Bearing rattle?

[SDsurfrider]

91 is less volatile than 87.

“Pinging” or engine “knocking” comes from pre ignition of the fuel (igniting before the spark) due to high pressures in the combustion chamber. This is why they recommend stepping up the octane if you experience pinging. Similar reasoning for higher performance engines; they are running higher pressures in the combustion chamber. So they have a need for higher octane requirement.

That is why 87 is more energy efficient because you get the same bang for less energy put into the system.