What does the first number signify in the fuel rating of aviation gasoline below grade 100?

The first number in the fuel rating of aviation gasoline below grade 100 signifies the performance in a lean mixture. Aviation gasoline, particularly 100LL (low lead) or the other grades, is rated based on its ability to resist knocking (detonation) during combustion, which correlates to its performance under varying mixture conditions.

In a lean mixture, which is when there is more air than fuel, the fuel's ability to ignite and burn efficiently is tested. Hence, this rating reflects how well the fuel performs when the engine is operating with less fuel in the air-fuel mixture. A higher number indicates a better performance capability in such conditions, which is crucial for engine efficiency and overall aircraft operation.

Understanding this rating helps mechanics and pilots choose the appropriate fuel for their aircraft's specific operating conditions, ensuring optimal performance and reducing the risk of engine knocking.