Listers, This one is long but boring (haha). Feel free to use it (with
credit to "me" if you reprint it please)
I have seen several responses and several more "mee toos" about the oil
pressure thingy on TR6's. I draw my following commentary on:
1) 6 years and 45000 miles of ownership of a '6
2) 2 years and 10000 miles of threahing my GT6+ in race and street
3) complete rebuilds on 4 motors (and only God knows how many club member's
motors I have looked at in "consultation")
4) the fact that everyone is entitled to my opinion!
Please read on to unlock the mystery....
First, what is the correct pressure for a '6 motor to run cold? My most
recent complete rebuild gets about 95 psi cold and 70 at idle. I
clearanced the oil pump to the tight end of spec. but the pump was 2
seasons old. This is, of course, with fresh 20/50 oil, a cut crank, new
relief valve, and fresh bearings everywhere.
The above suggests that 70 psi is a good indication of cold idle and relief
cutoof values (BTW, my TR4a had similar numbers suggesting a "standard" for
TR's). Very quickly, however, the high end pressure tails off and I get
about 50 and 75 respectively. This fact suggests three things. First,
the "truth" that multigrade oil gets "thicker" as it warms seems to be
false (any engineers out there that can elaborate?!?) Second, if the
above is true, it stands to reason that the hotter the oil, the lower the
pressure will be. Finally, the interval between idle and running seems
relatively stable at about 20-25 psi.
All the above being correct, we should expect a car driven hard to have
higher oil temperatures and lower oil pressures. In fact, the general
guideline is that for every 1,000 revs you extend your redline, you DOUBLE
the heat generated by the oil. Thus, an oil cooler is needed for
sustained high speed operation. This same thought extends to the
highway/city question posed by one lister...
At highway speed there is a lot of heat generated underhood but most of it
can be expelled via airflow. But, sort of the way a roast keeps cooking
after you remove it from the oven (Chef Boy-ar-Dave here) so does a motor.
And, it is at that very same time you have removed the cooling effect of
60 mph air. Do you still wonder why the pressure drops after a highway drive?
Part 2: Testing and solutions...
I installed an oil temperaure gauge in my TR6 because I was too cheap to
buy an oil cooler. It was nothing more than a cheap electric water
temperature gauge taped to the steering column between the speedo and tach.
A bit bush league, I know, but I also had no money at the time for a
Smith's safety gauge. All you need to do is get a pipe bushing adaptor
(may be included with the gauge) and fit the sener unit to the oil drain
plug. Voila, you now know your sump oil temperature.
The oil coold half the motor so it should be no surprise that its
temperature is often the same as the water temp - between 165 and 185 f.
If you use a spare TR6 temp gauge (I dunno, ditch the voltmeter maybe?) it
should read midway. I like a numbered gauge though.
It will become apparent that the oil warms up more slowly than the coolant.
This is why racers preheat their oil, etc in "warmup" sessions! Back to
the city situation, your ol often never fully warms up. Contaminants,
crud, and WATER (which does not boil out until over 200) build up in the
oil. This reduces lubrication properties of the oil which then affects
it's cooling abilities (well, actually, increases friction and robs BHP).
On the highway you will find slightly higher oil temperatures. When
returning to the city, you will notice your temperature skyrockets to
nearly (or over) 200f. This may actually help remove some of the
impurities in the oil (and bake some others on to components).
Mystery solved.
Finally, your brain as a computer and oil change intervals...
My experience is that NO oil is good for more than about 2500 miles in a
TR motor. Period! And I wouldn't spend tha cash on synthetic (well,
anymore. It really does leak more - like we need help).
Pick an engine speed (I used 2,000 and 3,000 being roughly 40 and 60 mph
respectively) and note a cold oil temp reading. Do the same under "normal"
city driving conditions. My guess is a temp of 140-150 and a pressure of
about 55 at 3,000. Figure your baseline out with fresh oil on a normal day
in your area.
AS SOON AS YOUR PRESSURE DROPS FOR A GIVEN OIL TEMPERATURE YOUR OIL IS
FINISHED. CHANGE IT!! (BTW, this will be of the order of 5 psi). This
is most easily noticed at highway speeds.
IF YOUR TEMPERATURE RISES (5-10 DEG) FOR A GIVEN OIL PRESSURE - YOU ARE
RUNNING LOW ON OIL. CHECK IT!
It is very rare that you will actually need an oil cooler (where you get
snow in the winter) despite what some people say. If you do get a
cooler, be sure to get an oil thermostat (they only activate the cooler at
70 C or higher) so the oil is cooled only when needed. You may be better
served buying a fresh set of bearings and oil pump.
(geez now I feel like an engineering prof).
Questions, comments, concerns? Race meeet #1 is Friday next so my Nomex is
washed and ready for "action"
Dave Terrick
Winnipeg
...the only substitute for cubic inches is cubic dollars
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