Definition
A rating system used for aviation gasolines with anti-knock qualities greater than that of pure iso-octane. Iso-octane is assigned a value of 100, and fuels rated above 100 are expressed as performance numbers, indicating the relative engine power that can be produced without detonation compared to a reference fuel. For example, a fuel rated 130/100 will allow an engine to produce 30% more power without detonating than it could on pure iso-octane.
Plain English
A number above 100 that tells you how much more knock-resistant a high-grade aviation fuel is compared to a standard reference fuel. The higher the number, the more power the engine can safely produce on that fuel without the fuel igniting too early.
Context Anchor
Seen in piston-engine fuel specifications, older aviation gasoline ratings, engine operating limits, and maintenance references for high-power aircraft engines.
Derivation
The term was coined because the older 'octane number' scale only goes up to 100 (pure iso-octane). Once aviation fuels were developed that out-performed iso-octane, a new scale was needed to describe them. 'Performance' refers to engine performance — specifically, how much power the fuel allows the engine to make before detonation sets in.
Why Pilots Care
Selecting fuel with the correct performance number prevents detonation that can destroy an engine in flight.
Intuition Check
Do not read Performance Number as a general score for the airplane’s climb, speed, or runway performance. Here it means a fuel’s resistance to engine knock at high power.
Example Sentence 1
Grade 100/130 avgas has an octane number of 100 on the lean side and a performance number of 130 on the rich side.
Example Sentence 2
High-compression engines require fuel with a higher performance number to run safely at full power.