Michael D. Porter wrote:
> jonmac wrote:
> >
> > Listers
> > I know this thread comes up for an airing from time to time
> > but I thought I'd post some 'off the record' comments that
> > have come into Gaydon over the last few weeks from oil
> > companies about modern oils - i.e. those which have
> > synthetic properties in them and others which are full
> > synthetic.
> > There's been a bit of a hue and cry in recent months in the
> > UK as to whether a 'synthetic' oil is suitable or better for
> > a classic engine. [snip] While acknowledging that modern oils are overall
>better (?)
> > than their forebears (they have to do a much more demanding
> > job anyway) the oil companies I've spoken to at some length
> > feel classic car owners will be doing themselves no favours
> > by using these lubricants in older engines.
>
> There are those who swear by full synthetics, but whenever this subject
> comes up, I generally repeat the following:
>
> When Mobil 1 was first on the market, I was working at a city
> engineering department on a survey crew. The guy running the crew raced
> snowmobiles and tried Mobil 1 5W-30 in his sled one weekend and burned a
> bearing. Not sure if the engine was tired, anyway, he put the newer
> engine from his brother's sled in his for the next weekend, filled it
> will Mobil 1, and burned a bearing.
>
> Not too long after, there was a letter in the technical tips column of
> R&T, from a man in Germany who'd had his Mercedes 380SL rebuilt by the
> factory (only hints about the true cost of that!), and shortly after
> breaking it in, loaded it up with Mobil 1 5W-30, took it out on the
> autobahn, up to about 130 mph, and promptly burned a bearing.
>
> Much, much later, a fellow working for me, with a Pro Stock car, tried
> Syntec 5W-50 at the drags one weekend... as he went through the traps
> and let off the throttle, and best guess is that the oil in the pan went
> forward suddenly, the engine lost prime to the oil pump and he spun a
> bearing. Never a similar problem when he went back to 20W-50.
>
> Both of these oils are full synthetics and have one characteristic in
> common. They are, quite simply, 5W oils. Most people assume that when an
> oil is rated 5W-30, it has the _viscosity_ of a 30W at operating
> temperatures and pressures. In fact, it has the lubricating capacity of
> a 30-weight oil, and there is a world of difference between that and
> viscosity. The latter figure indicates the approximate shear and film
> strengths of the lubricant when hot.
>
> But, viscosity is critical for maintaining oil pressure, especially when
> the engine is pushed hard. Mechanical forces go up by the square of
> engine speed. Double the speed, and the kinetic energy of the parts
> flying around is four times as much.
>
> Full synthetics, because their viscosity is much lower (it's one of
> their main attractions--easier starting in winter, and less friction)
> simply run out of spaces such as between bearings and crankshaft much
> faster. Therefore, it takes a higher-capacity oil pump to maintain full
> pressure at the extreme ends of the oiling system. If pressure isn't
> maintained, the thickness of the oil film is reduced, and the oil has
> less ability to resist metal parts contacting each other. For those who
> say, "I run synthetics and the oil pressure's great," it's worth
> mentioning that, on Triumphs, the oil pressure sender is located in the
> main gallery fairly close to the pump. The sender isn't telling you
> what's happening at the bearings.
>
> The synthetics have superior oil film strength and shear strength, but
> not enough to make up for not enough oil at the bearings when the engine
> is really cooking, unless all the engine clearances and the pump volume
> are adjusted for its use. And, John Macartney's remarks about newer oil
> additives attacking older non-metallic parts (i.e., seals, gaskets,
> etc.) is quite true.
>
> For people who have reasonably fresh engines, don't push the car hard,
> there may be some benefits to full synthetics, but those benefits
> probably don't outweigh the additional current and future costs.
>
> Cheers.
Michael,
I respectfully disagree with the notion that synthetics do not provide proper
lubrication
to engine bearings. In your example, you point out that the oils in question
were 5
weight oils, and it is by this very fact that they are not suitable, not
because the the
oil is synthetic. Choosing the wrong weight oil is human error, not the fault
of the
oil. I've run Mobil 1 in all of my cars including Mobil 1 15W-50 in my TR's
since the
early 90's without incident. In the TR I find oil pressure to be more
consistent from
cold to hot and more stable throughout the RPM range.
Further, 'when the engine is really cooking', an engine oiling system in good
condition
should be capable of blowing the pressure relief at 70 psi. 70 psi is 70 psi
regardless
of the viscosity of the oil and at 70 psi a lower viscosity of oil will flow
past the
bearings faster at the same pressure offering better cooling capacity that a
heavier oil.
I believe the problem you experienced with the 5 weight oils was a fairly
typical flow
problem. As liquids (including oils) are generally considered to be not
compressible
materials, the pressure in the oiling system should be constant throughout the
system.
Pressure is not the same as flow. Flow rates at individual bearings may vary
due to wear
in any number of components even though the system pressure is the same. If
the combined
flow rate is greater than the pump capacity, then you will see the result on
the pressure
gauge as a low reading. What you are seeing is engine wear and should not be
mistaken for
'bad' oil. If your engine requires a high viscosity oil to maintain pressure,
it is
because clearances are excessive and lower viscosity oils are flowing past the
bearings
faster than the pump can adequately supply more oil to the galleries. If you
need to run
this high a viscosity to maintain good pressure, and it is not available in
synthetic,
then use conventional oils, but don't blame synthetics when the problem really
has to do
with the weight of the oil, not it's composition.
Regards,
Brian Schlorff '61 TR-4 '64 TR-4 '72 TR-6 '79 Spit
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