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Re: [GMCnet] Fan Clutch [message #61578 is a reply to message #61560] Sun, 25 October 2009 13:05 Go to previous messageGo to previous message
Tin Gerbil is currently offline  Tin Gerbil   United States
Messages: 236
Registered: October 2006
Location: Vancouver Island, B.C.
Karma:
Senior Member
Roger;
I see I am not totally alone in my finding that a Light Duty fan burns
less fuel. That a Heavy Duty fan is close coupled tighter that I found
in my research.

I wonder if you actually have exhaust gas temperatures to indicate the
difference in the cylinder while using a 185 or 195* thermostat.

After working with 427 and 454 GM gravel trucks that burned valves on
long hard pulls, we installed twin EGT gauges and put a red line on them
for the drivers. This solved the problem of every time we had a new
driver, we had burned valves and/or burned pistons. The first attempt
at solving the problem was to lower the thermostat temperatures. This
had no effect on burning valves as it only controls the low engine
temperature. It also had very little effect on lowering the EGT when
the engine was not at WOT. I was always under the impression that EGT
and cylinder temperature is a function of compression ratio and water
temperature has very little effect on it. If water temperature was able
to control cylinder temperature, we could all increase our compression
ratio and just change thermostats to get away from ping. Conversely we
can increase our compression ratio one full point if we switch to
aluminum heads, as they can transfer the heat to the water faster and
eliminate the hot spots that appear in cast heads. Changing the water
temperature does not change the rate of heat transfer. Changing the
material does. I guess what I would like to know is does changing the
water temperature 10* change the temperature of the fuel burn, 20*, 50*
or 100*?
What is the correlation between thermostat temperature, EGT and cylinder
temperature? I only know the correlation between CHT AND EGT on
aircooled aircraft engines and they are very linear. I have been not
been able to observe the difference between water temperature and CHT.

Some people mistakingly believe that a 185*F thermostat will cause
moisture to accumulate in the oil. As the oil temperature in a
naturally aspirated engine is at least 50* hotter than the water
temperature due oil picking up this heat from the bottom of the piston,
this is clearly nonsense. With a turbocharger this oil temperature is
at least 100* hotter.
Thank you for the information;
Gordon


Roger P. Gleason wrote:
>
> Having read the message link I think Tin Gerbil provided a very good
> description as to the differences in the fan clutches. The only thing
> I might add to his input is that the 65% vs. the 95% stands for the
> decoupling when the temperature of the thermostat in the clutch is
> below the engagement temperature. This explains why the standard
> clutch is less noisy when it is satisfied. On the other hand, the 95%
> clutch only slows down 5% between being satisfied and to hot. Most
> probably would not be able to notice a difference in the noise level.
>
>
> This also explains why GM chose the 65% clutch since gas mileage was
> a major issue at the time. This is a rather large fan and the larger
> the fan the more horsepower it takes to move the fan. All that
> translates to is lower gas mileage. As long as the radiator system is
> rejecting the heat of the engine it is doing its job any additional
> heat rejection simply closes the engine thermostat retaining the
> water in the engine longer.
>
> Now for a question, several have lowered their engine thermostats
> from the recommended 195 degree to 185 degrees, what is the purpose
> of this? The newer gasoline’s are formulate where it takes
> higher temperatures to burn them completely some of the newer engines
> are using as high as 210 degree thermostats to accomplish a complete
> burn. Engines I have seen torn down with too low of thermostat leave
> unburned deposits in the heads leading to catastrophic failures.
>
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Gordon '74 Canyon Lands "Tin Gerbil" Vancouver Island, B.C.

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