Alrighty, I'm following now. So you might have the actual spark at, say, 12' BTDC, but you don't get actual combustion or full burn or whatever you want to call it until a bit AFTER TDC? Probably like 0.001 seconds later .. I could do the math and figure out the exact time if I wanted, I suppose. Thanks!

 

Yeah, that's more like it. I was reading some of the earlier posts trying to make some sense...

Spark always happens before top-center. Starting with basic timing, it usually gets even earlier at higher rpm and higher load. But the chemistry of combustion takes some time, and it doesn't all 'ignite' at once. There's a flame front starting at the spark, and traveling across the combustion chamber even as the piston is moving. So the highest pressure happens nearly at top-center.

It's a trade-off between earlier combustion giving more power, but also giving you more risk of damage from detonation or pre-ignition.

Timing of the actual combustion is not easy to measure, so the engine designers worry about that. Particularly since it's not instantaneous. Spark timing is what you can measure with at light...

 

Very well stated!