My understanding of this subject is this:
It's not really oil PRESSURE that lubricates the bearings,
its oil FLOW. The Spit's oil pump (like virtually all of them)
is a positive displacement pump, which means that the flow is
mostly independent of the pressure at a given RPM.
The oil pressure that you observe is related to the flow rate,
the resistance to that flow resulting from the oil being
forced past the bearings, and the oil viscosity. As the bearings
wear, the resistance goes down, and thus the pressure. If
the oil pump itself is not badly worn, the flow rate will
not drop a whole lot, even if the pressure is way down.
Note that changing to a heavier grade of oil in this situation
will increase the pressure, but probably the flow rate will
not change. But at least a thicker oil is a better lubricant.
In a nutshell, low oil pressure is more a SYMPTOM of engine
wear than a problem in itself. Of course, if you suddenly
start reading low oil pressure on a engine in good condition,
there could be a totally different problem, like a clogged filter
or oil passages (or lack of oil!).
Doug Braun
'72 Spit
At 07:30 AM 5/15/01 , Richard B Gosling wrote:
>Oil pressure is important! Yes, it is true that much of the engine is
> lubricated by splash, but the bits that REALLY matter, your bearings, have
>oil
> supplied to them from the oil gallery at the full gallery pressure. If the
> pressure drops, you don't get enough oil to the bearings, the oil film will
> get too thin and your bearing will wear rapidly. This is not good. I should
> know, I analyse engine bearing perfomance for a living!
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