Abstract: A method of operating a multiple combustion chamber internal combustion engine includes advancing an ignition timing of a first subset of the combustion chambers from an operating ignition timing until a knock event is detected while concurrently operating the remaining combustion chambers at the operating ignition timing. A first knock margin of the first subset of combustion chambers is determined in relation to a difference between the operating ignition timing and the ignition timing at the knock event. A characteristic of a fuel supplied to the combustion chambers is determined in relation to at least the first knock margin.