I don’t understand most of the above. Maybe words/punctuation missing?
Higher pressure means higher CHT I guess, but I imagine that even if CHT is in check it might still be rougher on the engine to have high combustion pressure?
CHT is asanity check" that everything is fine, it works well when one operate near ISA (maybe a bad idea to use 360F CHT reference with -50C OAT as it’s likely to be your peak CHT 
)
ICP is very related to CHT, especially at max rated RPM and WOT: both peak at same time for same fuel flow about 50ROP and both will drop when you go leaner past peak EGT
However,
To lean under peak CHT, you use Fuel Flow or peak EGT reference
Here is typical shape of ICP curve versus engine cycle versus fuel flow when you lean at sea level max RPM, WOT (or max MP on turbo)
For lean of peak, if engine runs smooth and you are damn sure it’s well lean of peak with stable CHT, I would sit on it (the clue is fuel flow when power drops a lot with fuel flow, you can’t be rich of peak)
I am careful leaning to rich of peak, I prefer full rich or +ROP250
Is it really “just” about not surpassing a certain CHT? Or are there some other aspects that have to be considered? For example I think John Deakin talked about avoiding max combustion pressure settings. Higher pressure means higher CHT I guess, but I imagine that even if CHT is in check it might still be rougher on the engine to have high combustion pressure?
Before attending the church of John Deakin, I unknowingly was operating an engine in the “red box” once. CHTs were fine for a few minutes, until suddenly one cylinder’s CHT started approaching the red line within 20 seconds. Luckily my engine monitor gave an alarm and I could immediately increase to full rich mixture and all was good again.
I went back through a few of his articles and he specifically mentioned that the red box is based on WOT and high/almost full RPM settings. This makes me believe that I cannot just go lower RPM to reduce power percentage in the context of the red box.
In regards to minimum CHT, I went down that rabbit hole in the last few days, too. There is surprisingly little hard data! And the anecdotes are all over the place. My personal takeaway is this: min CHT in climb or cruise should be 250. If lower, play with mixture but stay outside of red box. JD himself says the piston airliners had 212 as min CHT and the water cooled aviation engines have the thermostat set to 240. This is where my 250 number comes from. During normal operations I will never have a max of 250 only, so even if say the engine operates 2h at 250, there will be minutes or hours where it goes above 300 and should burn off the bad stuff. I was also trying to find out what the “optimum” cht is, and it seems to be around 350 but I guess there is quite a wide “green arc”.
One thing I cannot seem to find a good answer to: how would the Red Box described by Deakin (RIP) change if you are behind a 10:1 compression ratio engine? It seems basing the red box purely off of power setting seems not suitable in this case, as the fuel air mixture is compressed a good 20 more than in a 8.5:1 CR engine. So logically, the power thresholds for high CR engines should be adjusted downwards? Eg he suggests
“At 65 power, use richer than 100 ROP, or leaner than peak EGT.” And maybe for a 10:1 engine it should be eg 55% power instead of 65%power? Somewhat related: should it not also matter HOW the 65% is attained? I can use low MP and high RPM or high MP and low RPM to achieve the same power. But in the high MP low RPM scenario, there will be more fuel and more air per working stroke, hence higher detonation/pre-ignition risk, no?
65% does not matter much but it’s a good proxy you can also talk about how you adjust that 65% with 1:8CR vs 1:10CR, turbo vs non-turbo, cowl flap ON/OFF, summer vs winter, high RPM vs low RPM…what matter is that your average CHT goes up with mixture then down, if that max is above 380F then it’s bad
That max CHT goes down with lower power combinations (especially those with low RPM)
The best way to find out is to try: reduce power, lean to some fuel flow and monitor CHT < 380F (or whatever value, very low is also very bad for stuck valves from deposits and high oil consumption from rings)
If the engine does not have CHT indication, well…
@HBadger, I wonder about the same thing. “Bleriot” and I fly a Franklin with a 10.5:1 compression ratio. But considering that peak EGT can be used at 75% power on other engines, I don’t really worry about peak EGT and the red box at 65% power.
Thx for the link @boscomantico
By the way, a nice description of Lycoming fuel injection systems can be found here.
NCYankee wrote:
the throttle linkage also controls the fuel flow
It is like this in Continental fuel injection systems as well as some if not all carburetted engines. I am not so familiar with the latter to confirm the full throttle enrichment is available in all carburettor models.
And yes, as you advance the throttle on take-off with a Conti fuel injection system, max MP is achieved well before maximum fuel flow, especially so on turbocharged engines. On those, one may need to throttle back on take-off to control turbo overboost with a cold oil on automatic wastegate systems. However, despite the throttle-driven fuel flow restriction coming then into place, one is then not hurting the engine , since that is compensated for by the aneroid in the fuel pump increasing fuel pressure as a result of high turbo air outlet pressure, so you end up with the same or similar mixture, when all is properly adjusted. However, if the throttle is slighty retarded from full, with nominal MP on take-off, you will be leaning the mixture, so a bad thing as @NCYankee says above.
On RSA fuel injection systems fuel flow works differently.
honza wrote:
There is one thing in his articles that confuses me. He agitates against decreasing power after take off claiming the engines have some mixture enhancement feature at full throttle and therefore any power reduction will lead to leaner mixture. Is this really the case? Which engines does it apply to and how does the feature work?
All engines I am familiar with. For example, on my Bonanza with an IO520BA, the throttle linkage also controls the fuel flow, but the initial movement of the throttle primarily leans the engine. Most would notice that with a constant speed prop, it takes a significant reduction of the throttle position before you start to see the MP come down off of full, but if you look at the fuel flow, it is coming down all the way.
Here are some pictures of the linkage in action:
Full Throttle:
Idle
One thing I cannot seem to find a good answer to: how would the Red Box described by Deakin (RIP) change if you are behind a 10:1 compression ratio engine? It seems basing the red box purely off of power setting seems not suitable in this case, as the fuel air mixture is compressed a good 20 more than in a 8.5:1 CR engine. So logically, the power thresholds for high CR engines should be adjusted downwards? Eg he suggests
“At 65 power, use richer than 100 ROP, or leaner than peak EGT.” And maybe for a 10:1 engine it should be eg 55% power instead of 65%power?
Somewhat related: should it not also matter HOW the 65% is attained? I can use low MP and high RPM or high MP and low RPM to achieve the same power. But in the high MP low RPM scenario, there will be more fuel and more air per working stroke, hence higher detonation/pre-ignition risk, no?
