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Lubrication - Oil, what it
does and how |
19
January 2001 |
It’s criminal. Folk spend fortunes putting together super-sonic motors,
only to skimp on the oil they use. Why? Oil’s oil right? Wrong. Even if it’s a standard
engine, it deserves TLC considering it's extremely hostile working environment.
 Diag 2. |
Click image to enlarge |
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Oil is literally the engine's life-blood. The opening few sentences are
astonishingly true. Oil isn’t there just to prevent all metal components within
an engine fusing together in the first few seconds of running, creating a total
melt down of
The most commonly uttered statement about engine wear is most wear occurs
within the first 10 minutes from start up when cold. True if cheap chip fat oil
is used. In performance and race engines, a considerable amount of wear is
created by heat, load, speed, and pressure. Again, cheapy oil won’t give
protection here. On the other hand, a good oil should give protection under
both extremes.
Oil has to perform four basic functions. Lubrication is the obvious one,
then there’s cooling, cleaning and sealing. Cooling and sealing capabilities
are controlled by the base oil quality, lubrication and cleaning by additives.
Then there’s the dreaded grading of oils - something that confuses most at
best, and related terms banded about by the ill informed - such as ‘viscosity’.
Cooling and sealing
I’ve lumped these two together as the oil’s performance in these
departments as previously stated is largely controlled by the quality of the
base oil - essentially just hydrocarbons. It cools by absorbing heat from the
metal components - such as crankshaft, piston crowns, piston skirts, camshaft,
etc - conveying this back to the sump where the heat is dissipated into the
bulk of the oil. Generally most oil from major manufacturers does this fairly
efficiently. However, it’s one of the functions that some fully synthetic oils
haven’t yet equalled - and there are one or two top brands particularly
inefficient at this!
Sealing is mainly concerned with the piston rings, providing a barrier
between the relatively rough metal surfaces of bore walls and piston rings.
Again, base oil quality determines how well this is achieved.
Cleaning
Now we start with the additives. The base oil can’t perform this
function on it’s own. In fact it contributes more to residual carbon deposits
(that blackening of engine components, particularly where regular oil changes
have been ‘forgotten’) than controls it. Detergents and dispersants are used to
help keep engine parts free of the ‘black death’. These are generally calcium,
magnesium, and nitrogen. Some oils are sold on their ability to do this more
effectively than others - basically containing more detergents than others.
This isn’t good in your Mini engine. High detergent levels cause premature
gearbox wear. Manufacturers of oils aren’t bound to list the make-up of their
oil like you find on cereal packets, so identifying this is a problem. Those
possessing high detergent levels, however, do actually smell very washing up
liquidy funnily enough. And even stranger they also tend to be green in hue! I
must point out that this green-ness doesn't automatically mean a high level of
detergent though.
Lubrication
So to the ‘big one’. Lubrication is actually multi-functional. The oil has
to reduce friction, making sure all engine parts are nice and slippery and
protect them from excessive wear. It’s also the only function that can increase
horsepower where more specialised additives are used. The majority of
commercial manufacturers use sulphur, phosphorus, and zinc. The more specialist
manufacturers, after extensive research and testing, have found that adding
quite ‘exotic’ chemicals containing such as phosphorus esters and molybdenum
increase power outputs as well as further reducing wear.
Wear
Wear is the permanent removal of surface metal caused by friction.
Consider the relevant diagrams (diag.1 & 2) - it’s far easier to illustrate
than explain in words.
Higher quality - and generally more expensive unfortunately - oils
protect against this far better than those at the budget end of the spectrum.
More specifically the additives used. At start-up, where oil has drained off
components during inactivity, it’s the additives that reduce wear here. Now the
engine is running, the oil has to deal with a multitude of situations created
by temperature. Higher engine rpm drastically affects running temperatures of
engine parts, particularly in the upper cylinder and valve train components.
The oil has to perform at differing film thicknesses on extremely hot surfaces
and far cooler surfaces. Therefore the oil is a different viscosity in the sump
than the upper cylinder, exhaust valve stem, inlet valve, cam followers, piston
skirt, etc. etc. As can be seen from this, the viscosity (explained later in
more detail) of oil is of ultra importance. What manufacturers call 'fluid film
lubrication'.
Too thin a film between two metal surfaces sliding against one another,
the surface metal (high spots on the surface - see diag. 2) get too close,
finally making contact. On extremely hot surfaces such as the upper cylinders
and exhaust valve stems, the oil film becomes so thin it can disappear
completely. This is where the additives used step in as a secondary lubricant.
