AVGAS - A Performance Fuel
The octane quality of a gasoline is best described as its ability to "wait for the spark". In the power stroke of a gasoline engine, the air and fuel mixture is compressed by the piston before being lit by the spark plug, whereupon, it must burn smoothly. Hence, the fuel must be capable of withstanding heat from the compression and radiation as the flame approaches, without spontaneously igniting.
If the fuel cannot withstand these effects, it explodes and this results in a characteristic "pinking" or "knocking" sound from the engine. This is very dangerous as these explosions can cause loss of power, blast metal from the piston crown, or at worst, result in total engine failure. Engine design relies heavily on fuel octane quality. High power to weight ratio, large piston diameter, high compression, and supercharging, all call for fuels of high octane quality.
AVGAS has a much higher octane quality than other aviation gasolines such as Mogas. The best quality Mogas in the world today is about 88 Motor Octane Number, MON being a measure of octane quality (0 for very poor, 100 excellent). The minimum for performance AVGAS, 100/100LL, is 99.5 MON. But this is not the end of the story because there is an even more severe test Mogas does not face: Supercharge.
Supercharge is a unique test requirement for AVGAS. The test was developed to ensure AVGAS would perform under the most demanding of conditions in high performance, high power output engines.
Basically, the AVGAS is run in a single cylinder, fuel injected, supercharged engine by adjusting the air fuel ratio to give maximum power. The pure chemical "iso-octane" which gives 100 MON, can only achieve 100 performance number for Supercharge.
The specification minimum for AVGAS 100/100LL is 130 performance number.