One small correction. The back pressure is not constant as engine speed
rises. It is this back pressure that is registering on your gauge and is a
result of trying to stuff the oil through the various oil passages.
The volume of oil delivered by the pump rises directly with the speed of the
engine. But as that volume of oil flowing through the passages increases,
the back pressure rises at a higher rate (with the square of the speed, I
think, such that a doubling of the speed results in a four fold increase in
the pressure). As stated elsewhere this continues until the relief valve
lifts, whereupon little further rise will happen.
The rules of thumb for oil pressure vs engine speed are mainly there to
ensure that the centrifugal force on the oil in the crankshaft does not
cause the main bearings to be starved of oil as the oil is forced out to the
big end (rod) bearings. Again this will be a 'square of speed relationship',
but within the range that most engines run (say 5000-7500 rpm) a linear rule
of thumb is good enough.
Neil
----- Original Message -----
From: "David Kernberger" <dkern@napanet.net>
To: <buick-rover-v8@autox.team.net>
Sent: Monday, September 04, 2000 6:13 AM
Subject: Muddying the waters?????
> 9/3/00
>
> To all,
>
> The subject of oil pressure has been talked about from many angles
> here, but I haven't noticed any purely theoretical treatments. I have
> limited experience, but a good understanding of physics, so I offer the
> following, but not as a contradiction of what others have offered. Please
> excuse my verbosity--I do not use words efficiently.
>
> An engine oil pump is a positive displacement pump--it delivers a
> certain volume of oil for each revolution. Assuming it is not worn, this
> means the output (gallons per minute) is dependent only on the speed of
> rotation. Double the engine speed and you double it's output. The pump
> itself cannot develop any pressure at all unless it encounters some
> resistance to it's output (back pressure). Close bearing clearances,
small
> bearings, small oil passages, high oil viscosity, etc. make for high
> resistance, and therefore high pressure. Loose bearing clearances, large
> bearings, large oil passages, low oil viscosity, etc. make for low
> resistance, and therefore low pressure. It is not possible to completely
> characterize a system as either "high pressure" or "high volume" as many
> have been trying to do. The key is the size of the pump output in
relation
> to the back pressure of the engine. A big pump in a small engine tends to
> produce high pressure. A small pump in a large engine tends to produce
low
> pressure. Back pressure is basically not dependent on engine speed--it is
> a constant in any engine except for it's dependence on wear level and
> changing oil viscosity with temperature.
>
> In Rover/BOP engines, as in all others, actual pressure tends to be
> low at low speed because pump output is low while back pressure is
whatever
> it is. This is OK because bearing loads are generally low also and all
> that is needed is to maintain a film of oil in the bearing to prevent
> metal-to-metal contact. A VERY high volume pump would be required to
> produce very much pressure at idle speed. As engine speed increases, so
> does pressure, because pump output increases, while back pressure remains
> nearly constant. This is good because bearing loads tend to increase with
> higher speed and more pressure is needed to maintain the oil film. So
> far-so good.
>
> Rule of thumb or not, engine designers make a decision to limit
> maximum pressure to XX psi, and put a spring loaded valve into the system
> which limits pressure by bypassing excess flow back to the oil pan
whenever
> it reaches that level; this occurs generally only at fairly high engine
> speed. As long as the valve and the return passage are both large enough
> to pass the excess flow, system pressure will never rise above XX psi. If
> you want to increase oil pressure at high engine speed, you need a stiffer
> spring on the relief valve. But this spring cannot have any effect at all
> on low speed pressure. If the pump does not produce sufficient volume,
> changing the spring may have very little effect, even at high speed. High
> pressure requires high volume in relation to the system back pressure.
>
> Other things being equal, the following relations are generally true:
>
> 1) Increasing pump capacity will increase pressure at all engine
> speeds up to the pressure relief valve setting.
>
> 2) A stiffer bypass valve spring will only raise maximum pressure
> and, even then, only if pump has sufficient capacity.
>
> 3) It is very difficult to define any particular system as
> inherently high volume or high pressure.
>
> 4) Increasing oil viscosity will increase pressure at all speeds up
> to the pressure relief valve setting.
>
> 5) A new engine will show higher pressures than a worn engine.
>
> The other factors of relative load on the drive gears, effects of
> closer manufacturing tolerances, old versus new oil recommendations, ete.
> etc. etc. have been well covered by others with more experience than I.
>
> Thanks for listening, if you did.
>
> Cheers,
>
> Dave K.
>
>
> ----------------------------------------------------------
>
> >Frankly Glen, I limit my comments to this and other lists because of
responses
> >like yours. I'm not speaking of list surveys nor am I guessing. I'm
speaking
> >from 20 years hands on experience with TR8s, street cars and a race car,
and
> >having built a number of engines. I currently have a fully built 4.6 and
> >a 4.2
> >short block on engine stands. The TR8 factory manual specifies 1.97
> >kgf/cm2(28
> >lbf/in2) pressure at 2000 rpm and 35 lbf/in2 at 2400. Given your
assertion,
> >that would mean that pressure at idle, 750 to 900 rpm, would drop as revs
> >rise,
> >a most unusual circumstance. One would also then have to wonder why
there
> >is a
> >pressure relief valve in the system. You apparantly do not understand
the
> >proper and careful building of an engine, often refered to as
"blueprinting".
> >It means in essence to optimize clearances and mechanical relationships
to
> >approach that intended by the designer, removing manufacturing variances.
A
> >blueprinted bottom end would give results reflecting just what the
designer
> >intended, in oil pressure and in other ways. That is the way that race
> >engines
> >such as those I have built, using stock components, are put together.
You
> >apparantly do not understand oil viscosities either. 20w50 is one of the
> >viscosities recommended for the TR8, but not the only one. It was a
> >common oil
> >in the 70s before the feds imposed fuel economy standards and before
> >manufacturing processes allowed much less variance in clearances as is
now
> >true. Engines running the same bearing clearances now use 5w30 because
it is
> >easier to move at cold start, resulting in less wear in that time when
> >most wear
> >occurs, and because it contributes to fuel economy. Increased pressure
in
> >itself does not incxrease protection or lessen friction. It is necessart
only
> >that there be sufficient pressure to maintain the oil film that separates
one
> >part from another. Smokey Yunick, one of the most respectred
"hotrodders"
> >ever
> >has often stated that an engine needs only 10 #/in2 per 1000 rpm. 20w50
> >and the
> >like is often used in engines with larger clearances due to wear. You
are of
> >course entitled to use that which you choose, but it is not helpful to
make
> >guesses without the background needed to understand what is actually
> >happening.
> >Am I a hot rodder? Perhaps, but I respect the advances made by those
much
> >more
> >capable than I. Do you still use the tires, lubes, fuel and coolants
> >originally
> >specified. I doubt it, because most are no longer available, having been
> >supplanted by newer products. John
> >
> >Glen Wilson wrote:
>
>
>
>
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