What indicates an engine may detonate more likely when using fuel?

An engine is more likely to detonate, or experience pre-ignition, when the fuel used has too low an octane rating compared to the engine's requirements. Octane rating is a measure of a fuel's ability to resist knock, which is a form of abnormal combustion characterized by sudden increases in pressure and temperature in the combustion chamber. If the octane rating is insufficient for the engine's design, it can lead to pre-ignition or detonation, compromising engine performance and potentially causing damage.

Engines designed for higher octane fuel rely on its ability to withstand greater pressures and temperatures without igniting prematurely. When a fuel with a lower octane rating is used, the risk of knocking increases, especially under high load conditions, leading to serious engine issues. Therefore, ensuring the fuel used matches or exceeds the engine’s octane rating is crucial for maintaining optimal performance and safety.

Other factors listed, such as high octane fuel, duration between fueling, and idle mixture settings, do not directly contribute to an increased likelihood of detonation in the same way. High octane fuel is actually beneficial and appropriate for engines needing it, while long periods between fueling and improper mixture settings may affect performance but are not directly linked to the risk of