What condition results from running the engine at high power with the mixture leaner than the engine manual specifies?
- Reduced fuel flow to the engine.
- Increased engine oil consumption
- Detonation and engine overheating
Explanation from PART66Online
At high power the engine relies on extra fuel for charge cooling. Leaning past that point removes the cooling and slows the flame front, so combustion carries further into the stroke, cylinder head temperatures climb and the unburned end gas is liable to detonate. This is why operating procedures call for full rich above roughly 75 percent power.
J
jon@efm.ltd asked · 1 Sep 2026
This question is worded badly. If the mixture is set lean of peak, the cylinder head temperature will decrease, NOT increase. "too lean" is very ambiguous. Please be more precise in the wording!
Community Comments (1)
P
PART66Online
2 Sep 2026
Thank you for the feedback. The marked answer is correct here, and it is worth setting out why.
Leaning at high power does not take the engine straight to lean of peak. Coming back from full rich, the mixture first passes through the best power and peak cylinder head temperature region, and that is where the detonation margin collapses. The excess fuel that was providing charge cooling at full rich is no longer there, flame speed falls, and combustion carries further into the stroke. That is the condition this question describes, and it is why the operating procedures for most certified engines call for full rich above roughly 75 percent power.
You are correct that well lean of peak brings cylinder head temperatures back down. That is a real technique, but it depends on fuel distribution good enough to support it, and it is not a high power setting, so it sits outside what this question is asking.
Of the three options given, reduced fuel flow is a cause rather than a result, and oil consumption is unrelated. Detonation and overheating is the only defensible answer.
If you find an item where the marked answer is genuinely incorrect, please do report it and we will check it against the syllabus.
Leaning at high power does not take the engine straight to lean of peak. Coming back from full rich, the mixture first passes through the best power and peak cylinder head temperature region, and that is where the detonation margin collapses. The excess fuel that was providing charge cooling at full rich is no longer there, flame speed falls, and combustion carries further into the stroke. That is the condition this question describes, and it is why the operating procedures for most certified engines call for full rich above roughly 75 percent power.
You are correct that well lean of peak brings cylinder head temperatures back down. That is a real technique, but it depends on fuel distribution good enough to support it, and it is not a high power setting, so it sits outside what this question is asking.
Of the three options given, reduced fuel flow is a cause rather than a result, and oil consumption is unrelated. Detonation and overheating is the only defensible answer.
If you find an item where the marked answer is genuinely incorrect, please do report it and we will check it against the syllabus.
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