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CHTs over 400 degrees F

PA28 -181 back …………. starting a new thread as Peter asks for.

I often have to do a steep climb to 5,000 feet to miss a portion of Airspace. Often in the climb, my CHT on the hottest cylinder will hit 420- 430 degrees F

More recently I have been been trying to be more careful to keep below the magic 400 degrees F. but it is not always possible. I change the oil every 25 hours and see no reason why my engine, which goes flying at least once a week, can’t make 3,000 hours. (Non turbo Lycombing A0-360/ Superior Cylinders/ Solid Crankshaft)

At the next Annual I will have a good look at the baffling. Obviously I understand that 460 degrees is getting quite hot but I’ve only been there once or twice. The Lycombing POH manual is 435 degrees in the cruise and a peak is 500 degree F (If I saw 500 F I would be trying not to panic as from memory valves melt at circa 550 F). Also I always cruise at circa 380 or less

So the question is – How bad is 430 degrees? Also is this a linear function from 400-500 degrees? Or more of an exponential function? I am guessing it must be an exponential /logarithmic function so the 480-500 degree range is REALLY bad but perhaps 410-430 is not worth worrying about?

Lycombing only answers for this thread as every engine must be different.

Last Edited by Archer-181 at 13 Oct 20:39
United Kingdom

Has nothing to do with engine manufacturers. It’s simply metallurgy.

Frankfurt (EDFZ, EDFE), Germany

Perfect answer @boscomtico – Thank you. A picture tells a thousand words!

United Kingdom

@Archer-181

Do you know what kind of EGT probe your hottest cylinder has been fitted with?

The hottest cylinder of a Lycoming engine has usually got the factory installed CHT probe installed. I assume that is not the instrument one would be able to get an accuracte reading from

Thereby I would think you have an EGT monitor installed with multiple EGT sensors.
Could it be that you don’t have the piggy back sensor fitted on that cylinder, but instead another type that fits under the spark plug?
If so, you would get a reading that is 50 degrees F hotter, than the factory installed location.

Please check.

It’s an EDM 700 and the probes are all in the correct place. I know as I installed it working with my engineer. The Lycoming CHT gauge was absolutely rubbish and never even used to look at it!

As the fuel gauges are equally rubbish the EDM 700 solved two problems in one go. I agree with a previous poster’s comment (was it Timothy) about it has the world’s worst user interface, but the fuel flow is spot on within 2% on 150 litres of use. The EDM could do with another 4 buttons so they could do dedicated jobs.

United Kingdom

This may or may not be relevant but I believe Bosco’s diagram comes out of a book sold by John Deakin which was written by Pratt & Whitney in the 1930s. I have it here too; I bought it from JD. There are many aluminium alloys and “we” simply don’t know if the cylinders in current Lyco engines use the same alloy.

There does seem to be a consensus that consistent operation above 400F is not a good thing, especially rapid cooling from such high temps.

The EDM user interface is OK; one rarely needs to touch it. I never use any of the fancy features like “lean find”. It would be different if you use it for fuel flow (an option) also, however.

Administrator
Shoreham EGKA, United Kingdom

So there are no sparkplug type sensors fitted?

As the old Piper gauge is required to be connected for certification purposes when an EDM 700 is fitted, it must be a piggy back sensor that is fitted to the hottest cylinder.
In that case it means that the reading usually slightly under reads, so 400F is more like 420F.
At the same time when the piggy back sensor reads say 400F, the aluminium at the spark plug would be 450-470F.

The recommendation is for a normally aspritated engine ito aim for CHT’s under 360F during cruise and for short phases such as climb below 380F.

It would be wise to watch the oil temps carefully too.
Strange things such as sudden detonation can occur in a cylinder when CHT’s are over 450F and the oil temps are say over 220F.
The cylinder will self destruct in 1-2 minutes.
Hot oil and hot CHT’s are both required for the detonation process to begin.

complex-pilot wrote:

The recommendation is for a normally aspritated engine ito aim for CHT’s under 360F during cruise and for short phases such as climb below 380F.

Good general advice but, depending on the engine installation, below 380F in the climb is not really a practical proposition in the real world. Our O360 usually shows CHTs over 400F in the climb (EDM 900, sensors all in the proper places), which can only be kept below 415F or so by parking the nose down, reducing the ROC and improving the cooling airflow. This is perfectly normal and well within Lycoming’s recommendations. If it was not normal, many flight school aircraft would be falling out of the sky

Last Edited by NeilC at 14 Oct 09:32
NeilC
EGPT, LMML

Good general advice but, depending on the engine installation, below 380F in the climb is not really a practical proposition in the real world. Our O360 usually shows CHTs over 400F in the climb (EDM 900, sensors all in the proper places), which can only be kept below 415F or so by parking the nose down, reducing the ROC and improving the cooling airflow. This is perfectly normal and well within Lycoming’s recommendations. If it was not normal, many flight school aircraft would be falling out of the sky Quote

We have enough data available to us these days to debunk such a myths.
CHT’s over 380 is not a good thing and over 400 is to be avoided whenever possible. Not a good thing to call such a poor operating technique standard, or normal.

I have spent time and money to educate myself, as I know many others have done well before me.

Please also invest some time and a little money and build your knowledge, as such a discussion has otherwise no common ground and is therefore pointless.

I can recommend the online APS course as a starting point on that journey.

Last Edited by complex-pilot at 14 Oct 12:27

Not a myth but practical experience. The APS course is excellent and I fly LOP with GAMIs in another aircraft

But in some aircraft you will see +400 in the climb and it just has to be managed sensibly.

NeilC
EGPT, LMML
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