Jim makes sense...but. I am sort of lost. Let's define something, detonation. We mean detonation from compression, not spark, right? So, basicly is or is it not possible to detonate low grade gas at a 10:1 ratio? If so, then we have a diesel engine, needing no SI. And we'd have engines starting up with just fuel.
Now, if the mix cannot be "detonated" by compression, how do we end up with a knock condition anyway? I understand there could be hot-spots.
To simplify, since compression (ratio) and detonation or knock are physically dependent on each other, how then does *electrical* spark ...matter? The resistance to detonation *itself* being the definition of octane.
The only way I can understand this is, like was said, the enlarging volume of the partly ignited fuel takes up a more and more space in the combustion chamber THUS changing the ACTUAL ratio of compression far beyond 10:1. The physical dimensions of the combustion chamber are just a starting point. Uh? Hmm, in this case then, the timing itself (sort) of controls the compression ratio, but starting the burn and changing the volume of physical gas in the chamber. Did I figure it out? lol.