This topic has been discussed ad nasuem both on this board and throughout the internet. I did an extensive search on the internet, asked professors, a nuclear sub engineer and they all said the same: high flow rates do not cause overheating.
Here are some excerpts from that internet search:
The thermostat temperature shouldn't matter if the equilibrated
temp is higher than the thermostat temp, i.e. once the thermostat
is open it no longer affects the temperature.
I'm no expert on this stuff, but with regard to your question 2)
I believe you want the flow to be as high as possible (qualifications
below). The important thing to note is that the average temperature
of the coolant is independent of the flow rate. If the flow is 0
then the engine is putting infinite heat into the coolant it
contains, and the radiator is removing no heat from the coolant it
contains. At the other extreme we have infinite flow. In this case
the engine adds 0 heat to each unit of coolant that passes through
it, and the radiator removes 0 heat. In other words the temperature
increase across the engine is 0 and the drop across the radiator
is 0, so all of the coolant is at the same temperature. Obviously
this situation is unattainable, but you want to get as close to it
as possible. Why? Because higher flow reduces the maximum temperature
of the coolant, so you reduce the chance of steam formation. So,
what are the drawbacks to high flow. First, you have to worry about
turbulence. I believe some turbulence is good because it breaks up
the steam layers that form on the engine surfaces, but I imagine
that too much is bad and it screws up the simple model I described
above. Second, if you try to drive your water pump too fast it will
cavitate, leading to reduced flow.
And finally, ethylene glycol may raise your boiling point, but it
also reduces the specific gravity of the coolant. This means that
not only CAN your engine run at a hotter temp but it WILL run at
a hotter temp. It would be interesting to know what the break-even
point is. I imagine one should determine how hot they want their
engine to run, then put in just enough glycol to ensure the coolant
doesn't boil at that temp.
Enough rambling for now.
In general, (a) is correct. Don't think about individual hunks of water; think about heat transfer out of the radiator. The radiator is most efficient when it is at a high temperature compared to the surrouding air that's blowing over it. Thus, the faster the water goes (and the higher the exit temperature), the more heat the radiator dumps per unit time. The limit is reached when the temperature of the water is the same as the temperature of the engine, and each little hunk of water only carries an infinitesimal amount of heat on each (very fast) trip.
Sorry, but that doesn't mean anything. Your statements about heat transfer
are true regardless of whether the thermostat is installed.
Also, many people have experienced overheating (after removing the
thermostat) under normal driving conditions. If increased flow rate was
responsible for the overheating, then cars should overheat if you drive at
normal speeds in too low a gear. That will increase the water pump speed
and increase the coolant flow rate without a corresponding increase in
airflow through the radiator.
But in practice, production cars will not overheat even if you drive around
in low gear. That's another indication that high coolant flow rate does
*not* cause overheating. In fact, high flow rates increase heat transfer
due to increased turbulence.
Physics conflicts with 'horse sense' in this case. Regardless of the
composition or thickness of the surfaces involved or the type of coolant, in an
automotive cooling system the faster the coolant flows thru the system the more
heat will be transferred.
My source: "An Engineering Analysis of Automotive Cooling Systems", a four-part
series in Skinned Knuckles magazine beginning in November 1990. Automotive
engineer Bill McLellan specifically analyzed Ford cooling systems, and used
hydraulic equations to deal with the questions of flow rate and water vs an
anti-freeze mix as a coolant.
Before I let this dead horse rest in peace, I'll take a few more whacks.
I have been running cooling system simulations. For any given heat input to the water jacket, and
radiator with constant characteristics, if the quantity of coolant flow is increased its temperature
drops. By coolant temperature, I mean at the thermostat location, the radiator outlet, and the
radiator average temps increase. This applies to cold start & run-up to temperature, with high flow
run vs. low flow run, and hot running with a sudden change of flow. Except for brief transient
fluctuations, the coolant temperature is always lower with higher flow. If the radiator is just big
enough for a certain heat input, (I.E. the coolant will overheat with any increase in input) its
capacity will increase with a higher flow.
"More flow = cooler engine, period."
The idea that it will run hotter is flawed. Elementary Thermodynamics,
higher flow = higher heat transfer. You may even get discussion about laminar
and turbulent flow, bla, bla, bla. The water pump generates plenty of turbulence
and the paths through the engine are quite narrow anyway.
However, with the thermostat out, if you drive under "severe service"
conditions, not letting the temperature come up and stay up before shutting it
off, you may need to change your oil more often. The contaminants (acids, water,
etc.) will not have time at temp to boil off, possibly causing accelerated
engine wear.
Also, although a lower temperature will enable you to advance the timing a
little further (if you are only making short runs, if the engine comes up to
temp, this will result in detonantion) and get a cooler, denser air/fuel charge,
this will NEHATIVELY effect efficiency; power will increase slightly during the
warm-up period, mileage will decrease, more than slightly during the warm-up
period.
The engine will still run the same after warm-up, which of course will take
longer without the thermostat. My advice, don't drive a STREET car without a
thermostat, unless the one you have is stuck closed and you have to take it out
to use the car to go get a new one . . .
