how r performance octanes(ie, octanes higher than 100%) graded?Extrapolation or some specific test method?

Answer 1

Octane number is the knock resistance of the fuel relative to chemically pure octane. It's measured with a single-cylinder, variable displacement motor called a knock engine. The motor design allows compression to be increased or decreased. The amount of compression needed to induce knock is used to compute the relative knock resistance. The range of possible values is 40-120.

The method is spelled out in two ASTM standards: D2699 and D2700.