AOPA Pilot Magazine - October 2021 - 91

hotter than usual for the first few hours
until break-in is complete.
Also, while we don't want CHTs to
be too hot, we don't want them to be too
cool, either. If CHTs get cool enough,
there may be lead scavenging issues that
cause excessive deposit buildup in the
combustion chamber and particularly on
exhaust valve stems, resulting in valve
sticking issues (especially in Lycomings).
For this reason, I suggest your mental CHT
green arc extend downward to about 350
degrees F for Lycomings or 330 degrees F
for Continentals, with a yellow arc below.
That'll keep you in the sweet spot.
Keep in mind there's nothing magic
Continental head-to-barrel separation.
cooler than optimal. I'm not sure where he
got the idea that he should limit his CHTs
to that extreme.
With some exceptions, I like to see
CHTs no greater than about 400 degrees
F for Continental engines and 420 degrees
F for Lycomings to avoid stresses that may
be detrimental to engine and cylinder
longevity. Statistical studies from my
company's database of engine monitor
data show that Lycoming CHTs run about
20 degrees F hotter than Continental
CHTs, and for good engineering reasons-
chiefly Lycoming's use of sodium-filled
exhaust valves that transfer heat from
the valve to the head more efficiently than
the solid-stem valves Continental uses.
Lycoming cylinders are built to take this
additional heat, with a more robust headto-barrel
junction.
This is reflected in Lycoming's higher
redline CHT of 500 degrees F compared
with Continental's redline of 460 degrees
F. It's important to understand that
these CHT redlines should be treated as
emergency values only. We never want
to let our CHTs get anywhere near that
hot. (I once reviewed engine monitor
data from a Continental-powered Cirrus
SR22 whose pilot allowed one cylinder's
CHT to slowly rise to 466 degrees F-just
6 degrees F above redline-at which point
the cylinder's head abruptly separated
from the barrel and the pilot found himself
flying behind a five-cylinder engine that
was running rough as a cob.)
Personally, I like to leave myself a little
cushion, so I'm inclined to set CHT targets
of about 380 degrees F for Continentals
and 400 degrees F for Lycomings. These
aren't not-to-exceed values, they're just
comfortable targets. A good way to think
of them is to imagine a CHT gauge with
a green arc that tops out at these targets,
followed by a yellow arc that extends 20
degrees F higher and terminates with a
redline. This is different from the actual
markings on your CHT gauge, which
probably has a green arc extending all the
way up to the manufacturer's redline of
460 or 500 degrees F, but it's good mental
image to use to ensure maximum engine
and cylinder longevity.
These numbers should be adjusted for
some special situations. They should be
adjusted downward by 20 degrees F or so
if you're flying in unusually cold outside air
temperatures (below ISA), and also if your
airplane has an extraordinarily efficient
cooling system (e.g., Cirrus, Columbia,
Diamond). Also, the not-to-exceed values
should be increased by about 20 degrees
F when breaking in new or newly honed
cylinders because CHTs naturally run
about any of these numbers. Nothing
terrible will happen if a CHT runs in the
" mental yellow arc " or if there a brief
exceedances above the " mental redline. "
I'm offering them only as suggestions, not
commandments or operating limitations.
Cures for hot cylinders
You ought to be able to keep your CHTs in
the sweet spot if you're paying attention.
But what if you can't? In that case, you
need to diagnose why your cylinders are
running too hot. There are three common
reasons for high CHTs, and your mission
(if you accept it) is to figure out which one
is the culprit.
One common reason is that the toohot
cylinder is running too lean. If it's a
fuel-injected engine, perhaps there's a
partially clogged fuel nozzle; if the engine
is carbureted, maybe there's an induction
system leak. This issue is super easy to
diagnose. Simply do a test flight in which
you first cruise with a rich-of-peak (ROP)
mixture and then switch to a lean-of-peak
(LOP) mixture. If a cylinder is running
too lean, it'll have higher-than-normal
CHT during ROP operation and lowerthen-normal
CHT during LOP operation.
Frequently such a cylinder will " switch
rank " from being the hottest cylinder when
ROP to being the coolest cylinder when
LOP. If you see this, you can be sure the
cylinder in question has a mixture issue.
A second common reason for a hotrunning
cylinder is that there isn't
sufficient cooling air passing over the
cylinder's cooling fins. That's generally
due to some issue with the engine's rigid
AOPA PILOT / October 2021
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AOPA Pilot Magazine - October 2021

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