Read with interest, the very correct remarks (so far) on this subject of PSI.
I would like to perhaps clear up some few points which may in the end cause
readers to rethink their psi ideas.
The oil pump pushes out a certain volume of oil per minute and this of
course varies at low rpm but is more or less a constant output volume at
higher rpm.
Any excess volume not allowed to pass through bearings is dumped by the
by-pass valve in the filter housing.
This oil is pushes into the main gallery and from that gallery we tap off
and read oil psi on the dash gage.
The vol. of oil in this gallery is then passed down 3 or 5 main bearing
drillings into the crankshaft which in turn has drillings to the rod
bearings. There are of course MANY other smaller drillings which feed oil
to other places (such as timing chain, valve rocker shafts etc.) but
because these secondary lube targets have little affect on psi, I will keep
my story confined to crank shaft and rod bearings.
Our Rootes engines are like almost all modern engines and employ "plain
bearings" of the thin wall shell design. Main and rods, when new, run
normally with .001" to .0017" oil clearence and IF the engine has this
average clearence on all its rod and mains, the psi in the gallery will
indeed be around 45-50 psi above 3000 rpm, with say and SAE 30 wt. oil on a
summer day.
The 50 psi is obtained because the oil pump is making MORE volume than the
bearing clearences can let pass, hence the high reading in the gallery.
I think everyone can see then, that is the rod bearings wear out and have
say .0025"-.003" average clearences, then lots more of the gallery oil
volume will gush out past each bearing...and the psi reading will be lower.
------------------
The owner could install a thicker oil and see an increase in hot oil
pressure reading....but the bearing wear would still be there and the psi
would be higher on the gage because the thick oil does not pass through the
bearings as easy as the thinner oil.!
----------------
Plain shell bearings stay alive and handle heavy loads because of an
engineering feature called "hydranamic wedge". If you could look into a
plain bearing while it is working, you would see that oil is injected
through the drilling in the crankshaft and the shaft is lifted away from
the bearing a little amount. Opposite the oil insert point, the shaft is a
bit closer to the bearing shell. It looks like an oval of oil or egg shape
configuration of oil. This oval or "wedge" as it is called, is what
prevents the metal to metal contact !
-----------------
Now this bearing does NOT EVER need a certain psi of oil coming into it !
What it ALWAYS NEEDS is an "adequate amount of oil volume" to keep the oval
of oil or the oil wedge intact!
Volume is the key word here.....not psi pressure.
On giant motor ships having huge diesel engines costing millions of
dollars, they have a volume "flow meter" for each bearing. The engine room
staff monitor gallons per minute through each bearing and there are alarms
sounded if a bearing starts to take too much volume! If that happened, it
means the bearing is loosening up its clearences and passing more oil volume.
-----------------
In a passenger car, airplane or other vehicle, we cannot have costly flow
meters on bearings, so we use the next best device, which is the main
gallery pressure readings. We know what is normal and should recognize
what a drop in psi really means.
----------------
As a plain bearing does wear a bit, it is normal for the gallery psi to
read lower (with the same SAE 30 oil in summer etc. etc.) but this does NOT
mean we are having eminent failure. When it wears a bit, the bearing
allows more volume to pass. As an example, say the mains pass one quart
per minute (each) at 3000 rpm. When they wear loose to say .0029" they may
each allow 1.5 quart to pass each minute. This extra oil flow cools the
bearing nicely and further wear MAY NOT occur, if other factors such as
good shaft smoothness, clean oil, reasonable loading and rpms ranges etc.
are not adverse. An engine which when new held up 50 psi hot at 3000 but
now has 90,000 miles and holds only 40 psi hot at 3000, may last for a
really long time running this way, if not abused.
--------------------
Race engine builders understand that engines need lots of cool oil through
bearings to suck away damaging heat. To assure this, we make the bearing
clearences very loose and on Alpine SV I always used full .0022" for both
rod and main clearences ! Racing in the summer I did use SAE 40 wt.
Valvoline but I never saw more than 40 psi hot above 3000 rpm. During
longer races in 100 deg temp. my psi would be down in the 30 psi range but
that engine kept producing its power and reving over 7500 reliably.
When rebuilding your street engine or "mild" hop up engine, stick with the
book data on bearing clearences...and...use a multigrade SAE 10W-30 in
summer and a 5W-20 or 5W-30 in winter. Never use anything like SAE 40 or
multigrade like 20W-40 or 20W-50 with a nice stock bearing clearence. The
thinner oil gets into the bearing faster when cold starting and when
running at full temp. these thinner oils send MORE VOLUME through those bearings!
------------------------
So I say onto you........if your engine is in pretty sound condition, fear
not the lower psi oil readings you may be seeing. DO NOT install real
thick oils to bring up the psi. This defeats the purpose of getting high
volume through bearings. Continue with the NORMAL weights of oil according
to your seasonal temp.
Like anything else, judgement and thinking must be applied and correct
evaluations made. That is why I have written this material. So you will
get a handle on the problem, the cause, the cure and the pitfalls which can
be avoided.
Dick T.
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