Looking at this graph I see that a piston engine provides less than best power when the mixture is richer than best power, and this is expected indeed.

I know that at full throttle, more than the required fuel is provided to the engine in order to achieve better cooling. Also, I know that - especially on supercharged engines - the amount of "cooling" fuel is quite high, that is, the engine is not surely providing its best power when at full throttle.

If an engine is rated 300hp when at 29 inches of manifold pressure (MAP) and 2700RPM at full throttle, does it mean that it is actually capable of providing more power if leaned to the best power mixture? It seems to be that it should be able to provide around 8-9% more power. Of course, I would expect it to suffer some serious damage.


1 Answer 1


Yes. The engine will produce its optimum amount of power for throttle setting when leaned to the correct stoichometric mixture. The primary risk during operation at peak EGT/TIT is of course the high temperature during operation and greater wear and tear. Engines are usually operated approximately 50° rich of peak for max performance or 50° lean of peak EGT for economy operations, depending on the model of engine, flight regiment, etc. Rich mixtures are preferred to keep the engine cool and minimize detonation risks, but it can also foul the sparkplugs with lead deposits, reducing engine efficiency, power and causing engine roughness during operation. Excessively lean mixtures can sip gas during cruise but risk fuel starvation or put the engine at a high risk of detonation resulting in severe damage. For these reasons, the leaning range I stated above is the most widely used.

Normally aspirated engines are often leaned prior to high-performance operations at high altitude‘s, such a short take offs from a mountain airstrip, etc.

  • $\begingroup$ So it right to say that if I lean the engine at takeoff for best performance, it can produce more than its rated power? $\endgroup$
    – SeeEn
    Dec 7, 2017 at 18:40
  • $\begingroup$ The risk of detonation occurs with excessively high temperatures, in other words, at or near peak. The further lean of peak the engine is operated, the further that risk is reduced. The danger lies in inadvertently running the engine too hot while attempting to run lean. This can occur if the fuel system is poorly tuned and especially with insufficient temperature monitoring available (e.g. six point EGT). Excessively lean mixtures cannot and do not increase risk of detonation, but insufficient lean lean mixtures can. $\endgroup$
    – J W
    Dec 9, 2017 at 21:58
  • $\begingroup$ Detonation is always a risk on the lean side of the stoichiometric curve and there is a notable risk of this occurring with an excessively lean mixture as well. Some engines can operate safely at Peak EGT, though running on the richer side for cooling purposes seems to be the industry trend. $\endgroup$ Dec 9, 2017 at 23:01

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