Hi EuroGA, this is my first time posting here, although I’ve been a long time reader and have learnt a lot from you, thank you for that.
I tried searching for this but while I found some leaning threads of course, I didn’t find a definitive answer to my question here. Please point me to it if this has been discussed before.
I’m not fortunate enough yet to be able to own, but I rent a lovely C182P with a Continental O-470 and a JPI 930 which gives me per-cylinder CHT and EGT values, as well as a precise fuel flow and other information.
Per POH the A/C originally shipped with something called Cessna Economy Mixture Indicator which if I understand correctly is really a single cylinder EGT probe. The POH has pretty detailed instructions for leaning, but most importantly it lists:
I routinely cruise at around 75% power or FT if higher, anywhere between 3000 and 9000ft. If I lean using the EGT+75 method with the JPI I get approximately the fuel flows and speeds by the book.
I realized, however, that most of the time I can lean considerably further without any roughness, all while keeping the CHTs under 380F. That can easily bring the FF down from the book 13.4 GPH to about 12.5.
Do you think it’s safe to ignore the EGTs and lean to CHT < 380F, assuming the engine runs smoothly?
Thank you for sharing your opinion.
Marcell
@Marcell there is probably a max EGT around 1600oF?
The issue for leaning at 75% is that peak places you in the ‘box’, and also plus 75oF ROP is arguably in the box where cylinder wear increases, and in theory there is a risk of detonation. With a non fuel injected -470 am guessing the EGT spread is also quite wide.
At 75% either quite ROP max power, or err on the side of quite LOP. It sounds like you are quite LOP and perhaps leaned below 75% power, so if CHT is good, and the engine runs smoothly in theory all OK?
Marcell wrote:
Do you think it’s safe to ignore the EGTs and lean to CHT < 380F, assuming the engine runs smoothly?
Isn’t that what happens on every takeoff?
Limiting absolute EGT accomplishes nothing useful other than limiting CHT, high absolute EGT values are more a topic for climate change and global warming rather than engine thermo-mecanical stress, relative EGT is what matters when it comes to ROP/LOP
I would be impressed if you can peak EGT to 1600F on Continental O-470 whilr running smooth? it’s the worst engine ever designed when it comes to fuel distribution (no offense)
Worth reading this,
https://resources.savvyaviation.com/understanding-cht-and-egt-2/
I’m definitely ROP, I don’t think a carburated O-470 would ever be able to do LOP, given the uneven EGT that you mention.
I have not seen a strict EGT limitation in the POH, other than the “within 25 of peak” as seen above, but EGTs are below 1600 anyway. Fortunately I found one picture of the JPI where I tried this:
This is at 3500 ft.
Can you please elaborate on “plus 75oF ROP is arguably in the box where cylinder wear increases”?
Marcell wrote:
Can you please elaborate on “plus 75oF ROP is arguably in the box where cylinder wear increases”?
Not sure about the number but roughly,
- At 100% you need to run +250ROP or -70LOP
- At 80% you need to run +150ROP or -50LOP
- At 60% you can run anywhere
Running +75ROP above 75% power is risky business, things gets twisted a bit if you are not sure (people prefer full rich)
You cannot have an EGT 250 degrees higher than peak EGT :)
You probably meant one should be 250 degrees F cooler than peak by going rich (using unburnt fuel to cool the cylinders) or 70 degrees F cooler by going lean (allowing extra air to cool the cylinders?).
Adding fuel to bring EGT down 75 degrees F likey isn’t enough at 100% power. You need to keep adding extra fuel it till the temp drops by ~250 degrees F.
Also worth noting is that counting for LOP starts when the last cylinder peaks, not the first. So the first one might have to end up being 170 degrees lean of peak.
Welcome to EuroGA, Marcell 
On a non-turbo GA engine, the EGT is limited by physics, to ~1600F. Its value doesn’t matter by itself, and anyway it will differ between cylinders. The highest EGT will be found (for Lyco type engines) at the 75% of max rated power point which is the highest power authorised for peak EGT, and it will be ~1600F. It obviously scales with the OAT, like almost everything else.
- At 100% you need to run +250ROP or -70LOP
- At 80% you need to run +150ROP or -50LOP
Do you have a reference for the above? It does not make sense e.g. because you won’t make anywhere near 100% power at any LOP setting.
At 100% power you can run indefinitely (on Lyco engines) at whatever EGT you have with the mixture fully forward. I see ~1300F at sea level which is probably around 400F ROP but this cannot be safely established because peak EGT is not authorised at full throttle at sea level. I would think it would be ~1700F (hence my 400F ROP figure) but I am not going to try it.
Do you think it’s safe to ignore the EGTs and lean to CHT < 380F, assuming the engine runs smoothly?
You should not lean for CHT. You should lean for EGT. I know some lean for CHT; an IR instructor I knew was doing that, on a PA28, but then you can’t easily blow those up.
The procedure is much simpler than one reads about: climb (with all 3 fully forward, touching only the mixture after a few thousand feet) at a constant EGT and when level for cruise, lean for peak EGT (or a bit LOP if you like). The method is described here and takes you to whatever operating ceiling your aircraft has, safely and with minimum fuss.
I recommend keeping CHT below 380F, or below 320F if you are going to practice engine-out (sudden throttle closures). In climb, adjust the speed to achieve CHT < 380F.
“Deep” LOP is generally pointless; you don’t get better MPG than at peak EGT. I fly just a little LOP.
With turbo engines it is different.
I notice your LH cylinders are a lot cooler – this is same for my installation
Disclaimer: don’t have a heart attack leaning LOP at sea level wide open throttle and max RPM of normally aspirated engines, it’s ok if you are very LOP
Peter wrote:
Do you have a reference for the above? It does not make sense e.g. because you won’t make anywhere near 100% power at any LOP setting
I was referring to max possible power from your MP*RPM combination not actual realized power from fuel flow, the Lyco advice is usually to set MP*RPM that guarantee power below 75% and lean LOP on that
But you are right, actual realized power goes down when you are LOP, intuitively power is zero when no fuel even with WOT and Max RPM
From Savvy testing of NA engines, I can lean -75LOP on WOT and Max RPM setting at sea level (power stays under 75% and internal pressure is even lower than full rich)
- Full Rich +250ROP, you get 95% power
- Lean +75ROP, you get 100% best power
- Lean Peak EGT, you get 95% power
- Lean -50LOP, you get 80% power
- Lean -100LOP, you get 60% power
-75LOP on WOT & Max RPM at sea level, seems very healthy for the engine
+75ROP on WOT & Max RPM at sea level, not wise unless trees are ahead
www.eaa42.org/misc/gregs_show.ppt
The above is so complicated that a) nobody can see if it is right and b) nobody can operate it when flying a plane.
Deakin has done GA a great service, dragging it kicking and screaming out of the 19th century, but GAMI have been running their $1000 seminars for a reason
Peter wrote:
GAMI have been running their $1000 seminars for a reason
Long story short: fuel and maintenance saving will pay for the seminar on flying aircraft, I fly using fuel flow I have an EDM that thinks I am always near -30LOP (between us the -100LOP is way too slow)
