Considerable information has been published about this on the BAC website,
available to all members with a search on 'mixture settings (leaning)'.
GAMI has done more research on this than practically anyone else, including
some of the manufacturers themselves. You can visit the GAMI website, and
you can read the Pelican's Perch series on AvWeb.

I am personally convinced that running in the range of 30-40 degrees Rich Of
Peak (ROP) is the absolute hardest point at which to operate the engine.
The pressure peaks the fastest, and at a time when rod angle is not optimum
for leverage. The result is continuous operation under circumstances that
are just shy of being called detonation (in terms of pressure spikes and
impact on parts). My personal four conclusions are that:

(1) On an unsupercharged engine, you cannot get the EGT 'too high'. You
CAN get the CHT too high. CHT is the driving force in engine management,
for our engines. Peak CHT will occur in the 30-40 degrees ROP range, for
the reason stated above (pressure spikes, fast burn times). You can manage
CHT with added fuel, added internal air (leaning), added external air
(airspeed and angle of attack), and power output level. All have a role to
play, under different circumstances. And you can spend a great deal of
money by always blindly following the OWT's, using only excess fuel for

(2) If you need to compromise, peak EGT is a good way to go, as long as you
can ensure that all cylinders are peaked, leaving none operating in the peak
pressure range. It will give closer to maximum power without the pressure
spikes, without the roughness often associated with Lean Of Peak (LOP)
operation on carbureted engines, and without the higher fuel consumption of
operating significantly ROP.

(3) LOP operation will definitely give the lowest CHTs, the lowest EGTs,
and the lowest fuel consumption (for many reasons). It is difficult to
achieve true LOP operation on the carbureted engines, though some seem to
come quite close when flying high and at full throttle. Sometimes a tad of
carb heat also helps. To run LOP, you need to be able to verify that every
cylinder has 'gone over to the lean side of peak'. That means all-cylinder
EGT-CHT capability. The much more efficient operation comes at the expense
of power output. There is a significant loss of power during LOP operation,
and with our engines we cannot compensate by adding back some manifold
pressure from a turbo. During LOP operation, you are providing internal
cooling with excess air, regardless of external airspeed and cooling. For
any given power level and conditions, in my experience LOP always results in
the lowest EGT and CHT. During LOP operations you can also use fuel flow to
directly calculate power output, regardless of other engine settings. Every
bit of fuel is being efficiently burned, and the pounds per hour are
directly calculable as HP.

(4) Running at least 100-125 degrees ROP will give the highest safe power
levels without the spikes, and will give CHTs in the normal range at those
power settings (assuming normal airspeeds, etc.). It will also result in
the highest fuel consumption, as you are doing internal cooling using excess
fuel flow instead of excess air.

Many people have witnessed all this for themselves, during the GAMI engine
management classes. Their instrumentation enables the attendees to see
exactly what is happening inside the engine, as different parameters are

As an aside, virtually everyone has heard the old stories about how running
too lean can cause overheating and detonation. I am convinced that the
problems in question were not caused by being lean at all. They occurred
when engines were normally operated full rich, or close to it. If someone
experimented by leaning
Down a bit, they moved the operation into the peak spike range that lies
just rich of peak EGT. They were 'leaner than full rich', but were never
actually lean at all. Once the ratio passes the optimum (stoichometric?)
air-fuel ratio, burn rate slows down; just like it does when you go rich of
peak. A 'real' (LOP) lean mixture burns slower and cooler, not hotter and
faster. As one example, automotive ignitions have had variable spark
advance for two generations, so that either higher vacuum or electronics and
sensors could advance the spark with a lean mix. The earlier spark helps
compensate for the slow burn rate. Guess what one of the characteristics of
the GAMI PRISM system will be, once they have it finalized.


From: []
On Behalf Of Dennis McGoldrick
Sent: Tuesday, May 16, 2006 1:07 PM
Subject: [musketeermail] Re: egt


If you read the poh, leaning to 25 degrees below the
highest temperature reached on your egt gives the
maximum use of the fuel in cruise.
Leaning to 75 degrees (from memory, the poh is in the
plane) below max temperature is max power.

However, there was an article about a piper twin in
Austraila a few years ago with two io360's, short on
fuel. The pilot used the 25 degrees setting to max
his fuel usage (per the poh) and burned holes through
two pistons. The pilot made a forced landing in the

One of the mechanics on the list might comment, but
after reading the Piper article, I started leaning to
125 degrees below max for cruise. This uses more
fuel, but puts the engine at a long-life operating

dennis N3691Q a23-24

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