Jack Radley wrote:
> Subject: Oil Pressure - follow up
> What does the oil pressure gauge measure? If it is oil pressure right
> off the pump, what does that have to do with the condition of the
> engine?
The pressure reading will indeed vary with engine condition no matter where
the gauge reading is taken, but due to friction (assuming some reasonable
amount of flow), it WILL be higher the closer you get to the pump.
Typically, however, pressure readings are taken sufficiently far away to
indicate enough about engine condition to be meaningful. The best is a tap
off an oil gallery, somewhere fairly close to where oil under pump pressure
enters the engine.
As the engine wears, the bearing clearances enlarge and oil flows through
more quickly, with a resulting drop in pressure. An increase in temperature
will lower the viscosity of the oil and similarly increase oil flow through
the bearings, and similarly lower indicated pressure. Filling with a lower
viscosity oil (say 5W-30 vs 20W-50) will similarly lower indicated pressure.
All the pressure concern aside, the main thing is adequate flow through
bearings. Most engines will function fine for a very long time with only
10-15 pounds of pressure, if there is a good supply of oil to the bearings.
The bearing surfaces (crank journal and bearing shell) actually create
hydrodynamic pressure by virtue of their relative motion. A cushion of
relatively high-pressure oil is formed by the motion. As this high-pressure
oil escapes out the sides of the bearings, the pump must replenish the
supply. As long as there is adequate oil SUPPLY to the bearings, the motion
will create adequate PRESSURE at the bearing surface.
The most critical lubrication need is during cold start-up. Then, the
bearings actually tough metal-to-metal until oil flow begins and
hydrodynamic pressure builds a cushion of oil. Conversely, bearings
operating constantly at an even load and speed, with a good oil flow, can
last virtually indefinitely. I've heard of power turbine bearings, run
constantly for years and finally stopped and disassembled, which show
original tool marks made when the bearings were scraped in many years
previously.
> If you have an engine that is 100 miles from blowing up, but that has
> had a new oil pump installed wouldn't you expect to get good pressure
> while at the same time you are driving a time bomb?
An engine generally "blows up" from oil pressure failure (pump breaks, pump
drive gear or shaft breaks, or sludge, etc. blocks the oil system
somewhere), an out right component failure (cracked rod, crankshaft, etc.),
or a gasket failure. The first failure mode (oil pressure failure) is less
likely with a good new pump, but not impossible (ie drive gear, etc. breaks,
sludge blocks pickup screen of internal engine oil passages, etc.). The
other failure modes have nothing to do with oil pressure.
If bearing clearances are a little excessive, a new (maybe high-volume) oil
pump can actually prolong engine life a bit by providing adequate oil
despite the greater clearances. Not generally very cost-effective, though,
and if you have it far enough apart to put in a new pump, new bearings are
usually in order.
> Do the differences in the tolerances found in a new engine compared to
> the tolerances found in a worn out engine affect oil pressure? Is the
> oil pressure gauge affected by the back pressure of oil being pumped
> through the tighter tolerances of a new engine?
Yes and yes. That's it exactly.
- Karl Vacek
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