An engine's octane appetite changes due to numerous parameters, so it is very important to have enough octane quality. There is no such thing as too much octane.
The following is a list of the most significant items that contribute to octane requirement. These must be kept in mind when thinking about engine octane requirement:
Compression Ratio - Higher requires more octane
Cylinder Bore Size - Bigger bore requires more octane
Cast Iron Cylinder Heads - Iron heads need more octane that aluminum heads
Restrictor Plates - Less restriction requires more octane
Lean Mixtures - Need more octane
Coolant Temperature - Higher needs more octane; 160 degrees F is ideal
Spark Advance - More spark advance requires more octane
Intake Air Temperature - Higher temperature need more octane
Humidity - Dry air (low humidity) requires more octane
Barometric Pressure - Higher needs more octane
Altitude - Closer to sea level needs more octane
The above information is a guide as to which way the engine octane demand will go as these eleven key parameters are changed. There are other things that impact engine octane requirement, but those listed above are the most significant.