AOPA Pilot Magazine - January 2023 - 94

RUDDER & WRENCH
full-power takeoffs, we use an even richer mixture for increased
detonation margin.
For best economy, you need an air-fuel ratio of about 16-to-1,
quite a bit leaner than stoichiometric and so significantly lean of
peak. Such a lean mixture burns very clean and reduces combustion
pressure and temperature, which is great for engine longevity
but at the sacrifice of some power and airspeed.
Whether you want to go fast, go far, or make your engine last
a long time, peak EGT is probably not where you want to operate.
There are better places to be.
The real-world red box is defined by CHT rather than PCP.
The red box
The key to longevity is avoiding excess stress-something that is
true for both engines and humans. For engines, the best measure
of stress is peak cylinder pressure. Operating at excessive peak
cylinder pressure is abusive and can shorten engine life. George
Braly calls this " operating inside the red box " and it's obviously
something we'd like to avoid.
How do we know if we're inside the red box? In a perfect
world, we'd have peak cylinder pressure sensors in our cylinders
and an " excessive PCP " alarm in the cockpit. In fact, that's precisely
what Braly has in his fancy test cell at GAMI. He uses special
spark plugs with pressure sensors in them, and can monitor peak
cylinder pressure in real time.
In the real world, our airplanes aren't equipped with peak
The multicolored box is a useful way of visualizing abusive mixtures.
cylinder pressure sensors and probably won't ever be because
the sensors are expensive, short-lived, and non-FAA-approved.
Fortunately, most of us do have a pretty good proxy for peak cylinder
pressure in our cockpits: cylinder head temperature (CHT).
Although CHT isn't a perfect proxy for peak cylinder pressure, it's
a pretty good one. Peak cylinder pressure and CHT are closely correlated
as a function of fuel flow. So, the best practical method we
have of staying out of the red box is to keep CHTs well-controlled.
To stay out of the red box, I generally recommend keeping
CHTs no higher than 400 degrees F for Continentals and 420
degrees F for Lycomings. If you plot CHT and EGT as a function
of fuel flow, you'll see the most abusive place to be (maximum
CHT) is about 40 to 50 degrees F rich of peak, right where lots of
POHs urge you to operate. The red box varies with horsepower-
it's wider at high power settings and narrower at reduced power.
If power is low enough-say 60 percent-the red box goes away.
There's nothing magic about these CHT guidelines. Going deep
inside the red box is more abusive than going slightly over the edge.
That's why I prefer to visualize a multicolored box. For Continentals,
I think of CHT above 380 degrees F as being in the yellow, above 400
degrees F as being in the red, and above 420 degrees F as being in the
magenta. For Lycomings, add 20 degrees F to these values.
If you find yourself inside the red box (especially the magenta
part), get out ASAP. There are three ways to get out: exit the left
side (by enriching a lot), exit the right side (by leaning to lean of
peak), or exit the bottom (by reducing power).
Finding peak EGT requires dawdling inside the magenta box.
EGT as a leaning reference
Let's circle back to why I don't use EGT as a leaning reference and
recommend that you don't either, and why I especially caution
94
AOPA PILOT / January 2023
AOPA Pilot Magazine - January 2023

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