I was thinking about how cooling systems work on the way home yesterday as I watched my needle rise in stop and go traffic. I find it intesting that people think a thermostat will fix the problem.
The idea of a thermostat is to keep the water in the engine until it reaches a certain operating temperature and then when it reaches that temperature allows the water to start circulating through the radiator until it cools down enough to close the thermostat again. If it doesn't cool down sufficiently to close a 198 degree thermostat, why would it cool down sufficiently to close a 160 degree thermostat? Is it easier to remove heat from 160 degree water than it is 198 degree water? No, of course not. If the temp keeps rising with a 198 it will keep rising with a 160. The benefit of a higher temp thermostat is that it will also keep the water in the radiator long enough to cool off. When the coolant starts flowing, the colder fluid will mix with the warmer fluid until it hopefully reaches an equilibrium temperature lower than the temp of the thermostat and the process starts all over again.
I suppose that based on this, overheating can be caused by several different reasons.
The thermostat is bad and stuck closed so that water never flows.
The waterpump isn't pushing the coolant through fast enough so the water is getting heated up in the block as it gets pushed through and is never able to obtain an equilibrium below the rating of the thermostat.
The radiator doesn't have enough heat exchange efficiency to keep up with the heat generated by the engine. This is probably where most of us fit.
Sorry, it's just been one of those contemplating type of days.
Rick has a good suggestion. Replace the water pump with a high flow pump. That will make up for the underdrive. I don't know if it would cause more drag, though, negating the effect of the underdrive.
A good, monster aluminum radiator and a stock water pump may work equally well and will not cause the drag problem. If you're running a 50/50 antifreeze mix, it may be a good idea to go to 75% water since it releases heat faster than anti-freeze. Keep the anti-freeze for lubrication and corrosion protection. you should also use a hotter thermostat. 180 minimum, maybe even something higher. Get a good high flow one, not a cheapy from the local autoparts store.
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351W 89 Mustang GT Convertible