The standard theory is that higher octane fuel is less volatile, so that
being the case, detonation should definitely not be a problem. Poor spark
with a high octane fuel may cause misfiring due to poor ignition. Detonation
- or more appropriately pre-ignition occurs when fuel ignites prior to the
proper spark timing. This is usually caused by carbon deposits in the
cylinder head, which are heated up during running, and retain a high degree
of heat, providing a source of detonation before the proper time. Av-fuel,
or any high octane fuel (leaded or not) should decrease preignition problems.
These problems are often characterized by a "pinking" sound, which is the
sound of the combusiton charge being detonated prematurely.
Lead was added to fuel for one reason only - it serves as a "cushion" or
lubricant for the valve to valve seat. Older vehicles had valve and seat
materials that were damaged easily by the combination of high temperatures,
and the pounding of closing under high forces thousands of times per minute.
Leaded fuel will not make a poor running engine run better by itself,
although it may reduce wear on the valves over time.
I believe that the altitude change will result in a lean burning engine.
This is due to the higher density of the air at altitude. I would recommend
checking you fuel mixture, after properly setting ignition timing. Most high
engine temps are attributable to incorrect timing or mixture.
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