7.11 What is the effect of altitude?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
7.11 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 [27]. Other studies report the octane
number requirement decreased by 1.0 - 1.9 RON/300m without specifying
altitude [38]. 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 altitude.
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.
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