High CHTs

How to keep the Cessna 170 flying and airworthy.

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High CHTs

Postby abushey123 » Tue Jun 21, 2022 6:34 am

Hello everyone. Running into some slight issues with my new to me motor. I recently I stalled an o300d model on my 170b. Motor is stock and has 400ish hours on new cylinders on a overhaul. Took it flying today for the first time and pretty quickly had the cht right at 525. It was about 65 outside and 800ft density alt. I checked to make sure there wasn’t anything crazy but didn’t see anything off hand. Baffling is a little tired but is seems to be air tight. I put a new gauge and cht probe in it recently so I don’t think that’s an issue. Im just looking for ideas as far as where to start. Motor runs fine and doesn’t diesel or ping even when it’s that hot.
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Re: High CHTs

Postby GAHorn » Tue Jun 21, 2022 12:58 pm

What type CHT system do you have installed…(spark plug thermocouple..?) …on what cylinder, and spark plug position…??

525-F is the limit (as you probably know) but to help reassure you.. if your baffling is in good condition, … since no adjustable cowl flaps are installed on this airplane, there is no requirement for a CHT gauge… meaning that it’s unlikely the engine installation (if proper) can exceed it’s operating limits.

I.E., new gauge installations ( replacing identical installations ) that read extraordinary would be suspect in my first thoughts. Depending upon quality, etc., a new gauge may not be calibrated…. just because it’s “new” doesn’t mean it’s “calibrated”.

Can you give a bit more info on the above?
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Re: High CHTs

Postby abushey123 » Tue Jun 21, 2022 2:35 pm

George, I installed a alcor cht gauge and probe that is installed on the top spark plug on cylinder 2. Reading through the papers that come with the gauge it does say that it’s calibrated at 500 deg so the thermocouple would read 8ohms. I’d be very surprised if the gauge was not accurate. The only thing that might be of interest is the baffle baffling on the motor has 172 o300 inner cylinder pieces while the rest is 52 and prior 170 stuff.
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Re: High CHTs

Postby GAHorn » Tue Jun 21, 2022 2:43 pm

Well, you are certainly correct to suspect the baffling condition, etc. Do you still have the “box” within the cowling which ‘52 and prior 170’s came with?

The Cessna installation was on the lower spark plug…but I would guess that might actually be hotter than the top… (don’t actually know that…just a suspicion.) Is your gauge-face actually calibrated/placarded in degrees F..?? or is it a scale of indexes with an (*) asterisk placed over one of the index-marks…??

Do you have items 59, 60, 61, and 64…?
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Re: High CHTs

Postby abushey123 » Tue Jun 21, 2022 3:47 pm

When I got the d model it didn’t have 1-7 installed. Rather it had the inner cylinder baffling found in the late model as seen in the second picture.
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Re: High CHTs

Postby abushey123 » Tue Jun 21, 2022 3:47 pm

Also for some reason it’s not letting me upload pictures but in the c170b manual on page 93 it shows which parts I’m talking about.
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Re: High CHTs

Postby GAHorn » Tue Jun 21, 2022 4:04 pm

abushey123 wrote:When I got the d model it didn’t have 1-7 installed. Rather it had the inner cylinder baffling found in the late model as seen in the second picture.


abushey123 wrote:Also for some reason it’s not letting me upload pictures but in the c170b manual on page 93 it shows which parts I’m talking about.


Pg 93 is a listing only….not a picture….so have no idea what you mean….but mixing and matching different cowlings from different installations is not going to be the best solution.

