incorrectly that a flat-topped graphic engine monitor display (with
all EGTs equal) is the mark of a well-balanced engine, and that
unequal EGTs are a sign that something is wrong. This common
misconception tends to be reinforced by digital engine monitors that
display a digital “DIFF” showing the difference between the highest
and lowest EGT indication.
As illustrated by the earlier anecdote about the front cylinders on
my Cessna T310R, differences between absolute EGT values are both
normal and benign. It is not uncommon for well-balanced fuel-injected engines to exhibit EGT spreads of 100 degrees, and
carbureted engines often have spreads of 150 degrees or more. In
fact, as shown in Figure 4, EGT spreads are usually smallest near or
just rich of peak EGT (the worst place to operate the engine), and
often significantly greater at leaner or richer mixtures (that are
much kinder to the engine).
The mark of a well-balanced engine is not a small EGT spread
(“DIFF”), but rather a small “GAMI spread”—defined as the
difference in fuel flows at which the various cylinders reach peak
EGT. Ideally, we would like to see this spread be no more than about
0.5 gph (or 3 pph). Experience shows that if the GAMI spread is
much more than that, the engine is unlikely to run smoothly with
LOP mixtures.
IT’S ALL RELATIVE
Al Hundere had it right after all: The only
important thing about EGT is its relative
value: how far below peak EGT and in which
direction (e.g., 100 degrees ROP or 50
degrees LOP). Absolute values of EGT (e.g.,
1,475 degrees) are simply not meaningful
and are best ignored. There is no such thing
as a maximum EGT limit or redline, and trying to keep absolute EGTs below some
particular value—or even worse, leaning to a
particular absolute EGT value—is simply
wrongheaded. Don’t do it. If you must fixate
on those digital engine monitor readouts,
fixate on something important, like CHT.
Mike Busch, has been a pilot for 44 years, logging more
than 7,000 hours. He’s a certificated flight instructor and an
airframe and powerplant mechanic with inspection authorization. Log in to join Mike for a webinar titled “Say No to
Useless Maintenance” on November 9 at 7 p.m. CDT at
www.EAA.org/webinars.
