What damage is likely to occur if an aircraft reciprocating engine that is designed to use grade 100 fuel is operated with grade 80 fuel?

• A. Detonation will occur
• B. Increased power
• C. No issue
• D. Fuel economy improves

Answer: (A)

Octane rating determines a spark-ignition engine’s resistance to detonation. An engine designed for grade 100 fuel relies on that high resistance to keep combustion controlled under its compression and timing. If grade 80 fuel is used, its lower octane means the mixture can ignite prematurely or burn in multiple pockets as the piston is still compressing the charge. That premature, uncontrolled burn creates detonation, with abrupt pressure spikes that place abnormal stress on the piston, connecting rods, bearings, and valve seats, and can cause overheating and power loss. So the most likely outcome is detonation, not improved power, no issue, or better fuel economy.