Bob, the VI number does not refer to oil viscosity (please re-read
original text I posted). This refers to the inherent molecular film
strength or in other words, to the ability of a layer of oil
molecules to resist pressure and maintain its layer intact under such
pressure from the metal parts it is trying to keep separated.
Obviously a higher VI number would be an oil which would have
superior ability to prevent metal to metal contact such as in
bearings, cam lobes, piston rings etc.
The actual viscosity (thickness like SAE 30, 40, 50 etc.) has NOTHING
to do with VI properties of the oil and in fact a certain 10W oil
with say 120 VI # would protect the engine much better than an SAE 30
oil which had a VI# of say 108. I confess not knowing exactly how
they determine this VI number but do know the lab test to determine
it is quite carefully done on each batch of oil being blended to meet
API and Mil specs. The blender must add costly VI improver
chemicals to bring a weak grade of oil up to the spec VI # required.
Pennsylvania crude has a natural VI of somewhere around 90 (?) when
refined. This is pretty high to start with and they have to add only
a small amount of VI Improver.
Back prior to about 1950, US engines mainly had low VI requirements
due to small valves in OHV engines which had mild spring pressures.
As engines were better designed, HP per Cu In increased,RPM increased,
bearing loadings and rpms increased etc. etc. Of course oil VI had
to increase in order to protect these engines. In time we had
emission controlled engines which required new oil qualities but
VI # has NOT had to be increased in recent years due to internal
stress NOT being increased, and in fact it is generally a bit lower
these days due to better designs (bigger bearings etc.)
I have no idea what your instructor referred to in those data your
quote and again confes no memory of such rating systems . I am
inclined to think that due to passage of years, perhaps the oil
viscosity as measured by Saybolt Universal Seconds test apperatus,
(SUS) from which we obtain the well recognized SAE oil vis. tests
performed at both zero deg F. (W numbers) and 212 deg.F. (non W
numbers)has gotten mixed into your oil viscosity index where of
course it is not supposed to be???
And by the way.........back in about 1927-29 when we first developed
the current SAE thickness system still in use, we used a Studebaker
Commander car to determine how hot oil ran , on a summer day at 60
Prior to this date, oils were rated with locomotive terminology like
winter, summer, thick, thin, medium and any and all sorts of terms
the maker might decide to use. Steam cylinder grades were then the
only common terms we could sort of interchange and even those were
not to strict test rules.
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