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29 Nov 2021, 01:59 [ UTC - 5; DST ]


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 Post subject: Question about CHTs on C-172 with O320
PostPosted: 22 Oct 2021, 10:43 
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Joined: 04/16/13
Posts: 1850
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Location: NW Oklahoma (6K4)
Aircraft: Bonanza G33
Flew a friend's plane yesterday. Not sure which subset of O320 but it's the 150HP variant in a 1972 C172.
It has a multiprobe CHT gauge with analog display.
In easy cruise (2200 rpm, 2500MSL and slightly leaned) it was showing 360-380 on all cylinders. Not sure how accurate this is but haven't flown a Lyc in years and those didn't have multi probes with the actual values shown.
I was fiddling with the leaning and it never stumbled but I got to a point it slowed the rpm a bit. Is that sort of LOP?
Normal? Problem?
Thanks!

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 Post subject: Re: Question about CHTs on C-172 with O320
PostPosted: 22 Oct 2021, 11:02 
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Joined: 03/24/19
Posts: 657
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Aircraft: Glasair Sportsman
That sounds about right, depending on the CHT probe type.

I personally consider 400F CHT as the top end of my comfortable range for continuous operation. Lycoming lists 500F as the upper limit but we know from experience that above about 425F there's a strong inverse correlation between temperature and cylinder life.

As for leaning, the old style of "lean until it gets a little rough, then richen it up until smooth" is about as good as you're going to get. If you had EGTs on all four cylinders you would likely find the engine is running right around peak EGT, some cylinders perhaps a little LOP, some perhaps ROP a little. This method of leaning gets you best economy, not best power. My O-360 runs quite well in this regime achieved by the same method of leaning.

You're doing it right - fly on!

EDIT: after re-reading your post... if you leaned until it slowed down a little you might have actually found an O-320 which will run a little LOP. Be extra sensitive to "roughness" rather than "stumble". You want your "twitchy butt meter" to be detecting the beginning of misfires which is making vibration - go just a bit rich of that point where you no longer can detect the vibration of misfires and you've likely achieved your most efficient mixture setting.


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 Post subject: Re: Question about CHTs on C-172 with O320
PostPosted: 22 Oct 2021, 12:06 
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Joined: 04/16/13
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Location: NW Oklahoma (6K4)
Aircraft: Bonanza G33
Great info Mark, thanks.
Seemed to run really well but there was a marked slow down that I was able to reverse by adding back the fuel. It has the 'clicker' mixture knob of course, not a vernier.

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