9/4/00
Neil,
Thanks for the excellent additional information. I would say it is
a large, not small, correction.
Cheers,
Dave K.
----------------------------------------------------
>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:
>>
>>
>>
>>
|