The better the additives (again, generally not in the cheaper oils) the more
protection against wear there is. Particular types leave protective deposits on
exposed metal surfaces even when the fluid film has gone. And as you will see,
power gains can be made.
 Diag 3 |
Click image to enlarge |
The most savage example of this in a normal running engine is the upper
cylinder - see diag.3. From these you can see as the piston moves down the bore
on the induction stroke, the rings scrape oil off of the bore wall leaving the
barest of smears. This is extensively washed off by the incoming fuel, leaving
the rings to slide on basically bare metal. The fresh mixture charge is
ignited, forcing the piston down the bore on its power stroke. The rings again
reduce the oil film on the bore walls to a mere stain, this then being burnt off
so on the exhaust stroke the rings are once again sliding on bare metal. So
every two strokes in your poor old Mini engine, the rings are sliding on
essentially unprotected iron. This is why 75%, yes 75%, of friction generated in an engine occurs in this relatively
small area. Astounding when you consider how thin the rings are. Consequently
there’s a massive amount of wear generated here, not to mention power loss.
Friction
Friction is the resisting force to motion. There are many forms of
friction - fluid friction, solid friction, rolling friction, sliding friction,
stick and slip friction, etc. The main items of interest to us are solid
friction and fluid friction.
We’ve had a brief insight into solid friction in the ‘wear’ section, and
as with practically all institutions the oil industry has a standard by which
friction is measured. This is used to determine a particular oils wear
properties (friction reduction). Diag.4 shows the basis of the test. Zinc (or
more precisely oil-soluble zinc - zinc dithiophosphate) is used as it is
recognised throughout the industry as one of the best anti-wear/anti-weld
additives that is plentiful and cheap, therefore widely used in budget motor
oils. Phosphated molybdenum, phosphated zinc, and phosphorus ester are far superior
in performance but are also far more expensive to produce and blend in exactly
the right proportions. The end product is therefore more expensive.
To give you some idea of what’s what here, the co-efficient of friction
of iron on iron is 0.51. The co-efficient of friction of zinc on zinc is 0.75.
The bigger the number, the more friction. Weird that zinc is used considering
it causes a massive 32% increase in
friction. So why on earth does the oil industry use this in its oils? Basically
for reasons outlined earlier - it’s very cheap, plentiful, and excellent at
wear reduction. The cost to you is a loss in horsepower to the tune of
approximately 3%. Not so bad really - but why loose it if you don’t have to? A
very few specialist oil manufacturers can re-coup that loss - at a reasonable
price. Particularly where increased or maximum performance in a fast road car
or any racecar is the goal. The more expensive and carefully crafted elements
mentioned produce a co-efficient of friction of 0.44, a massive 41% reduction. A bonus is a 40%
increase in wear protection at start-up. The best oils also increase fuel
economy through friction reduction.
Fluid friction
Fluid friction then is the internal friction within a stream or film of
fluid - in our case oil - and is related to its viscosity (there’s that word
again!). So for us viscosity is the oil’s resistance to flow. Conversely
viscosity is the oil’s ability to flow. Low viscosity oils have less flow-
resistance than high viscosity oils, are easier to pump, and circulate through
the engines oil galleries faster. Low viscosity oils give a lower running
pressure, but allow the oil pump to deliver a higher volume through the
galleries. High viscosity oils perform in the exact opposite way. As oil
temperature goes up, the viscosity goes down. The internal friction of oil thus
rises and falls with temperature.
There are definite power gains to be made by increasing the lubrication
systems efficiency utilising reduced fluid friction by lowering oil viscosity.
BUT the correct selection/application is imperative. Low viscosity oils produce
thinner fluid films when hot, to a point where the oil film becomes too thin -
allowing high spots on opposite surfaces to slide against each other. This
increases friction, power loss, and causes excessive wear.
Viscosity
For some reason, the mere mention of oil viscosities has all manner of
ill informed folk spouting all sorts of gibberish about cold performance,
thinning under temperature, the ‘weight’ benefits of oils, and so on. Let me
put you in the picture…
Motor oil is described by its manufacturers using basic information.
Supposedly to make direct comparisons with other oils easy - most of which are
multi-grades. We’ve all seen this on oil cans/bottles - SAE 20W50. So to kick
off with, we’ll deal with the lettering. SAE stands for ‘Society of Automotive
Engineers’, basically a body of people who determine how oils should be graded/compared
by fixed tests. ‘W’ is most definitely NOT ‘weight’. I’d love a pound for every
time I’ve heard some one refer to an oil grade by its ‘weight’. It actually
stands for ‘winter’. Simply because the grade implies the performance of oil in
sub zero temperatures, and applies to the first number.
