EGT and CHT

Hi Emmanuel –

As Steve mentions, the absolute number should always be taken with a grain of salt. And if you are getting one number each for EGT and CHT, you can only treat this is as a general guess for what the remaining cylinders are doing. Over time you will likely see a pattern develop, then when the temperatures depart from this pattern, it’s time to be concerned (or check to see if you have leaned at high altitude and forgotten to enrichen as you descend!).

Having said all that, I like to look at the manufacturer’s operating recommendations.

I believe this is Lycoming’s operator’s manual for that family of engines, including the IO-360-C:

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO%20%26%20TIO-360%20Oper%20Manual%2060297-12.pdf

The operator’s manuals can be wordy and confusing, because they cover a number of different engines. But I find it interesting to see what the manufacturer recommends.

Here’s a few snippets I found – before using this guidance you should have a look yourself:

Section 3 – Operating Instructions – General Rules

“For maximum service life, cylinder head temperatures should be maintained below 435°F (224°C) during high performance cruise operation and below 400°F (205°C) for economy cruise powers.”

“LEANING TO EXHAUST GAS TEMPERATURE GAGE.
a. Normally aspirated engines with fuel injectors or uncompensated carburetors.
(1) Maximum Power Cruise (approximately 75% power) – Never lean beyond 150°F on rich side of
peak EGT unless aircraft operator’s manual shows otherwise. Monitor cylinder head
temperatures.
(2) Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT.”

There is also guidance for oil temperature on page 3-11 – for ambient air temperatures above 30 degrees F, the “desired” oil temperature is 180 degrees F, and the max is 245 F.

Boiling it all down, if it were me, I’d start by operating conservatively and recording some data:

– Can you takeoff at full power (and full rich) and keep CHT below roughly 400? If not, I’d be visiting my shop to discuss the situation with them, perhaps there is an engine cooling issue.

– For cruise, try operating at 65% power (2350RPM, per page 3-13), and leaning to peak EGT. How many gallons per hour are you burning (do you have a fuel flow gauge?). From the book, it should be about 9.5 gallons per hour. What happens to EGT if you enrichen to 10 gallons per hour? I’d be tempted to operate slightly rich of peak in exchange for slightly cooler temperatures and possibly longer engine life, but that’s just me, and opinions vary.

– What is your nominal oil temperature and pressure during takeoff and cruise? Record these values, then you’ll have a heads-up if they change dramatically one day.

-Frank.

Hi Emmanuel,
The EGT value as a number is not very important due to many factors–distance the probe was installed from the cylinder exhaust flange, orientation of the hole relative to the exhaust port, etc. The important factor about EGT is the actual number that signal peak EGT because that peak number (it’s different for each cylinder due to many factors) is a marker used to successfully lean the fuel air mixture.
In a nutshell, all leaning is done off the FIRST cylinder to peak. This may not be the cylinder with the highest EGT number.
The common thought on CHTs is that 400 deg or lower are the desired max in cruise. The numbers may creep slightly above 400 during climb or during high air temperature operations.
Best,
Steve