I have read and been told that the richer the mixture the lower the egt's and same for leaning out. the leaner it is the lower the egt, which means that higher EGT does not mean leaner like most of us think. I belive that stoich is where the EGT is at its highest. Now my question is, how come if one leans out too much, why does it cause detonation? if your running lean shouldnt temp's be lower and therefore not cause hotspots within the cylinders?

 

Anybody know?

 

Gasoline cools the air/fuel charge and so cools the cylinder, as it enters the cylinder. Cooler resists detonation and pre-ignition better. Richer is cooler. Richer resists better. Leaner does not resist as well. Lean plus heavy load (mash gas at low speed) equals bad.



Excellent article on detonation/pre-ignition, very many words from an engine designer

http://www.contactmagazine.com/Issue...ineBasics.html



Fancy graph and some theory from an engine manufacturer



http://www.lycoming.textron.com/supp...es/SSP700A.pdf

 

Its a good question and not one that I found could be answered by google. I actually asked this exact same question on another forum back in march: http://hyperaustralia.com/forums/vie...6194&hilit=egt



The conclusions I have drawn:

-A lean mixture requires more ignition advance because fuel is scarce within the cylinder: the flame front is slow. That means that there is far more time for heat to be transferred into the head and combustion chamber walls.

-Oxygen is in excess in a lean reaction, perhaps this creates more heat to be released. This ties in with the slow flame front, ie the hot reaction basically dumps all of its heat into the cylinder head. A quick, stoich reaction will carry its heat into the exhaust.

-This also works with the detonation theory: it heats up the cylinder far more.



This is by no means comprehensive and these are just my own ideas. Although it is true that a lean mixture burns slower.

 

Retarded timing will increase EGT too.