Installed oil pressure gauge -- sludge problem

[Kent McLean]

Arthur Marks wrote:
 > Haven't found a good physics description of pressure and flow
 > through the lube system.

What do you consider "high" and "low" oil pressure?  I'll give
you my 2 cents, based on what I learned driving a bunch of
British sports cars (Healeys, Triumphs, MGs) as a kid...

In the "old" days (single weight, non-synthetic oil), the oil
pressure would be high on initial start-up of the car, then drop
as the engine warmed.  At idle, it was about 20 psi. Revving the
engine would cause the oil pump, geared off the crankshaft, to
pump more oil, raising oil pressure to the neighborhood of 60 psi.
If the car was driven hard, stressing the engine would overheat
and thin the oil, and oil pressure would drop, reading real low
(5-10 psi?) at idle, and maybe 30-40 with revs.

Fast forward to the new millennium. Multi-weight and synthetic
oils handle wider temperature ranges, but you should still see
something like 60 psi (4-5 bar) while running, less than that
at idle.

As for the oil pressure relief valve in the filter, cheap
oil filters might not have one, good filters will. You'd
have to do a web search to see if yours has one. But if it
does, it's purpose, as previously stated, it to keep oil
flowing when the filter itself clogs and can't pass oil
any longer. It the pressure relief (by-pass) valve will
let oil flow.

If your engine was poorly maintained (few oil changes, too
long between changes, cheap oil, no filter change, etc.) and
has sludge built up, then cleaning the inside of the engine
may release sludge, causing the oil filter to clog quicker.
Synthetic oil tends to cleanse the internals, and there are
engine cleaner additives that will cleanse things faster
(the "idle for 5 minutes then drain" additives).  In either
case, if you fear a clogged filter, change it. And keep
changing it more often (1000 miles? 500 miles?) than what
is usually "recommended" until the engine internals are
clean. When is that?  Remove the oil cap and look inside.
Reach a finger in and wipe the underside of the valve cover.
Or be real extravagant and spend some money for a new gasket
and remove the valve cover to look at things (replace the
gasket when you reinstall the valve cover).

<snip>
 > So I would guess upstream oil pressure could drop by the
 > amount required to open the valve?

My understanding, and I could be wrong, is that the engine
without an oil filter should produce a given oil pressure.
That is, free flowing, no restrictions. Let's call it 5 bar
(~75 psi), and also assume it moves a gallon of oil a minute
(that's just a big guess on my part, as a basis for this
discussion).  Adding in a clean oil filter shouldn't change
that much at all. The filter's job is to filter, not restrict
flow.  But over time, the filter does it's job, particles
clog the filter, and it does restrict oil flow.  The oil
pump still wants to move 1 gallon a minute, but the oil
filter won't flow that much. That would increase the oil
pressure between the pump and the filter, while the passages
after the oil filter would have next to no pressure (when
the filter is totally clogged).  To combat that, a good
filter will have a pressure relieve valve which lets the
oil flow past (bypass) the filter, reaching the passages
behind the oil filter.  Yes, the oil is unfiltered, but
that is way better than no oil reaching critical parts.

OK, that was more than 2 cents.  If anyone wants to correct
my view, or update my knowledge, please don't hesitate. I
have been wrong before.

--
Kent McLean
'94 100 S Avant, "Moody"
'89 200 TQ, "Bad Puppy" up in smoke