Engine Failure Sequence
The pilot reported an RPM overspeed accompanied by an oil temperature rise. Throttling back alleviated the overspeed condition, but upon turning back toward a potential emergency landing airstrip, the remaining power was rapidly lost. The propeller continued windmilling until impact.
Post-Accident Examination
Examination of the engine's pistons revealed that pistons 1, 2, 4, 5, and 6 had few combustion deposits on their combustion faces. Piston 3 exhibited evidence of detonation, including a hole approximately 1 inch in diameter and a combustion surface with an eroded appearance.
Fuel Specifications
The engine, as originally manufactured, required a minimum octane of 91/96. Teledyne Continental Motors subsequently amended the recommended fuel to 100 octane after 91/96 octane fuel became obsolete. Placards on the airplane's fuel tanks specified that minimum fuel required was 100 octane. No Supplemental Type Certificate for automotive gasoline use was found for the subject airplane or engine.
Fuel Analysis
A fuel sample removed from the airplane after the accident was tested. The Research Octane Number was 94, the Motor Octane Number was 83.8, and the anti-knock index was 88.9. The pilot stated that automotive gasoline had been used in the airplane for approximately 700 hours.