2948CE5B-24D3-4E90-BF08-E6CEC11AAE2D.png


0D94A1AE-F1F5-4211-BDB4-09DBC55C7E3D.png
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Re: High CHTs

Postby c170b53 » Tue Jun 21, 2022 4:46 pm

Maybe take a picture, or a few of the baffling. For pictures, they to be formatted to fit the size requirements of our site. If the picture is too large it will not be uploaded. Easiest way to resize is send the picture in an email to yourself, save it in your photos and rename it, then select the file size you want to save as in small, medium or really big and crop the picture to fit into the size that can be loaded. Then most of the time, everyone will jump at looking at pictures
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Re: High CHTs

Postby abushey123 » Tue Jun 21, 2022 10:57 pm

I have 33-36 installed from the late b model and the rest of it is all early model. While mixing and matching baffling may lead to some things, I suspect the inner cylinder baffling, seeing for all o300 it purpose is the same, should not affect CHT by 100 deg. It might be worth noting that the plugs were a light to very light brown suggesting it may be running a hair on the rich side. With this being said, I am curious if there is a way to richen the motor on the top end even after the mixture control is all the way at the stop. I am getting plenty of fuel flow to the carb and the carb is a recent overhaul.
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Re: High CHTs

Postby GAHorn » Wed Jun 22, 2022 2:03 am

If this were a mixture issue…then other cyls should also display high temps. It would be necessary to instrument all cyls to determine that.

I am suspicious of this CHT gauge firstly, …and THEN would search for other answers to the question. Have you an Infrared thermometer with which you can make a comparison? …or another identical CHT gauge to install on an adjacent cyl…?? … or failing that, have you confirmed the present gauge is actually calibrated AS DESCRIBED in the document earlier mentioned? (I.E., I do not trust this gauge.)
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Re: High CHTs

Postby abushey123 » Wed Jun 22, 2022 4:25 am

I have a few other gauges I’m going to try tomorrow along with installing an egt gauge in the same cylinder. Will report back.
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Re: High CHTs

Postby GAHorn » Wed Jun 22, 2022 11:50 am

While only an unproven personal theory…. I believe the reason Cessna decided the hottest location to monitor is the lower position of #2… is because of the clockwise rotation of that engine/propeller… which puts a clockwise spiral airflow into the cowl. That airflow likely pumps more cooling air into the right bank of cylinders (as the left bank will receive an upward-flow while the right bank receives a rearward/downward flow) similar to the phenomena of “P-factor”, where the “downswing” side produces more thrust than the upswing side of the prop.
The #2 being the farthest aft presumably receives warmer air (it having passed over other cyls beforehand) and the lower spark plug being shielded by the cylinder… probably runs hottest.

That’s how my imagination works anyway.
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Re: High CHTs

Postby cessna170bdriver » Thu Jun 23, 2022 2:49 am

My #2 CHT (late B-model baffling) is also the hottest (measured with a UBG-16 6-cyl CHT/EGT system) but not by very much. The way my imagination works (at least on the later B models) is to notice that the cabin air drawn off the rear baffle isn’t warm, meaning the air passing over the top of the cylinders picks up very little if any heat on the way back. My theory is that as each cylinder draws air off, there is a pressure drop, so the rear cylinder has the least pressure forcing air down around it.
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Re: High CHTs

Postby abushey123 » Fri Jun 24, 2022 7:00 pm

Alright. I think I've narrowed it down to incorrect gauge installation. I had my doubts from the start seeing the engine did not exabit any characteristics of over heating such as dieseling or pinging in flight. I replaced the gauge with a known good one and it again read 525. I unfortunately did not have a replacement probe but when running at wot on the ground and the gauge reading around 500 I was able to take a reading right on the head next to the spark plug with my IR gun and the most I ever saw was just shy of 325. After reading the instillation manual for the ALCOR CHT gauge and probe, (Shame on me for not reading the first time) It says the gauge is calibrated to a specific length lead set which I did not use as the gauge and probe did not come with it. I will be contacting my local avionics shop today in hopes they have one!!
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Re: High CHTs

Postby DaveF » Fri Jun 24, 2022 7:37 pm

Also make sure you’re using the correct probe type for the meter. A J-type thermocouple will give high readings if used with a gauge that expects a K-type.
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