I notice this when I run higher octane: The engine shuts down quietly when you turn off the key, no 'dieseling' or 'run on' with higher octane fuel. Now a lot of the 'run on' problem can be cured by simply having a low idle rpm, which pretty much closes off the carb throats 'starving' the engine. But with MY cam AND dual fours I have to keep my idle rpm around 1000. Thats 'just enough' for the engine to continue to draw in a little bit of 'fuel\air' through the carbs and theres 'just enough' hot carbon in the combustion chamber to ignite that fuel\air and cause the dreaded 'run on' condition.
When I'm using 'race gas', ZERO problem, shuts right down, even at 1100 to 1200 idle rpm. Why? Higher octane fuel is hard to ignite, another way to say it is, it's more stable. You COULD surmise that it 'burns cleaner' leaving fewer deposits to ignite said fuel, maybe. But no, I think whats happening is it's simply 'harder to ignite'. I routinely shut off the key and 'dump the clutch' with the car in gear when shutting down. I've done it so long I don't even think about it anymore, until I let someone ELSE drive and they DON'T, then I'm freaking out when the motor 'runs on'!
Using octane boosters has NOT helped this condition, even using well above the recommended amounts. Conclusion, octane booster aint doing squat for me!