I’ll post this in the xjs section, since this is where most people seem to worry about the engines. It applies to all cars, v12 or not.
This is an excerpt from a datalog of a car being driven into the garage and left for five minutes. The top graph shows rpm. The bottom graph shows coolant temperature at the cylinder head water rail in light blue, coolant temperature at the bottom of the radiator in dark blue, ambient engine air temperature in yellow.
At t=1107 seconds, the car is driven into the garage (rpm>0) and the radiator fan is on.
At t=1133 seconds, the key is turned off and back on, revs drop to zero, the radiator fan turns off. Coolant temperature is 85’c and the radiator water temperature is 70’c.
At t=1273 seconds (140 seconds later), the coolant temperature has risen to 87’c and this triggers the radiator fan on (temperature 65’c)
At t=1407 seconds, the radiator fan is still on, the coolant temperature peaks at 87.3’c and the radiator water temperature drops to 34.6’c. As the engine is off, there is almost no water circulation and the fan’s efforts are mostly wasted as it is only pushing coldish (mid 30’s 'c) air over the engine block.
At t=1443 seconds the engine is deliberately restarted.
At t=1486 seconds the engine is shut down again.
The fan ran for 170 seconds to not much effect but it is the 43 seconds of engine running which swaps the 31’c cold water in the radiator with the 86’c water in the heads. The radiator water temperature rises to 71’c and the cylinder head temperature drops from 86’c to 74’c.
Basically, you can drop your hot shutdown temperature by 12’c if you wait five minutes and run the engine for 45 seconds then shutdown again.
Running the engine for a longer time than 45 seconds is counter productive as once the bypass closes, you are simply pumping hot water back into heads like normal.
Running the fan on its own does little to cool the heads. The purpose of running the fan is to generate a reservoir of cold water in the radiator so that you have something to cool the heads with.
Now stop worrying about heat soak, fancy current-hungry upgraded fans, recored radiators and simply use what you have and never worry about dropped valve seats ever again.
EDIT:- The green trace is one of 12 EGT sensors embedded in the exhaust ports. It is showing ambient head temperature when the engine is not running and comes down to about 90’c on the graph. This probably represents your heatsoak potential in this case. It’d be worse with a closed bonnet and no radiator fan running, but the same cooling strategy can be applied.
kind regards
Marek