Now a little more oiling system information.
The stock oil pump in good condition can pump up to 56 quarts per minute
at 7000 rpm engine speed (I learned that the hard way). At relief
pressure (50-80 psi at road speed), about 12 to 20 quarts of oil per
minute (depending on engine condition) goes through the engine, and the
excess gets dumped over the pressure relief valve directly back into the
sump. An oil pump has to be in excessively bad condition not to be able
to pump at least 20 quarts per minute at 3500 rpm, so an engine in decent
condition will always be dumping some oil over the pressure relief valve
at road speed. The pump is a positive displacement type, meaning that it
always moves a certain volume of oil regardless of the pressure (except
for a little internal leakage).
Oil pressure is a result of two things, oil flow volume from the pump,
and resistance to flow throughout the engine. Smaller oil flow passages
provide more resistance to flow, and therefore higher system pressure.
Larger passages provide less resistance, resulting in lower system
pressure. This is why worn out bearings result in low oil pressure.
Maximum system pressure is limited by the pressure relief valve. Minimum
system pressure is determined by how slow the engine idles and the
condition of the bearings (all of the bearings). Larger clearances
result in more (and easier) oil leakage from the bearings. Loose main
bearings will dump too much oil, reducing oil flow to the rod and cam
bearings. A loose rear cam bearing will further reduce oil flow going to
the rocker shaft. With really bad bearings the oil pressure will be
quite low and the rocker shaft will get very little oil.
A certain amount of power is required to drive the oil pump, and power is
energy. None of the oil flow does any useful mechanical work, so all of
that energy turns into heat when the oil pressure returns to zero as the
oil escapes back into the sump. When oil is passing over the relief
valve, all of that oil flow represents heat being generated directly in
the oil. The greater the pressure drop, the more heat. The greater the
flow, the more heat. If you increase spring force on the relief valve to
get higher pressure, you get more heat, but only a little more flow
through the engine. If you install a higher volume oil pump, you get
more flow over the relief valve, and more heat, and no more flow through
the engine.
The only time a high volume oil pump increases flow through the engine is
at low speeds and low flow rates when there is no oil going past the
relief valve. This can increase pressure at idle but will not affect the
pressure at all at higher engine speeds, and is generally not required
with an engine in good condition. Higher flow and higher pressure in the
system take more power to run the pump, heat up the oil, and produce more
wear on the drive gears.
Heat is also generated in the oil by friction of the moving parts. Oil
flow through the bearings is important to carry away the heat generated
by friction in the bearings. High volume oil pumps are generally
applicable to racing engines where increased bearing clearances are used
to increase oil flow in the bearings for improved cooling of the
bearings. When oil flow is too little to carry away the heat, the oil
will overheat and the oil film will break down allowing metal to metal
contact and catastrophic failure of the engine. This is why an engine
will give up the ghost very soon after an oil pressure failure or if it
runs out of oil.
A large increase in pressure will NOT give you a large increase in flow.
Pressure increases exponentially with flow. This means that a large
increase in pressure will only get you a little increase in flow through
the engine.
One other point. The pumping pressure in the oiling system does not
support the load on the bearings, it just drives the flow of the oil.
The load on the bearings is carried on the thin film of oil trapped
between the bearing surfaces. And, the working pressure on the
connecting rod bearings (just one example) far exceeds the oil pumping
pressure. No pumping pressure is required to support the load, only to
provide the flow. I hear that certain Rolls Royce engines run nicely on
4 or 5 psi system pressure. Also, many early auto engines had no oil
pump at all, and therefore no pumping pressure, where oil is supplied
only by splash, spray and gravity. I'm not saying that these engines had
no oil pressure, just no pumping pressure; there is always pressure on
the oil film from the working load.
Barney Gaylord
1958 MGA with an attitude
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