Well, someone will fill in the huge gaps in my explanation.. but basically in the instance of spark occuring too early, you'd have the force of combustion pushing downward on the piston at a rod that is trying to spin upward toward TDC. The increase in force on the rod is sort of like the difference between a car traveling 70mph on a highway bumping into the back of a car doing 60 versus the same car doing 70 into a car traveling in the opposite direction.... and if it really is the same car, this dude needs to learn how to drive :frustrated:
So what were we talking about again? Oh yeah, bending rods? :wtf:
Since igniting a mixture too early is actually not that common (it is harder to burn a less compressed fuel/air mixture), detonation is probably a more likely cause of bent rods (and burnt pistons..). Detonation is not a normal combustion by any means.. the cylinder pressure can reach insane levels during such an event, so high that the forces on just about everything in the engine are several times above normal levels. If you manage to get detonation a bit before TDC it makes the situation especially bad for the rods in my stupid "if a train travelling west...." example above. This relates back to the ignition system when you're on crazy boost and the spark fails to start a normal burn in the cylinder and peak cylinder pressure reaches the detonation point.. ping!
I could be wrong, that's just how I rationalized it.
Marc, yes, I agree, IMO the reason for these bent rods in 3B's is likely due to inaccurate timing calculations thus causing predetonation at the worst possible time, the liklihood of a spark jumping some 144degrees to the next dist contact point and actually igniting an uncompressed A/F mixture is unlikely IMO. Predetonation kills motors commonly, and its a reality that dist. systems have inherent weaknesses here.