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Running Lean of Peak EGT

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Running Lean of Peak EGT    1 year 5 months ago#433

·         Paul

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Everyone seems to agree that at full throttle, the mixture must be very rich, approx 11.5:1 Air to Fuel ratio. But at cruise power there are 2 schools of thought. One side says to run approx 50 degrees rich of peak EGT, the other side says it's better to run lean of peak EGT by about the same EGT.

The data I've seen for cruising lean of peak is pretty convincing. The benefits include running cleaner, less fuel use, less peak cylinder pressure, lower CHTs, and combustion event happening later in the crank shaft rotation. So, as long as the cylinder head temps are good, and the engine is running smooth, running lean of peak sound good to me.

There is data supporting running lean for Lycoming engines, but I haven't heard about any Jabiru engines running lean of peak for any length of time.

Anyone doing this. Has Jabiru tested this?

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Running Lean of Peak EGT1 year 4 months ago#440

·         Doug Smith


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Hi Paul,
Lean of peak is not, in principle, a bad thing and I'm sure there are folks out there in Jabiru-land that have done it (though I'm not aware of any directly).

The problem as I see it is that we're working with the limitations of the altitude compensating carburettor which has been set up as a best compromise to suit the Jabiru engine(s) in various different airframe installations, running a variety of different propellers, intake and exhaust configurations. For what it is, it does a great job, but it's not really clever enough to reliably identify when engine loads are low enough to safely run lean and then adjust itself to suit. In theory, with time, care and the right instrumentation you could customise a needle shape to suit your particular engine, propeller and airframe combination and get it close to the mark but you'd be trying to get an abacus to do ipad-level calculations so there will be a limit to how far you could drag it down that path.


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After adding a hacman mixture adjusting needle valve you have better control of the mixture and can accurately obtain lean of peak mixture when cruising at altitude and still have full rich full power for climbs.

What are the recommended EGT temps to operate at?

my 3300 peaks at around 750 deg c at peak temp is reached