On the diagram showing the power output of a piston engine as a function of mixture ratio, best economy on rich side of the EGT top is shown at:
Refer to figure.
The air to fuel ratio that ensures complete combustion is about 15:1 by mass. It is called the chemically correct ratio or the theoretical best economy. In reality, this mixture is not the best suited for normal operations because of the high combustion temperature-induced and the increased risk of detonation. This is why the best is to set the mixture about 10% richer so that the excessive fuel absorbs some of the latent heat and cools the charge below the detonation value.
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