AOPA Pilot Magazine - February 2023 - 90

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it during the leaning process, and focus
solely on CHTs ( " Obsessed with EGT, "
January 2023 AOPA Pilot). Although redline
on your cylinders might be 460 degrees
(Continental) or 500 degrees (Lycoming),
the goal is to keep the CHTs below 400 on
most Continentals and 420 on Lycomings.
Second, forget that 50 degrees rich of
peak EGT is the best mixture to operate.
In a column written many years ago for
AVweb, John Deakin makes a compelling
argument as to why. In it he references
a chart developed by Continental that
plots fuel flow, brake horsepower, EGT,
and CHT of an IO-550 on a test stand
on one graph. Key to understanding the
chart is knowing that the throttle and
rpm were held constant throughout
the test. The only thing that changed
was the fuel flow via the mixture control.
EGT peaks at about 1,520 degrees
Fahrenheit, which corresponded to a
cylinder head temperature of about 410
degrees. Critically, enriching the mixture
to 50 degrees rich increased CHT to its
maximum value, around 425.
The lesson is simple. Because most of
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us historically have had only rudimentary
EGT gauges with which to monitor leaning,
the handbooks described a set value
based on some faulty assumptions and the
spare instrumentation that what was available
at the time. Focusing only on EGTs
ignores the critical nature of CHTs to longevity,
and the chart shows that the former
is definitely not a good proxy for the later.
Third, and thanks again to the graph,
flying lean of peak can make your engine
run cooler and make the top end happier.
CHTs drop off sharply on the lean side
of peak EGT, and while there is a small
horsepower decrease in a normally aspirated
engine, cooler temperatures mean
a longer engine life. If you're new to flying
lean of peak, maybe you're concerned
about flying leaner than the airplane manufacturer
recommended. But remember,
we generally only talk about EGT as part
of the leaning process because that's what
was available at the time the airplane was
made, not because it's the best measurement
of stress on the engine.
Finally, there is no one right way to
lean. Although leaning with an engine
90
AOPA PILOT / February 2023
monitor and properly tuned fuel injectors
is optimal, it's not necessary, according to
Busch. Pilots of carbureted engines can do
it, as can anyone without an engine monitor
or an EGT gauge. If that's you, Busch's
advice is simple: lean to the onset of roughness
and enrich only enough to smooth
it out again.
For those with an engine monitor,
Busch recommends not using the
Lean Find function and instead focusing
on CHTs and fuel flow. Here's how he
describes his personal leaning procedure,
which he used over thousands of hours,
including taking his engines well beyond
200 percent of time between overhauls.
If he wants to go far, Busch leans to a
specific fuel flow he knows will give him
the range he wants. Some horsepower
is lost in a normally aspirated engine
when you go well lean of peak EGT, but
that's an acceptable tradeoff if your goal
is range and cylinder longevity. A GPScoupled
fuel totalizer will do all the math
for you. If Busch wants to go fast, he simply
enriches the mixture until the hottest
cylinder is less than 400 degrees (420 in
a Lycoming).
The experts at GAMI developed a
concept as part of their Advanced Pilot
Seminars presentations on engine operation
called " the red box. " The red box
is a no-go zone where pilots shouldn't
ever operate the airplane for an extended
period. At higher power settings the red
box is large, and at lower power settings-
usually somewhere between 60 and 65
percent power-it goes away completely.
Essentially, the box corresponds to an area
where CHT values will be more than 400
degrees. It doesn't matter if you're climbing,
cruising, or descending, you can lean
to whatever you like so long as it's not in
the red box.
Thanks to the work of the engine manufacturers
and independent testers like
GAMI, we know more than ever what
consequences our leaning process has on
engine health and longevity. Operating
lean of peak doesn't burn valves, and operating
50 degrees rich of peak is often right
in the middle of the red box. The old way
of doing things is not always the best way.
ian.twombly@aopa.org
http://www.aopa.org/financemytraining
AOPA Pilot Magazine - February 2023

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