Altitude above sea level is not something that most of us need to consider really except for the few that live at 600-1000ft above sea level. Up there has upto 1% less oxygen available for combustion.
An engine perfectly tuned for this altitude will run weaker when thrashed on a run out to the coast, potentially leading to piston melts, seizures, recovery truck, embarrassment and depression

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Always best to set them up on the rich side if you live up where the air is rarefied.