EGT is relative to the temperature of the charge as it burns within the
cylinder. CHT measures the temperature of the engine cylinder.
The highest EGT is achieved at a ratio of 15 parts of air to one part of fuel.
If the ratio is reduced ie rich the charge temperature reduces due to excess
fuel being left in the cylinder. If the ratio is greater than 15:1 ie Lean or
Weak the charge is cooled by excess air. To appreciate why CHT increases with
lean mixtures we must consider the speed at which the fuel burns, ie its flame
rate. lean mixtures burn slower than rich mixtures.
This means more heat is transferred to the engine components with lean
mixtures.
Example. If you move a lighted match quickly under your
hand, no problem, but move it slowly and you will burn your hand! Slow burning
lean mixtures will increase CHT.