^^ The above posted by is correct, but does not tell the whole story. Anyone who is good with electrics finds this is an easy analogy to the world of electricity. It is all a matter of thermal resistances (like electric resistance), heat flow (like current flow), and temps (like voltage) .
Excluding the heat out of the exhaust (which is actually a major exit path for combustion heat), the two paths for heat flow (like current flow) are: through the block and other engine surfaces directly into the air; and through the cooling system. These are like 2 parallel current paths with different resistances from different 'voltages' (temperatures). (The cylinder wall temps are different from the head interior temps and thus the analogy of the 2 different voltages.) The cooling system path has several thermal resistances in series and parallel; the block and head paths to the coolant are in parallel, and both combine and are in series with the coolant and radiator.
If the heads are AL with its roughly 4 times lower thermal resistance than iron, the internal head surface temp will be lower (unless there is a 4x increase in heat flow, which there is not). With a lower interior head surface temp, there will be more heat tranferred from the hot combustion gases into the head, so you don't get as much head temp drop as one might think. The net result will be higher heat flow through that path into the coolant and to the radiator along with lower internal head temps.
The thermal resistance of the radiator does not change if it is the same radiator, so the increased heat flow through the lower thermal resistance of the AL heads raises the radiator and coolant temps a bit; the actual increase in coolant temp is dependent on how much extra heat gets transferred to the AL heats from the hot combustion gases.
This HAS to happen if the interior surface of the AL heads to be cooler, which we know they are; this is evidenced by their better resistance to detonation. The rest of the engines surfaces may a bit hotter since the coolant temp is a bit higher.
What we have achieved with the AL heads is a cooler head interior surface, particularly on the raised or pointed spots on the head, where the detonation will tend to occur.
Oh yeah, I didn't stay in a Holiday Inn Express last night!
All in all, a very good discussion, especially about the radiator being the largest thermal resistance, and how to make it work better with shrouds, electric fans, etc.
