Date: Mon, 1 Jul 2002 15:57:41 EDT
Reply-To: FrankGRUN@AOL.COM
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Frank Grunthaner <FrankGRUN@AOL.COM>
Subject: Max Engine Oil Temperatures
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As I've indicated on the list in the past, the whole issue of motor oil
operating temperatures and the use of synthetic vs. conventional oil is a
very involved subject. I hope all of you know my predilection to data and
measurements as opposed to hearsay passed on as wisdom. There are many myths
about synthetics and I've tried to address a few recently. For those who want
an excellent introduction to the experimental facts, I highly recommend the
Redline website. Specifically:
http://www.redlineoil.com/products.htm.
There are several key issues here including the design of the engine, the
cooling system and the lubrication system. I'll only address the oil
characteristics here. The maximum operating temperature for a lubricating oil
is limited by the flash point, the change in viscosity (loss of lower
molecular weight fragments and oxidatively induced polymerization - sludge
formation) and the degradation of additives.
The flash point is a reported number for nearly any oil brand (required for
the MSDS sheets). This is largely given by loss of low molecular weight
fragments. For conventional oils this number is around 200 - 210 C depending
on the exact nature of the base stock. For synthetics, this number is between
240 and 270 C. So first take home lesson ... synthetics have higher thermal
stability than conventionals.
The next question is the change of viscosity and film strength with
increasing temperature. Now, as nicely detailed on the Redline site, is turns
out that the applied pressure across a thin film of oil causes a major change
in the film viscosity. That is, the effective viscosity of the oil in the
sump at 275F is nowhere the same as that viscosity in the con rod or main
bearing shell (where the oil is doing a high pressure standoff!). A so called
50 weight oil can have a real film strength of a 20 weight oil under these
conditions. In all cases, this load induced reduction in viscosity is greater
for conventional oils than for synthetics.
The oxidation onset comes on very quickly as the temperature is increased.
Simple oxidation of the oil increases the viscosity, generates abrasive
particulates and leads to a number of complex processes that create tar and
sludge. All oils have additives to combat the effect, but the additives
actually scavenge the decomposition intermediates from the oil - not the
oxygen. The additive package to control oxidation and maintain film strength
for conventional oils can be consumed in less than 1000 miles. Synthetics are
naturally more resistant to oxidation processes and can generally operate at
300 F for more than 10000 miles.
So to summarize and conclude: Operational specs on high stress engines can
easily run to 315 F. Higher for air-cooled racing applications (AC vanagon
Westy, fully loaded, uphill, 3rd gear, 30 mph). Mobil 1 is rated to provide
full engine protection to 204 C (400 F) and Redline has been tested to 500 F
for continuous duty.
So, I say, use a synthetic for full protection, and an oil cooler to keep the
engine from softening! Redline has my money for the turbo application, but
Mobil 1 is my current choice. Oh, yeah, you AC junkies ... I'd use Mobil 1
for 4 stroke AC motorcycles. Even better high temp additive performance.
Frank Grunthaner
