quantity to prevent oil breakdown. It has no other 
effect on bearing life.

This is the main reason the newer oils can be so "thin" and still provide 
the necessary protection. The oil needs to have high resistance to sheer to 
do its job. It doesn't have to be especially viscous.

Now older engines were designed to run with heavier viscosities so fooling 
around with oil that is just too thin for the designer's specification is 
not a good idea. Valve guide clearances tend to be a bit large in older 
engines so thin oil can disappear like water down those. Rings operate 
differently from bearings as do cam followers and so on. Otherwise there's 
no such thing as oil that's too thin.

Cheers


Michael Smith
White 1991 164L
Original owner
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