Thanks .. I'm still not totally getting it though... or not communicating it well enough.
Lemme try it this way .. you said:
spark ALWAYS happens after TDC. when you set your ignition to 12 degrees before top dead center, by the time fuel ignites, the piston has passed TDC.
You said the spark always happens AFTER TDC, but in the next sentence you said you set it to 12deg BEFORE TDC. That's what's sorta confusing me. Are you saying that the physical spark occurs BEFORE TDC, but that the mixture doesn't fully ignite until AFTER TDC? What am I missing?
if you retard the spark, the piston has moved away from TDC... therefore being less compression on the mixture compared to when the piston is at TDC. if you have less compression, you have less chance of detonation. when air is compressed, it heats up due to friction in the air molecules. if you over compress it, it will auto-ignite. this is how a diesel engine works... they have no spark plugs as they rely on auto-ignition for combustion.
I totally get this .. my question was just the following: whether the spark happens at 12 or 14 degrees AFTER TDC, you're still PASSED TDC. If the mixture were going to detonate, wouldn't it detonate at EXACTLY TDC, where compression is the greatest? I don't understand why a mixture would detonate at 13' after TDC, for example, but not have detonated at exactly TDC? Think of it this way .. (1) you start before TDC. Compression is low. (2) You move on to TDC, and compression increases. (3) You hit TDC, and move away, and compression starts to DECREASE again. And NOW it detonates? Why didn't it detonate at TDC? Does that make any sense?