<< Message: [MV] M37 Running Hot -Reply >> Bruce is correct about the =
themostat. When coolant is allowed to flow
unchecked trough the cooling system there is not enough time for the
heat transfer to take place at the radiator. As I learned in Physics
classes, we must account for themal lag. That is, put simply, we must
give the heat time to escape from the liquid before it is pumped back
into the block. In past years, while working as an auto mechanic, I have
seen some vehicles that would overheat without a themostat, and others
would never even heat up enough to run the heater. The second case has
potential to cause you problems also.=20
--=20
Mike Klaput
Hi, Mike,
I think you made a good point. I agree totally with you that removing =
the thermostat can sometimes cause overheating...or uneven cooling...or =
undercooling. However, in terms of overheating, I think that the flow =
rate causes a different set of problems that result in overheating. See =
if you agree with my analysis:
If a given unit of water passes more quickly through a radiator, you are =
right insofar as that particular unit is not cooled as much due to the =
shorter time spent in the radiator. However, there is a mitigating =
factor, and that is that there are more units of water passing through =
the radiator in the same period of time, which has a tendency to balance =
the equation. There is another factor involved here as well. Heat is =
more quickly transferred when there is a greater temperature =
differential than when there is a smaller temperature differential, thus =
the rate of heat transfer changes as the water passes through the =
radiator tubes. More of the cooling is done at the beginning of the =
tubes than toward the end of the tubes. If you pump the water too slowly =
through a radiator, it can actually reduce the net heat transfer due to =
the decreased temperature differential between the water and the outside =
air over a larger percentage of the tube length. A smaller percentage of =
the tube is being used efficiently.
As far as velocity-related problems are concerned, I agree with you that =
pumping the coolant too fast can sometimes cause overheating, but I =
think that this is primarily due to turbulence patterns that create =
uneven distribution in the water jacket.
A relative of mine is a flathead V8 fanatic and has raced in several =
antique classes. Flathead Ford V8s are known for a number of cooling =
'ideosyncrasies.' A few years back, he did some experiments using =
various pumps and restrictors of varying sizes and shapes in the water =
passages to alter the water flow velocity. At higher velocities, the =
engine overheated. However, it was apparently not the velocity through =
the radiator that caused the problem. He substituted a Chevy truck =
radiator with almost twice the cross-sectional area, effectively slowing =
the water through the radiator to something about normal, and the engine =
still overheated to about the same degree. Here's what is interesting: =
He borrowed an infrared viewer (like those used by home inspectors to =
determine areas that need insulation) and found that the engine was =
developing both HOT spots and COLD spots. With this viewer, he was able =
to adjust the coolant velocity to the point where cooling was relatively =
even...problem solved. Apparently, when he increased the velocity of the =
water, it caused some turbulence patterns that allowed some parts of the =
water jacket to receive more coolant and other parts less. If I think =
about it, I'll give him a call and ask him if he remembers the details. =
He lives in Reno now.
That said, I wouldn't guarantee that a higher velocity wouldn't have =
exactly the opposite effect on some other engine.
Anyway, while the increased flow rate without a thermostat could affect =
the M37 cooling, I somehow suspect that what may be just as significant =
in the case of this M37 is the net rate of thermal transfer from the =
water to the air. If there is a layer of rust and scale in the radiator =
tubes, or too much paint on the fins, this decreases the rate at which =
heat can be transferred out of the water, reducing the net cooling to a =
substantial degree.
Let me know if I made any logical errors. I try to think things through, =
but I'm always happy when someone points out some problem with my =
analysis.
Regards,
Alan Bowes
===
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