Just some random thoughts on oil pressure :
> Enough pressure is more than sufficient as
> it will lubricate the engine.
It actually takes almost no pressure at all to lubricate the engine. The main
problem is that the oil must also cool the bearings. Which requires an adequate
flow of oil through the bearing, to carry the heat away. If the flow of oil
stops, the bearings overheat fairly quickly (within a few miles) ... the usual
result is that the bearing material actually melts and welds to the crank. I
helped rebuild an engine that was driven some 20 miles with NO oil pressure
(after the pump drive failed) ... we literally had to chisel a bearing insert
off the crank journal. (But the crank cleaned up at .010" under so it ran
again.)
Unfortunately, flow is not directly related to pressure. The clearance in the
bearing also plays a large part. So engines with somewhat worn bearings can
survive with much lower oil pressure indicated than those with new, tight
bearings. I've driven old engines a long way that carried less than 5 psi at
hot idle ... one of them carried less than 5 psi (idiot light on) at hot cruise
!
> but seriously, I wonder how
> accurate these gauges are,
Not very.
> and what the REAL tollerances of oil pressure
> really is.
To me, the oil pressure gauge has two primary uses :
One, any sudden unexplained change indicates a problem. That can be either too
low or too high; but is always relative to what your engine normally does under
the current conditions. If it normally carries 70 psi on the highway, and you
look down and only see 30 psi, there is something wrong. But if it normally
only carries 30 psi, then 30 psi is probably fine. And seeing 70 psi means
there is a problem.
Two, it can provide a general indication as to how worn or loose the bearings
are. Of course, it's affected by a lot of other things, like oil grade and
temperature, but it still gives some indication.
> I will monitor it everytime I start
> the engine and check it when I turn the car off.
Occasionally checking it as you drive is also a good idea, IMO. To me, it's the
sign of a good driver to scan the gauges from time to time. And I always watch
for it to come up before putting any load on the engine.
Randall
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