RE: How hot is too hot for oil?

["John Fielding" <johnf@domain.elided>]

In AD V8 #835 Robert Watson queried the safe oil temperature of his  GTV.  Well funny
thing is that only the other day I came across an old technical publication I purchased
when I was in the UK.  It deals with the development of the Jaguar V12 motor designed for
Le Mans and later detuned for the UK car market.  The XJ13 mid engined prototype is a
beautiful car, shown on the front cover.

One of the very interesting parts deals with the oil temperature and the design of the oil
pump and oil cooler.  The first couple of motors used a large oil cooler incorporated into
the water radiator and the oil temperature ran around 90C.  Later this was changed to
increase the oil temperature!  The explanation is that at the higher oil temperature the
oil system actually works better!

The graphs published show oil flow and pressure at the two different temperatures, at the
higher temperature the oil flow and pressure increased by as much as 25%.  And before
anyone says they were using an exotic oil, nope the test were done with SAE 20W50 grade.

In my reply to Robert I pointed out that too low an oil temperature is wrong, the oil
needs to exceed 100C to boil off the water content which gets absorbed from various
sources.  It is also a well known fact that too many modern motors are overcooled.  For
efficient running the motor needs to run at above 100C, hence the use of pressurised
cooling systems and anti-freeze solution to raise the operating temperature.  Somewhere
amongst my thousands of technical articles I have a paper on the optimum engine
temperatures for various fuels, perhaps Eric Storhok has an up to date version he can give
details from? Also modern oils can tolerate much higher temps than older oils, a figure of
200C seems to be about the safe maximum today from the manufacturers literature.

Another interesting fact in the paper is the discussion on cylinder head shape.  As the
development of the racing motor was cancelled late in its development the Jaguar company
decided to convert the motor for road use.  To quote from the paper-

"Investigations indicated little advantage with any particular combustion chambers shape
except that simple shapes devoid of traps for unburned hydrocarbons appeared best and NOx
production proved considerably less with the flat type chamber than one of hemispherical
type.  Tests on variations in bore/stroke ratio also appeared to influence the results far
less than was originally expected, except that the long stroke/small bore favoured low NOx
emission."

Thought you would all like to know!

John
Durban
South Africa
Alfetta 1.8L turbo - which runs 140C oil temp at full load on the track, with a cooler.
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