I see what you mean, i take it this also can happen if there are hotspots in the chambers to ignite the mixture before the spark?richie00boy said:Basically as you have an explosion going on in the chamber and the piston is still rising up against it, the force of the explosion stresses the crown and blows bits out of it. Just like throwing a grenade really. Add to this that the mixture has probably not been fully fueled yet so you get a series of explosions as more fuel/air is fed in - hence the pock markings.
As usually the piston is about to go down at the time of the explosion, the force exerted on the piston is less so it doesn't break.
So if the sparks fires to early that is DET but if the mixture ignites before the spark thats pre ignition?rodgerq said:thats pre-ignition which is different again