Re: 87 octane vs 89 octane

If you live considerably above sea level, another thing to consider is altitude. Keep in mind that octane serves essentially to reduce the chance
of detonation. Generally speaking, reduced air density provides lower combustion temperature and pressure, so that higher altitudes require less octane.
If you live in Denver, for example, I've no doubt that you could run 87 octane and your Charger's PCM would never retard the timing at all. But I don't know where you cross the magic altitude line, though, that allows you to run 87 octane with zero detrimental effect. 1500 feet? 3000 feet?
With older engines, as air density decreases, the stoichiometry tends to move to rich, with a lower octane number requirement. This may not be applicable today, with carefully monitored air-fuel ratios.
Also with older engines, manifold vacuum controlled spark advance, and reduced manifold vacuum results in less spark advance, though I don't know if this would have any parallel effect on modern engines.

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