What is the most reliable indicator of preignition in an aircraft?

The most reliable indicator of preignition in an aircraft engine is higher exhaust gas temperature (EGT). Preignition occurs when the fuel-air mixture ignites prematurely in the cylinder, prior to the spark plug firing. This leads to an increase in combustion temperatures, which is reflected in the exhaust gases. As a result, monitoring EGT provides a clear indication of what's happening within the combustion chamber.

Increased fuel consumption, loss of power, and engine roughness can be associated with various engine issues but are not specific indicators of preignition. Increased fuel consumption can result from a variety of inefficiencies in fuel delivery or engine tuning rather than preignition alone. Loss of power may indicate engine problems or other inefficiencies, while engine roughness can be a sign of ignition issues or fuel irregularities, not necessarily preignition. Higher EGT directly correlates to the abnormal combustion conditions caused by preignition and thus serves as the most reliable indicator.