Octane rating

A rating of the anti-detonation characteristics of a reciprocating engine fuel. It is based on the performance of the fuel in a special test engine. When a fuel is given a dual rating such as 80/87, the first number is its anti-detonating rating with a lean fuel-air mixture, and the higher number is its rating with a rich mixture. Open angle. An angle in which sheet metal is bent less than 90°.

Aviation Maintenance Technician Handbook — Airframe, p. G-24