Octane rating relates to how well the fuel burns in a controlled manor. For example: A 90 RON fuel will burn under control in a standard engine with X compression. If you then raised that X compression ratio,lets say a high performance engine, the fuel will no longer be within its stable combustion range.. and will tend to spontaneously combust..i.e. det. But if you put a fuel in the 'high compression performance engine', that would combust stably under those conditions, it would be of a higher RON.. i.e. 99 RON. This fuel will of course combust fine in the standard engine with X compression. The number just refers to a range of compression or conditions the fuel will combust under. Mitch