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Under ideal conditions, the flame front advances from the spark plug. What is important, though it that it IS less controlled in that the timing of the ignition is before the intended ignition. This causes a pressure wave that has to be absorbed by the piston, cylinder walls, head, valves, and plug. That affects power and durability. If unchecked, it can raise the heat level to a point where you can get pre-ignition and that's when you blow holes in pistons, bend rods, or throw them through blocks (which can be quite dramatic and entertaining if it's not your car).
When the knock sensor detects knock (basically, the knock sensor is a microphone that picks up the little pings through the metal block), it tells the ECU, which switches to a more conservative engine map. This would be retarded ignition timing (cools things down inside the cylinder, but is lousy for power or fuel efficiency) and possibly increased fuel, which cools the engine even further. I think some really paranoid ECUs might even lower the rpm limiter, but that's just what I heard and I don't know for sure. Not that I doubt it, but it could have just been the retarded timing and rich fueling naturally limiting the maximum rpms. Anyway, if the knock sensor stopped working in my old Subaru, it was supposed to go into a 'limp home' mode which did the ignition cut and the fuel enrichment (I heard of people getting 12 mpg from 2.2 liters like this!). Unfortunately for me, the early Legacys had knock sensors not up to the rest of the car's durability standards and my knock sensor went, but the ECU never noticed and I ended up yanking a real ringer of an engine (the N/A in that car was as quick as some stock turbo versions up to around 70) with a relatively young 250k miles because of it when I lost a cylinder (still maintained 85 mph). Even more unfortunate was that I had a turbo engine I was rebuilding in my kitchen and so I just swapped it out the next day (running the lower compression block normally aspirated. Went down to 23 mpg from about 26 and lost any breathing above 5k rpm) and all the ended up being wrong with the 'dead' motor was a melted $12 exhaust valve! I was pretty ticked at that point (especially when I saw how beautiful, even almost new, the rest of the internals looked), but committed to the other engine. So anyway, the point is, don't try to save a couple dollars by risking the engine. I'd play it safe and go with whatever the minimum octane is that works until you can monitor what's going on. Really, you shouldn't notice a difference, but if it feels slower or is getting worse mileage, then you're too low. If the recommended octane seems too low, you might have a problem. Could just be something other than knock is setting off the knock sensor (bad sensor, odd vibrations from something) or the plugs might be the wrong heat range or bad.
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