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Where does oil pressure come from ??

Posted: Sat Jul 06, 2019 4:50 pm
by jondee86
Copied from the SYNFORCE web site and worth a read :)


A customer changes their brand of oil and then complains of low oil pressure ... is this perception
or reality and is low oil pressure inherently bad, why don't customers complain about high oil pressure?
Customers who change brand and experience an increase in oil pressure usually show delight,
when in fact they should show concern.

The first thing we have to understand is the word 'pressure' ...Pressure is resistance to flow

In the simplified diagram below, the oil in an engine is picked up from the sump by the oil pump
through a filter screen, and then pumped through the oil filter and on through the oil galleries.


A pressure sensor is usually placed down stream of the filter to monitor the oil pressure the read
out being the oil gauge in your dash.

The most important criterion for good lubrication is OIL FLOW, not oil pressure.

** Low oil pressure is not necessarily bad, in fact it can be very beneficial within limits
** High oil pressure is not necessarily good, in fact it can be cause for concern
As pressure is resistance to flow, an engine with wide oil galleries and using a low viscosity
oil will have a faster flow rate and lower oil pressure, a very desirable condition for minimising wear.

Conversely, an engine with smaller galleries and a higher viscosity oil will have less oil flow
and a higher oil pressure, resulting in less efficient lubrication. In extreme cases, the oil would
never get to the critical parts to do it's job of lubrication and reducing wear, and in some
instances the oil would 'by pass' through a valve fitted to most engines, that is to allow the oil
to return to the sump, reducing the amount of oil getting to the those moving parts.

High oil pressure (too high) can actually cause damage to seals and components, and should
be seen as an indication of poor flow.

Cold start, When an engine starts from cold, all of the oil is in the sump.

The oil pump cannot deliver oil pressure until it has sucked up the oil and has begun pushing
the oil through the galleries, hence cold start lubrication is best achieved from an oil with good
cold flow Properties. This is essential as wear is most experienced at cold start due to the lack
of oil available to engine components.

You may have noticed that your oil gauge reads higher at cold start up, this is telling you have
more resistance to flow, and probably at a point in time you need faster flow (less pressure).At
this point and until the engine warms up and the oil pressure stabilises, you do not have proper
lubrication. Until stable oil pressure is obtained, wear rates are high due to inadequate lubrication
and a sign of a good oil is one that gives stable oil pressure the fastest.

So when you change brands and experience higher cold oil pressure, think of the above.

Normal operation
Under ideal circumstances, oil pressure should be stable within reason, and any large increase
or decrease should be investigated for possible mechanical damage. Consult your Vehicle
handbook or experienced service centre to learn of the correct oil pressures your Vehicle
manufacturer designed the engine to run on, both cold and hot.

In Summary
** Watch for any large decrease or increase in pressure from normal, this usually suggests
that you have a fault and it should be investigated.
** Remember, Correct flow is more important to lubrication than pressure
** Choose a good quality oil with a high Viscosity Index to minimise thinning at temperature.
** Cooling is an important function of any engine oil, and flow rates effect this.

Do not be fooled by those oil companies building a 'thicker' oil under a standard viscosity rating so you
will endorse their product over others, based on the false premise of higher oil pressure is better.

Beware of high oil pressure, it may cause damage.


Cheers... jondee86

Re: Where does oil pressure come from ??

Posted: Sat Jul 06, 2019 10:21 pm
by sirdeuce
Too bad there isn't a flow metering device to measure the flow rate instead of, or in addition to, pressure. In the past I've built my 1/4 mile engines with looser tolerances and high volume pumps (VW days). Increased clearances and notched rod thrust surfaces to allow oil to flow. No real way to measure volume, but as a general rule we liked 10 psi per 1000rpm. Though I typically shimmed my pressure relief to 75 psi max. Back then paraffin based oils seemed to work best for our race engines, even though I refused to use them in my street engines.

Good subject! One that doesn't get much attention.

Re: Where does oil pressure come from ??

Posted: Sat Jul 06, 2019 11:28 pm
by jondee86
Surprising what you can find if you poke around the interweb. The below is lifted from
a really old post on MR2OC talking about a 16V 4AGE build.

"Drain backs are on the right rear of the block. Oil pools there on high G left hand turns
on the front engine rear drive cars.. Front wheel drive layouts like the MR2 and Corolla don't
have to worry about that as the oil is spread across the rear of the banks and does not pool.
With the switch to the SC high volume pump gear across the range (rev 1 used a lower
volume pump) you had much more oil up top.

