Ask Paul: What is the best EGT?

Question for Paul McBride, the General Aviation News engines expert: Hey Paul, I have a 1964 Mooney M20C carbureted. What is the best EGT on my cruise at 24 inches times 2400 RPM prop?

David Woodall

Paul’s Answer: I will begin by throwing out another reminder to our readers that it really helps if more specific information is provided with all of your questions.

I took a wild guess that the engine in your 1964 Mooney M20C is a Lycoming O-360-A1D and I’ve based my response to your question on that guess.

Yours is not an easy question to answer because of the distribution of fuel on an engine using a float-type carburetor, but hopefully I can offer some thoughts on the subject that will assist you in operating your engine.

Let me begin by telling you a very simple rule when it comes to leaning.

It’s important to remember that Lycoming recommends that you should only lean the engine when you are at 75% power or less.

Leaning an engine that incorporates a carburetor is pretty straight forward, but you must keep in mind that the Lycoming recommendations with regard to leaning are based on calibrated instrumentation.

I think we can all agree that the accuracy of instruments has been known to decline over time. This not only goes for the instrument, but also the EGT probe.

If you’re confident in the accuracy of your instrumentation, then we can move forward.

Once you’ve established your cruise altitude and are not exceeding 75% power, you may begin to lean.

Slowly leaning, at some point, will cause a slight engine roughness that indicates the leanest cylinder has become so lean that it’s beginning to miss. This is also a good indicator that you are near the peak EGT point.

No damage will occur to the engine provided you are at the recommended cruise power setting.

Keep in mind that this is typical of an engine with a float-type carburetor.

At this point you should enrich the mixture for smooth engine operation.

You must remember that the peak EGT on the gauge will also vary with different power settings, changes in altitude, and ambient temperature.

David, I think you’ll probably notice by now that I haven’t provided you with a “best EGT temperature” which you requested.

I’m going to let you do some experimenting on your own, using the information mentioned here, but I think you’ll find that your engine will probably operate smoother at +25 to +50 degrees rich of peak EGT. This is fairly common with engines using a float-type carburetor.