On 27 June 2012, J H Cole in Hampshire, UK wrote:
"I had a leak from the bottom hose of the radiator that drained away a lot of the coolant that I did not detect. I first noticed it when the the temp gauge went off the scale past the 230 mark. Having replenished the coolant to get home I drained the radiator and replaced the hose. I normally run with 25% antifreeze but this time I refilled with plain rainwater. What surprises me is that my engine now runs at least 10 degrees lower temp from its usual 190 degrees down to 180 degrees. Why should this be so? Is it that the thermal conductivity of water is better than antifreeze or has the high temperature water boiling disturbed something such as the stat or temp gauge"?

The short answer is, I don't know why you see a 10dF drop of coolant temperature, because changing from antifreeze/coolant mix to plain water should not make that much difference. I can tell you what the difference should be (if you have enough patience to tolerate the explanation).

The Constant pressure heat capacity of ethylene glycol is 2.42 J/g K.
The Constant pressure heat capacity of water is about 4.20 J/g K.
Specific heat of water is defined as 1.0 as the baseline for the specific heat unit.
Specific gravity of water is defined as 1.0 as the baseline for the specific gravity unit.
Ethylene glycol is about 10% heavier than water (depending on operating temperature).
So specific gravity of ethylene glycol is 1.10.
This makes specific heat for ethylene-glycol to be 2.42/4.20x1.10 = 0.634.
This means that ethylene glycol (common antifreeze) carries 37% less heat per unit volume.
This is not nearly as bad as it might sound.

When engine coolant is a 50/50 mix (by volume) of ethylene-glycol in water, the resulting specific heat of the mixed fluid will be about 0.82. For this fluid to carry away the same amount of waste heat from the engine, the temperature rise in the engine and temperature fall in the radiator must be 1/0.82 = 1.22, or 22% greater change of fluid temperature. This also is not nearly as bad as you might think.

This does not mean that the radiator needs to be larger. Cooling capacity of the radiator depends on heat transfer from internal fluid to external air. This heat transfer function through the radiator wall from liquid to air is similar regardless (almost) of what fluid is inside the radiator. Also the radiator will need to dispose of the same amount of heat, regardless of what fluid is inside. As the fluid temperature increases, the external temperature difference increases, and the radiator becomes more efficient, requiring slightly less temperature rise to dispose of the same amount of heat.

As a starting baseline with plain water in the cooling system, assume temperature is 190dF at top of radiator core and 160dF at bottom of core for a 30dF temperature difference (temperature rise in engine and temperature drop in radiator). Also assume that ambient air temperature is 100dF, and that the thermostat (perhaps a 180dF opening thermostat) is wide open (no restriction to flow). Also assume the entire system is in a condition of dynamic stability with temperatures neither rising or falling with time. That is, coolant temperature has risen and stabilized at 190dF (gauge reading near top radiator hose) to carry away all waste heat as required.

Then change the internal fluid from plain water to 50/50 antifreeze mix. Without changing coolant flow or air flow, temperature rise in the engine will then need to be 1.22x30 = 36.6dF, so you might "expect" to see a 6.6dF increase of coolant temperature near the engine fluid outlet (thermostat housing or top of radiator). But this is not what happens.

Keep in mind that total waste heat is still the same, and ambient air temperature and air flow has not changed, so the difference between air temperature and radiator average core temperature must also be unchanged. The trick here is that with higher temperature at top of the core heat transfer to air is increased slightly, so more heat is transferred from the top half of the core. This leaves less waste heat needing to be transferred from the bottom half of the core, so the lower core temperature will be slightly lower, and fluid outlet temperature will be lower. This may at first seem to be counter-intuitive, but that is what actually happens.

In the end, for the required 6.6dF increase in fluid temperature "difference" between inlet and outlet, temperature will be about 3.4dF higher at top of the radiator (as seen on the gauge), and about 3.2dF lower temperature at bottom of the radiator (in the lower radiator hose). So the end result of change from water to 50/50 antifreeze is about 3.4dF increase of temperature as seen on the gauge. Not so bad, huh?

In another scenario, if the system is over-cooled, or if ambient air temperature is lower, it may operate with the thermostat only partly open to restrict fluid flow (in order to maintain proper minimum engine operating temperature). In this case when you change from water to 50/50 antifreeze mix, the thermostat will open more to allow more fluid flow. When fluid flow through the radiator increases about 20% it will carry away the required amount of waste heat with very little change of fluid temperature, perhaps only 1 or 2dF change showing on the gauge. The condition of greater temperature change on the gauge (due to fluid change) only happens when the system is at full dynamic heat transfer capacity with the thermostat wide open.

So for the question, I don't know why the fluid temperature might be 10dF lower after you changed to plain water. I suspect we don't have complete information about other changes of conditions. Maybe ambient air temperature dropped. Maybe you blew some bugs off of the radiator core to increase air flow. Maybe you had coolant on the fan belt causing slippage and low fan speed before the fluid change. Anyone else want to offer some theories?