LogFAQs > #979747592

91+ octane fuel burns more slowly than 87 octane fuel. This helps mitigate engine knock (when the piston slams into the ignition flamehitting the flame too early, causing a knocking sound as it's jostled in the cylinder instead of being pushed straight down) in high-compression engines. Most engines nowadays are high-compression for better performance and emissions control.

When you use low octane, a modern car will adjust the timing of the spark plug ignition with the help of a knock sensor, so that the flame doesn't happen too early. It's unlikely that this sensor will ever fail. Cars without this knock sensor face damage from too much engine knock. I'm not sure how much is too much (10,000 miles of engine knock? Would it survive?). The damage is burned/soiled pistons or valves. In extreme cases, a piston might melt or develop a hole, after which it loses compression and becomes useless.