Cyl/Exhaust Temps

How to keep the Cessna 170 flying and airworthy.

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n2582d
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Re: Cyl/Exhaust Temps

Post by n2582d »

bgiesbrecht wrote:... For the 170, it should be the lower plug, #2 cylinder. ...
I'm of the opinion that the CHT probe should be on the hottest cylinder. Looking at the history of Cessna Accessory Kits it seems that this cylinder was rather amorphous. Here's a screen shot from AK172-16.
Click to Enlarge
Click to Enlarge
What's weird is that one would think the change in cylinder location between #3 and #2 would correspond with the change from the "doghouse" style engine baffling to the later style baffling without the "doghouse". The change in baffling happened between the 1952 and 1953 models at s/n 25372. But this Accessory Kit calls for moving the CHT probe from the #3 cylinder to the #2 cylinder location at at the end of the 1954 model year (s/n 26504). On the early C-172's AK172-27 uses cylinder #2 for the probe but later, on the 1961-62 C-172 models, in AK172-41B Cessna spells out cylinder #6 for attaching the CHT probe.
Gary
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GAHorn
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Re: Cyl/Exhaust Temps

Post by GAHorn »

The problem with Accessory Kits is that they may not consider all the factors involved. Accessory Kits sometimes take “short cuts” purely for ease of access or to accommodate assembly/disassembly issues on equipment previously installed.

The TCM Overhaul Manual addressed the engine and only the engine by the engine mfr’r. For that reason I prefer to follow that guidance. (The AK172-16 was also cancelled.) I agree with you Gary…it’s curious that the instructions don’t seem to follow the associated cowling… I am of the opinion it’s because the engine mfr’r disagreed.

It may be helpful to remember that CHT gauges are only required equipment on aircraft which have cowl flaps. (certification requirements) The airframe mfr’r had to meet the limitations of the engine mfr’r when the airplane was offered for airworthiness certificate. So it might follow that if the engine mfr’r stipulates the CHT probe should be on No2…then the airframe mfr’r should not countermand that.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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4583C
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Re: Cyl/Exhaust Temps

Post by 4583C »

The strangest things thing is in the last sentence of what Gary posted where it says to Move the CHT to number 6. 5&6 are in the vicinity of 100 degrees cooler on my engine than the other four. I guess with a single point CHT that would cure the problem of excessive cylinder temps. :cry:
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n2582d
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Re: Cyl/Exhaust Temps

Post by n2582d »

The only thing I find regarding CHT probe location from the Continental Overhaul Manual is on page 35: "To assure that the specified maximum cylinder temperature will not be exceeded: Install a spark plug gasket type thermocouple in place of the regular gasket under the down-stream spark plug of the hottest cylinder. Locate the cylinder by experiment."

Baffling (no pun intended) why the specified location changed as it did. Here's a similar issue from Service Letter SL75-12 regarding the position of the sensor on the C-206:
Click to Enlarge
Click to Enlarge
Gary
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GAHorn
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Re: Cyl/Exhaust Temps

Post by GAHorn »

But Gary, the attachment you posted for a C-206 would be for an IO-520 Engine (and which used an entirely different type probe.)

See Table VIII of the O-300 Overhaul Manual, Section 2, Page 5. (link below)

download/file.php?id=17100&mode=view
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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cessna170bdriver
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Re: Cyl/Exhaust Temps

Post by cessna170bdriver »

I thought the “normal” location for a cylinder head thermocouple was under an upper spark plug. I don’t recall seeing them on lower plugs, but I haven’t actually seen very many. My ECI cylinders have a bayonet connection for CHT probes, and the instructions say to lower the redline by 25 degrees, indicating to me that the spark plug location is that much hotter than the bayonet location.
Miles

“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
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bgiesbrecht
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Re: Cyl/Exhaust Temps

Post by bgiesbrecht »

GAHorn wrote:
bgiesbrecht wrote:
9976AJds wrote:I have read articles and told by AP/IA's on updraft engines the CHT Probe should be on a top spark plug, mine is on No #6 Top. Appreciate any tips on Oil Additive, Cam Lube for LYC, how about AV Lube for Cont? Marvel Mystery in Fuel? Thanks JD Smith
I realize this is an old thread, but found in a search as I'm exploring putting in an engine monitor. At any rate, our birds (170) are downdraft cooling. The Globe Swifts - another bird commonly found with the O-300 - uses updraft cooling and there your approach would be correct. For the 170, it should be the lower plug, #2 cylinder.

Also, the spark plug gasket CHT probes can read 25-50 degrees hotter due to their location next to the plug than if you had a "normal" CHT probe in the cylinder. Therefore, if you see a reading of 375 degrees, it is likely the "actual" CHT is closer to 325-350 (that is, the temp you'd see if you had a normal CHT probe).
We are discussing Cylinder HEAD Temperatures.
Continental specifies the use of a spark plug gasket type CHT probe. (the only other cylinder temp specified is a cyl base temp…not ordinarily instrumented except in test-cells.) From the X30013 Overhaul Manual:
947C9AA9-05F3-472C-BEBD-41AC05FA51E4.jpeg
Yes. What did I say that was different?
Former owner of:
1953 170B
N1977C
s/n 26122
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GAHorn
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Re: Cyl/Exhaust Temps

Post by GAHorn »

bgiesbrecht wrote:
GAHorn wrote:
bgiesbrecht wrote:

…. the spark plug gasket CHT probes can read 25-50 degrees hotter due to their location next to the plug than if you had a "normal" CHT probe in the cylinder. Therefore, if you see a reading of 375 degrees, it is likely the "actual" CHT is closer to 325-350 (that is, the temp you'd see if you had a normal CHT probe).
We are discussing Cylinder HEAD Temperatures.
Continental specifies the use of a spark plug gasket type CHT probe. (the only other cylinder temp specified is a cyl base temp…not ordinarily instrumented except in test-cells.) From the X30013 Overhaul Manual:
947C9AA9-05F3-472C-BEBD-41AC05FA51E4.jpeg
Yes. What did I say that was different?
I was only emphasizing the central point of the topic …because the discussion appeared to drift into variances based upon probe type & position. There is no allowance in the TCM manual for a differing CHT-limitation using a different type probe or in a different location… It seems (to me) to be inappropriate to lead folks into allowing a 25-50-degree variance from TCM specification, and the Table I posted also included data on cyl base temps which I didn’t want to become distracting. Also, the reference to a “normal” probe might confuse…but I believe you are considering the gasket-probe to be “normal”. (No other inference to your posting….no intent to criticize.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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bgiesbrecht
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Re: Cyl/Exhaust Temps

Post by bgiesbrecht »

GAHorn wrote:
I was only emphasizing the central point of the topic …because the discussion appeared to drift into variances based upon probe type & position. There is no allowance in the TCM manual for a differing CHT-limitation using a different type probe or in a different location… It seems (to me) to be inappropriate to lead folks into allowing a 25-50-degree variance from TCM specification, and the Table I posted also included data on cyl base temps which I didn’t want to become distracting. Also, the reference to a “normal” probe might confuse…but I believe you are considering the gasket-probe to be “normal”. (No other inference to your posting….no intent to criticize.)
Ahhh, understood! Agreed - important clarifications.
Former owner of:
1953 170B
N1977C
s/n 26122
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