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.
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>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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