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I have been trying to understand the exact details of an EGT gauge, specifically to understand why on a recent flight it was (for a short time) reading much lower than I would have expected.

I would usually reference the EGT gauge during leaning, and from experience of the aircraft, am aware that peak RPM usually coincides with between 1400-1500 °F. This is a useful cross check, and reference during leaning.

During a flight yesterday the EGT was reading about 1200 °F when leaned just rich of peak. This felt unusual, and I wondered if there might be something amiss. The OAT was around 10°C (50°F) degrees, and I was in a thin layer of cloud at the time.

Some 20-25 minutes later I was clear of cloud and the EGT was reading 1400 °F which is more what I might expect.

Is there any obvious reason for this?

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    $\begingroup$ An EGT is fairly useless unless it provides readings for all cylinders. You normally get large variations between cylinders. Also the objective isn't a specific temperature, it's a temperature above or below the peak temperature, whatever that happens to be. $\endgroup$
    – John K
    Commented Sep 12, 2022 at 12:42

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You've probably just witnessed the reason high performance piston engines (especially those for warplanes in the last years before the jet era) were equipped with water injection.

Adding water to the intake air, either in the form of vapor or fine droplets (like cloud/fog) will lower combustion temperature and can retard the onset of preignition ("knock" or "detonation") which can damage the engine. For turbo- or supercharged engines with very high manifold pressure, this can make the difference between literally destroying the engine and simply needing an inspection and oil change after a combat flight with prolonged War Emergency Power.

In a light plane, like most modern GA aircraft, you don't have to worry about manifold pressures in the range of 2-3 atmospheres melting your pistons -- but if your engine is ingesting water, you'll still see a drop in combustion temperature, therefore a similar drop in EGT. This might be seen as a small loss of power, as well, but if the ingested water is liquid droplets its conversion to steam may actually add a small amount of power while carrying away combustion heat.

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  • $\begingroup$ Modern materials are also better at handling heat than WW2 materials. $\endgroup$
    – Kevin Kostlan
    Commented Sep 13, 2022 at 3:08
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    $\begingroup$ Piston GA engines aren't made with "modern" materials. Aluminum pistons, for instance, won't take as much heat as the cast iron in most WWII radial engines. Jet engines are a whole different animal. $\endgroup$
    – Zeiss Ikon
    Commented Sep 13, 2022 at 11:14

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