The EGT will give you the combustion cylinder thermal picture , so you can make adjustments to mixture to a max EGT or see the effects combustion wise .
The CHT will give you the cylinder head thermal loading , now you can see the rich / lean effect on the head and the cooling system efficiency
The stock head has its coolant passageways , designed around a maximum designed thermal capacity , areas in the head require more heat extracted ,and hot spots avoided .
The head is tested at manufacture and using various pressures and flow of water the cooling system can be sized - sized for pressure and most importantly flow .
Pressure has a limit as aeration occurs , air bubbles are disaster for cooling and the water must pass at a controlled rate. Some leeway is given for say low water quantity , oil quantity etc .
All the data is done around stock power levels and stock heat transfer , so what happens if you multiply that power by 3 or 4
There is extra capacity for this , if flow is increased . The problem is we only measure at one point , the water temp entering the head - this is not CHT , but the water temp .
Once water system is fully loaded (for that flow rate) it cannot absorb any more heat , so heat now passes to the oil system and oil temp rises .
This hides what is happening to the cylinder head internally temp wise and everything is on a delayed cycle .
A £50 CHT guage shows you what's happening to the head very quickly .
As cooling is a flow based system , fixed quantity of water , fixed rad size , variable pump speed and variable stat flow - the radiator has a very minimal effect as does the quantity of water .
Flow has a massive effect - take the stat out and the engine runs mega cold , so we have capacity for cooling , it's just the control of it .
Most people simply fit an oil cooler and accept that the oil has gone up by 30 degrees but the cooler has sorted that , it's because the water has maxed out on cooling that the oil temp rose , and now your CHT has also shot up - but by how much ? - CHT gauge.
Is it all worth it ?
Heads get valve guide wear, exhaust valves burnt,cracks around the spark plug holes,valve seat wear , warpage and gasket problems,hotspots and detonation all through excessive heat - it's all Dollar to a stingy bugger like me - lol