CHT and EGT Settings

J

JackFulford

New member
Hi all,
I am looking for suggestions on where to set the Max/Min values for CHT and EGT on the engine analyzer for the O-470 powered 7655B. A person who shall remain nameless (i.e. me) managed to wipe out the settings that were in there originally. A call to Continental hooked me up with a guy that said max 460F for the CHT. He hemmed/hawed around and eventually suggested 300F to 330F for the minimum. He refused to even venture a guess on the EGT. Anything you guys can suggest is greatly appreciated.

Jack
 
JackFulford said:
Hi all,
I am looking for suggestions on where to set the Max/Min values for CHT and EGT on the engine analyzer for the O-470 powered 7655B. A person who shall remain nameless (i.e. me) managed to wipe out the settings that were in there originally. A call to Continental hooked me up with a guy that said max 460F for the CHT. He hemmed/hawed around and eventually suggested 300F to 330F for the minimum. He refused to even venture a guess on the EGT. Anything you guys can suggest is greatly appreciated.

Jack
Click to expand...
Hi Jack. Some general comments.
Though I have an A&P cert and done research re combustion and fuels, like the Continental rep, I don't know either. The last word for max oil and cyl head temp is the engine Type Certificate Data Sheet. For your O470, its probably 460 F like the man said. I have seen minimum oil temp for takeoff in some refs, maybe a POH or engine TCDS, but I didn't notice any min temps stated in your O470 TCDS.
EGT is not mentioned in the specs at all. EGT should have a max reference established by, say, what temps your setup produces by mixture peaking the EGTs at high cruise. Variables would include probe location and outside air temp. OAT rises by 10F, the EGT would theoretically also rise by 10F. Again, no real min temps.

If the result of the analyzer detecting an out-of-limit is an alarm of some sort, I would suggest making the normal ranges quite wide to avoid distractions. Unnecessary distractions can be unsafe if you are IMC or LAX busy. And besides, who wants a bitchy airplane anyway? :)
 
Thanks much for the quick response! I will probably set the CHT at 300F min and 470F max. For the EGT, I am thinking 0F for the min and maybe 1800 to 2000F for the max. I agree with you completely regarding the distraction of the flashing red LED. It is enought to drive you nuts (which I guess is the idea) when it is false alarm. I finally just put it in a mode that does not "alarm"
 
Maybe I'm nuts, but I think there is actually a theoretically correct EGT based on optimum stochiometric mixture ( this involves counting electrons in the moleclues of oxy and fuel).

Metalurgy and murphy enter into the equation.. but I'm thinking you should be looking for around 1600-1700 degrees for normal operation.. and 2000 degrees is WAY into the red zone .

Running a bit rich is part of the normal cooling of an aircooled engine.... and that knowledge should influence what you think of as "normal" or optimum, and also colors what acceptably leaning looks like.

Different engineering offers up different correct answers.

If you have ever seen badly burned (think burned away) exhaust valves.. or a sucked valve you will recongize the value of erring on the side of caution.

there are schools of thought about which side of lean you "should" operate on...
with maybe as many right answers as there are situations... climb, descend, high alt. cruise,
and what engine and fuel.

the advice you got to find peak lean temp.. and set your alert or normal 'red line' very close to that
makes good practical sense. 100 or 200 degrees above that is maybe too much.....
 
Check out the December Sport Aviation. Mike Busch wrote a great article on EGT/CHT. He also has a great seminar available from EAA website. You might also just want to go to the source, and call the engr dept of Continental, and see what you can get.

Aerospace Logic told me that the numbers they suggested for my Franklin were:
Low Level 1250°F
Normal 1430°F-1480°F
Red lights start1575°F (warning)
All lights (high)1625°F (high temp)
 
"there are schools of thought about which side of lean you "should" operate on...
with maybe as many right answers as there are situations... climb, descend, high alt. cruise,
and what engine and fuel."

There is only one school of thought with carb engines.. Dont even try to run lean of peak. Leave that to FI engines with known balanced injectors.
That is, unless you live is SoCal, I make a lot of money on carb'd engines where the owner read the kool-aid from unknowing owners trying to run their Carbs LOP.
I dont always agree with Mike Busch, but his manifesto on LOP is gold.
As far as temp limits go...
Page 19 in your manual has really good guidance for setting your alarms.
I have my CHT MAX set for 425. Sounds too low? Under climb power, my IO-470 rarely exceeds it so if it does, chances are something is wrong. Could be as simple as my mixture isnt set right but I'd rather be nagged than find out later I hurt something. I set CHT MIN at 250 - if the fire goes out in a single cylinder you'll be notified.
EGT MAX on a carb engine depends on you're particular plane. Get stable in cruise, sneak the mixture lean until it peaks and set your MAX alarm there. Think of it as a "do not cross line". If the alarm goes off you should pay attention and get back rich of the line.
I cant remember if 55B has the carb temp hooked up, that would be awesome.
Then again, you could just turn all the alarms off and fly old-school. Seems kind of silly to me though, since you have them already.
I cant recall for certain but I *think* your UB-8A does not have an alarm cancel feature. If it doesnt, you could add a master caution with an alarm silence feature to temporarily silence an external alarm when appropriate - if you find alarms a nuisance.

I put a UBG-16 in my plane with a voice annunciator. Once got a "Check Temperature" alert in my ear and discovered it was complaining about the cabin temperature! Seems my wife read the manual and set it for 60 so, she'd know when to turn on the heat... what a woman :)
 
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