CJ750 toolbox

Cajun Mike on predetonation

From the e-mail that I have received it is clear that combustion chamber detonation needs some further explanation. What you want is a flame front that travels down from the sparkplug ignition point in waves of flame burning all the air and fuel and spread equally across the top of the piston.

When the fuel mixture burns unevenly or detonates, small fires start independently of the sparkplug ignition point before the sparkplug ignites the fuel mixture. These fires fight each other for the last bit of oxygen left in the combustion chamber. This last area of oxygen is going to be on top of the piston and a fire ball will be concentrated at one spot on the top of the piston surface generating tremendous temperatures on that particular area.   It may not be hot enough to burn a hole in the top of the piston instantly but over time it will melt enough aluminum from the piston to weaken it and in time melt through.   If you have the cylinder  head off and you clean the piston and see small craters in the top of the piston these are melted areas caused by detonation. Iin some cases you may see droplets of liquid aluminum that have melted and stuck to the top of the piston .   Besides lean fuel mixtures and high compression, one of the most common causes of detonation is using the wrong type and heat range sparkplug  for the engine.  

Cajun Mike

Beijing 2003