What is the primary difference between 100 and 100LL grades of aviation gasoline?

The primary difference between 100 and 100LL grades of aviation gasoline lies in the lead content and color. Specifically, 100LL (low-lead) gasoline contains significantly reduced amounts of tetraethyl lead compared to the regular 100 grade. The "LL" designation indicates that this fuel is formulated to be lower in lead content, addressing environmental and engine compatibility concerns.

While both fuels serve similar purposes in aviation, the reduced lead in 100LL helps to minimize lead emissions and potential lead-related engine deposits, making it a safer and more environmentally friendly option. The color difference is also notable, as 100LL is often colored blue for identification, while traditional 100 grade fuel may appear different due to its higher lead content.

The other options, such as volatility, density, and BTU value, do play roles in fuel performance, but they do not define the fundamental distinction between these two grades of aviation gasoline as directly as the lead content and color do.