With the high volume pump and with the pressure increased with the spring, you can run
looser bearing clearances without the chance of loading the head with oil. Looser bearing
clearances makes for a faster engine.
To put the importance of bearing clearances into perspective, consider this:
by simply doubling the space between the bearing and the crankshaft, as much
as four times the volume of oil can be distributed.".


Cheers... jondee86

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 12:48 am
by jondee86
Oil Pressure Tech by Kennedy’s Dynotune
Oil Pressure Tech

Is oil pressure important? In a sense, NO. But that’s an exaggeration to get your
attention. What IS really important is oil flow. The plain bearings in an engine work
as follows. There is no metal to metal contact except at startup and shutdown.
What separates the shaft and bearings is a film (actually a wedge) of oil. Oil is
constantly flowing out from the edges of the bearings, so a constant flow of oil is
needed to replenish this.

The classic definition of a bearing is a device or system which can transmit a load
through two elements of the system moving relatively to each other. The other
obvious requirement is to allow this relative movement with the minimum amount
of power being used, in other words, with the least friction. There are many ways
of achieving this, but one of the most efficient and cost effective is to separate
the two surfaces of the elements of the bearing with a thin film of pressurized oil.
This oil pressurization could happen in two ways: we could supply oil to the bearing
at sufficiently high pressure to force the surfaces apart, or we can use the geometry
of the two surfaces and their relative motion to generate the pressure. This latter
method, which is known as hydrodynamic action, is that which is found in the
bearings of an internal combustion engine.
The term “plain bearings” for this type
of bearing signifying that there are no balls or rollers separating the two surfaces.


How does the hydrodynamic action work?
The diameter of the part of the connecting
rod which connects to the crank is slightly larger than the diameter of the crankpin.
This means that when the centers of the crankpin and big-end are slightly offset, a
wedge-shaped space is created between the two. Oil, being viscous like syrup, is
drawn into this wedge shaped space and is squeezed or compressed, causing a
pressure to be generated in the film of oil between the surfaces. If the speed is high
enough, the pressure will be sufficient to push the two surfaces apart.

The oil is supplied by a pump, which is of the positive displacement type. The volume
the pump flows is directly proportional to the rpm of the pump gears. The oil pump
does not produce any pressure!!!!! It produces flow. The oil coming out is at the same
pressure as when it went in to the pump – zero relative to the oil in the sump.

So why can we measure oil pressure above zero? Well, the pump is flowing a fixed
volume of oil into the oil passages of the block and out through the bearings. This
causes resistance to flow and the result is pressure. Oil pressure is thus a surrogate
way of measuring oil flow – the more oil flow (from higher pump output), the more
pressure will be produced. If we increase the resistance, by using a more viscous oil,
the pressure will also go up because of the increased resistance to flow. When the oil
is cold, it is more viscous, and there will be more oil pressure.

How much oil pressure is enough? Keep in mind that what counts is flow. A time
proven rule of thumb is 10psi/1,000rpm. This assumes the normal range of engine
bearing clearances and it is well proven that 10psi/1,000rpm will supply enough oil
flow when normal clearances and parts are used. Since we have no convenient way
to measure flow, we use pressure as a surrogate. If the bearing clearances are larger,
we need more flow to keep an adequate film of oil between the bearings and the shaft.
Do we need more oil pressure? No, but it will take more volume to produce the same
pressure because the resistance is lower. So, we need a high volume pump to maintain
oil pressure. This costs hp – more power is needed to drive a HV pump. The reasons
to choose large clearances are beyond what I want to go into now.

What if the bearing clearances are tight? Well, we will see more pressure with the
same output but we don’t need it. In that case, we can use a lower viscosity oil with
less friction and less resistance to pumping. This will lower the oil pressure and gain
hp because of the lower pumping losses and less friction. This is what the OEM’s are
doing to improve mileage and performance. Some new vehicles come with a
recommendation for 5W-20 oil. There are real hp gains from using a low viscosity oil.
Some hard core racers will use 0W-10 weight. The problem is that there is a general
relationship between viscosity and shear strength. Low viscosity oil may not provide
enough resistance to shear to protect bearings (avoid metal to metal contact) under
very high loads (high boost blower cars, heavy nitrous use, etc.). Many racers are
using 5W-20 for the same reasons but with a little more protection.