 Diag 5 |
Click image to enlarge |
Higher numbers describe higher viscosities, i.e. thicker oils. Some oils are described purely as straight grades (i.e. SAE40) because they can’t pass the low temperature viscosity test. If it did meet one of the ‘W’ tests it’d be a multi-grade. These are generally used in race engines that are unlikely to come up against sub zero temperatures. Take a good look at diag.5, this shows how the basic SAE is measured - essentially how fast a fixed amount of oil goes through a specific sized hole at an equally specific temperature (centistoke - cst) and is the basis of comparing the main (high temperature) multi-grade component - the second figure in the specification. Diag.6 compares oil grade against outside temperature (centipoise - CP, believe me you do not want to know how this is determined!!) and deals with the sub zero performance of the smaller oil component - the first number in the specification. Diagram 7 gives the exact data performance for each component.
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Click image to enlarge |
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Miracle additives
I don’t want to dwell on this one as full diagnosis is a books worth on
it’s own. Suffice to say many after-market additives exist. If they were as
good as they claim, commercial oil manufacturers would be blending them into
their base product. Try and find a commercially- manufactured oil that contains
PTFE for example. Some are potentially disastrous as they are so thick they
stay like that when added to the sump oil - causing possible oil starvation by
blocking the pick-up pipe. Most of these are ‘old’ names that were around when
oil was in its infancy, degrading in a very short time so needed ‘thickness
boosters’. Basically my advice is buy the right oil, and avoid additives like
the plague.
Conclusion
I’ve deliberately avoided dealing with synthetic oils (a very
science-orientated subject) as this was conceived to deal with the basic
functions of motor oil - which encompasses synthetics, as they have to do the
same things and conform to the same tests. Oil technology has out paced
mechanical technology by a long, long way. Superior lubricants will cost more.
BUT any self respecting advanced fully synthetic oil will last up to eight
times longer than a cheap mineral multi-grade oil - so is actually cheaper
long-term. Particularly when considering the right choice of oil will reduce
friction, boost power, and therefore fuel economy. Look carefully - there are
some astoundingly good oils out there. Ask the professionals.
Useful part numbers:
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MOC10ROW |
10-row
oil cooler, 1/2-in BSP male fittings |
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C-ARH221 |
13-row
oil cooler, 1/2-in BSP male fittings |
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MOC16ROW |
16-row
oil cooler, 1/2-in BSP male fittings |
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21A1794 |
Original
spec S oil cooler mounting brackets that fix to subframe (sold individually) |
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C-AHT3 |
Mini
Clubman steel braid, swaged end oil pipe kit |
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C-AHT4 |
Mini
steel braid, swaged end oil pipe kit |
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MOC1016 |
'12A'
engine/1992 onwards steel braid, swage end oil pipe kit |
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MOC1017 |
Pre-engaged
starter type up to 1992 steel braid, swaged end oil pipe kit |
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MOC100102 |
Mini
Clubman rubber, swaged end oil pipe kit |
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MOC1013 |
Mini
rubber, swaged end oil pipe kit |
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MOC1014 |
'12A'
engine/1992 onwards rubber, swaged end oil pipe kit |
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MOC1015 |
Pre-engaged
starter type up to 1992 rubber, swaged end oil pipe kit |
Note: All pipe kits are
1/2-in BSP screw-on female fittings at cooler end. Pipe lengths in inches for
each kit are listed as block to cooler/filter head to cooler:
Clubman 14/24, Mini 12/14, Pre-engaged to 1992 15/22, 12A engine/1992 on
15/22
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MOC1018 . |
Stainless
steel braided oil transfer pipe replacement kit for the standard '12A' post
1992 engines with 'O'-ring block fitment |
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MOCOT1 |
Mocal
oil thermostat (80 degree C) |
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MOCOT2 |
Mocal
oil temperature sender adaptor for oil cooler pipe fitment |
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MOC207 |
1/2-in
BSP female cooler union, 90 degree push-on oil pipe fitting, 1-in Long after
bend |
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MOC208 |
1/2-in
BSP female cooler union, 45 degree push-on oil pipe fitting, 1-in Long after
bend |
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MOC209 |
1/2-in
BSP female cooler union, 45 degree push-on oil pipe fitting, 2-in long after
bend |
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MOC01 |
1/2-in
BSP male cooler pipe union fitting for early 5/8-in UNF block fitting |
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MOC02 |
1/2-in
BSP male cooler pipe union fitting for early 1/4-in NPT filter head fitting |
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MOC03 |
Pair
push-on adaptors for early block/filter head thread sizes and 1/2-in bore oil
pipe |
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C-AHH8537 |
1/2-in
bore rubber cooler pipe sold by the foot |