What is the effect of altitude?
The effect of increasing altitude may be nonlinear, with one study
reporting a decrease of the octane requirement of 1.4 RON/300m from sea
level to 1800m and 2.5 RON/300m from 1800m to 3600m. Other studies
report the octane number requirement decreased by 1.0 - 1.9 RON/300m
without specifying altitude. Modern engine management systems can
accommodate this adjustment, and in some recent studies, the octane
number requirement was reduced by 0.2 - 0.5 (R+M)/2 per 300m increase in
This article is from theGasoline
by Bruce Hamilton with numerous
The larger reduction on older engines was due to:- - reduced air density
provides lower combustion temperature and pressure, Fuel is metered
according to air volume, consequently as density decreases the
stoichiometry moves to rich, with a lower octane number requirement. -
manifold vacuum controlled spark advance, and reduced manifold vacuum
results in less spark advance.