Our preference is a little more bearing clearance and a heavier oil for a hi-power race
motor. The idea is that with high loads, more clearance and greater shear strength is
needed to avoid metal to metal contact when there is parts deflection. In most of our
race cars, we use a semi-synthetic 20W-50 and a HV pump because of the bearing
clearances. In our blown Hemi, we use a straight 50 weight non-synthetic oil. Our
preferred brand is Brad Penn, which we sell at a competitive price. Street and street/
strip cars with standard clearances can use 5 or 10W-30 oils as recommended by OEM’s.

CAUTION: Some cars with stock oil pans should NOT use a HV pump. Enough oil may
be removed from the pan to uncover the pickup with hard cornering or braking with
resultant engine damage.
A little more detail for those who are interested :)

Cheers... jondee86

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 2:41 am
by jondee86
And finally, about the best article on the engine lubrication system I have
ever read. Written for the Audi 1.8 turbo engine but the basic principles
apply to all internal combustion engines. ... ooting-DIY

This is a sample from the article...


Gets into a reasonable amount of tech stuff but with plenty of quality photos
and diagrams to make it a worthwhile read if you are serious about how to let
your engine live long and prosper :)

Cheers... jondee86

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 4:02 am
by davew7
Thanx jondee86
The oil requirement chart is very enlightening relative to the amounts needed. I did not realize that the main bearing needed that high of a percentage when compared to the rod ends. The one thing you did not mention is the actual cooling of the bearings. Thinner viscosity oils will have a higher flow rate and therefore carry more heat away from the bearings. davew7

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 9:07 am
by sirdeuce
Great read! Thanks for this, it's an important issue any gearhead should pay attention to. Can this be made a sticky so it doesn't get lost in the pile of posts?

If you want another interesting subject try lubrication chemistry or HP additives. See how hot it gets when you talk about adding TSP to your oil.

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 3:38 pm
by jondee86
sirdeuce wrote:If you want another interesting subject try lubrication chemistry or HP additives.

Not sure if I want to open that can of snake oil :) My opinion is that if you use
a quality oil with the correct grade, you should not need any additives. However,
when I rebuilt the engine I am running now, I did use Penrite 15W-40 high zinc
(mineral) running-in oil for the extra wear protection provided by the zinc.

The way I see it, the big oil companies are always looking for an edge to make
their products more attractive than the competition. So if there is some miracle
additive out there that can do that, they would have it in their oils faster than
you can say "New and Improved" :)

Cheers... jondee86

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 4:55 pm
by sirdeuce
Phosphates and sulfur have been added to oil for quite some time for their HP qualities, their abilities to adhere to metal surfaces and reduce friction. I have read some studies on the subject, NASA did a decent test on the subject. I used TSP in my VW engines. Only thing that made it seem like it was doing any good is the way the bearings wore. After around 50 runs there was a noticeable decrease in wear. Never put it into use on the street though.

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 6:04 pm
by jondee86
Agreed. There is no doubt that additives work, and the big oil companies spend
a lot of time and money on researching and evaluating the apecific additives that
they put into their oil formulations. The confusion arises when small bottles of oil
additives are sold over the counter with spurious claims that they will cure almost
any engine problem. Vary rarely are these claims supported by any credible form
of independent testing. Most rely on anecdotal testimony from "happy" customers
and uncontrolled "tests" performed to impress naive Youtube viewers.

By way of contrast, here is an unbiased explanation of how oils are formulated
and how the additive package is tailored to the intended use. These people don't
sell or endorse any particular oil and all the basic information is covered... ... -additives

As a side note, in the course of my work I came across several examples where
industrial users had been sold expensive "specialist" oils by non-mainstream
suppliers, and experienced ongoing problems. These problems were solved by
switching to the correct grade and type of lubricant supplied by a company such
as Mobil or Shell at a considerably lower cost. I learned from that :)

Cheers... jondee86

Re: Where does oil pressure come from ??

Posted: Sun Jul 07, 2019 7:46 pm
by sirdeuce
After reading that bit I realize how long I've been away from the whole thing, I can't even get the terms right, I've been calling it 'HP' when I meant 'EP'. I have recommended the use of phosphates in machinery, engines and gearboxes. It's been 20 years since I went that route.

Re: Where does oil pressure come from ??

Posted: Fri Jul 26, 2019 5:36 pm
by allencr
Thanks Jondee.
Lube be good.