What is the fuel-air ratio for maximum power output?

• A. Rich 1:12
• B. Lean 1:16
• C. Stoichiometric 14.7:1
• D. Rich 1:6

Answer: (A)

Maximum power in a spark-ignition engine comes from a mixture that is slightly richer than stoichiometric. More fuel in the mixture increases the energy released during combustion and the peak cylinder pressure, boosting power, up to a point. Around 12 parts air to 1 part fuel (a fuel-to-air ratio of about 1:12) is a commonly cited optimum for maximum power at wide-open throttle because there’s enough fuel to maximize combustion energy without flooding the engine or starving it of air.

The other options don’t maximize power for different reasons: a lean mixture (too much air) provides less fuel energy per cycle, reducing power; the stoichiometric ratio (about 14.7:1) is ideal for complete combustion and emissions, not for peak power; and a very rich mixture (more fuel than optimal) can flood the engine, cause incomplete combustion and cooling effects that actually lower power.