Rocky Mountain 356 Porsche Club
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Rocky
Mountain 356 Porsche Club
OVERSTEER
November
2006 An Oil Temperature Odyssey From
the time that my car was new, it never had a problem with high oil
temperature...or so I thought. Even cruising across the desert southwest in
summer, the oil temperature never exceeded 240°F on the gauge. In
1975, a broken ring necessitated replacement of the pistons and cylinders. Steel
86 mm big-bore jugs replaced the original 82.5 mm aluminum cylinders with chrome
dimpled walls. By that time, the car was used primarily to drive 8 miles to work
and seldom encountered the sustained high-speed driving. A few years later, it
entered semi-retirement without revealing any propensity to overheating. Following
restoration of my instrument cluster in 2001, and installation of the oil
temperature sending unit supplied by Palo Alto Speedometer, I was surprised to
see the oil temperature 20 to 30 degrees higher than I was accustomed to seeing
under similar conditions on the unrestored original gauge and sending unit. I
had read that any oil temperature above 260°F was considered
"overheating" (Up-Fixin der Porsche, Vol II). My oil
temperature was nudging 275°F at a sustained 75-80 mph on a hot day. The upper
limit of the gauge is marked "280." An
article in the September, 2004 issue of Hot Rod Magazine corroborated the
Up-Fixin statement, and further recommended that at "...oil
temperatures (at or above) 270º - 290º F...(a)n oil cooler or synthetic oil
(emphasis added) is recommended. A
comprehensive article by Ed Hackett (edh@maxey.unr.edu) defined the various
terms used to compare motor oil performance and included a brand-by-brand list
of the most popular oils. The bottom line is that the higher the flash point,
the less is the tendency for the oil to suffer vaporization loss at high
temperatures and to burn off on hot cylinder walls and pistons. A minimum flash
point of 400° F is recommended by the author. From the chart included in his
article, Mobil 1 and Valvolene Synthetic in grade 10w-30 both have flash points
of 450° F. I
made a precautionary switch to Mobil 1 at the next oil change. I
checked off the list of possible causes: Fan belt tension, obstructions to
airflow, ducting seals, ignition timing and carburetor cruise mixture. I even
checked that the oil pressure relief valve spring was within specifications. A
dipstick thermometer (itself calibrated in boiling water and adjusted for
Denver's altitude, see <http://www.biggreenegg.com/boilingPoint.htm>)
established that my VDO gauge at 180 was within about 5 degrees of the actual
oil temperature. Because the oil temperature decreased so rapidly at idle, I was
unable to obtain valid dipstick thermometer readings at 240 and 280. I
needed proof and devised an experiment reminiscent of a Physics lab...or the
three Witches in Macbeth, invoking, "Double, double toil and
trouble; fire burn and cauldron bubble." In
a nutshell, I drained the engine oil and removed the oil temperature sending
unit. With the sending unit held in a pan of engine oil by a homemade fixture, I
heated the oil with a camping stove. The electrical lead from the car was
connected to the sending unit with a jumper, and the fixture was grounded to the
engine to complete the circuit. I checked the temperature of the oil with my
dipstick thermometer, and then would turn on the ignition to check the VDO gauge
temperature in the car. The experiment was run twice, each time recording the
temperature as the oil was heated and once again after the stove was turned off
and the oil cooled. Thus, a total of four readings were made for each point. At
the gauge maximum reading of 280° F (I've never seen it that high while
driving), the actual temperature of the oil was 287° F. Two other identified
points on the VDO gauge, 180 and 240, were each 5 to 7 degrees below the
measured temperature of the heated oil, which is remarkably close for an
automobile gauge. 7 Rocky Mountain 356 Porsche Club
OVERSTEER
November 2006 Oil
Temp Odyssey – Con’t
In
a related matter, I've known for some time that, in spite of careful adjustment
of the linkage, some heat was leaking through the carburetor pre-heat tubes.
After driving on the highway for several miles, I once measured the temperature
at several points in the engine compartment with a Raytek Raynger ST2L IR
temperature gun, and learned that the carburetor pre-heat ducts were about 40 to
50 degrees hotter than the surrounding sheet metal. At the conclusion of
my sending unit calibration, I filled the tubes with crushed aluminum foil and
repeated the test. The carburetor pre-heat ducts are now about 5 degrees cooler
than the adjacent sheet metal. I am cautiously optimistic that this will solve
the overheating problem. Personal
note: The pre-heaters are only needed during warm-up in cool, humid
conditions--conditions in which my Porsche will be in her heated garage under
her warm, fuzzy electric blanket, enjoying a bowl of popcorn and watching a
football game on TV. Skeet Gifford, October 2, 2006
8
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Rocky
Mountain 356 Porsche Club
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October 2006 Submitted by Bob Little, Oil Change instructions for Women:
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