The ‘red box’ is around 50F ROP.
Nothing special about it but it is where the CHT peaks.
The ‘red box’ is around 50F ROP.
Nothing special about it but it is where the CHT peaks.
Interesting that this is the mixture recommended in the POH for fuel-injected C172’s!
Yes… 
There are those who think the “red box” is an artificial creation and I tend to agree; there isn’t some shark sitting there which will bite off your cylinder heads. But there is no point in being there, because you get the highest CHT and you aren’t doing more MPG. You are just hammering your engine, and if you are already on the limit then 50F ROP could push it over the edge. But yes a lot of people fly at various ROP settings, for various reasons or no reasons…
In general, the only places worth sitting at (for cruise) are
Additionally, on tightly cowled installations (not enough cooling) one could go deeper LOP, to get still lower EGTs (and thus lower CHTs). My engine doesn’t like it, but I have heard that PA46 pilots use it because of the CHT management issues they have.
I used to think that best power is 80F ROP and have seen it widely written but now think that is wrong; many flights make it clear that 130F ROP is nearer the mark. I wonder if anyone knows the theoretical figure?
You seem to imply that high EGTs lead to hight CHTs, Peter, but from all I read (especially Mike Busch’s articles) this does not seem to be the case – and I think the way he explains these thiugs makes sense.
There isn’t a totally direct connection (because various factors drive CHT) but obviously there is a connection.
I have never seen a situation where you are steady-state and increasing the EGT doesn’t increase the CHT.
Well, going from 50ROP to peak EGT would definitely lower CHTs. Not by all too much, but still.
going from 50ROP to peak EGT would definitely lower CHTs
Yes, sure. But that is just playing between the peaks of the two curves. Once you are say peak EGT then going more lean lowers both EGT and CHT.
I tried to work out the best-power ROP value from your graph but it isn’t clear because the BHP curve is shown as very flat. It could be just 80F ROP but I am pretty sure (from my countless attempts to probe the ceiling, in ISA+15 conditions and carrying the whole kitchen sink ) it is more than that.
Not any engine behaves like that in my experience.
Agreed, a confroming engine when LOP with a tight GAMI spread will produce lower EGT’s and CHT’s, sure.
On the other hand, when leaning past PEAK, a non conforming engine will reach a point, usually very near to PEAK EGT, of suddenly rising EGT’s again on one cylinder due to productive combustion process in that cylinder having been lost.
As one works step by step on the issues of poor combustion distribution between cylinders, one can lean further and further from PEAK EGT.
A conforming engine will lean very deep into LOP, way past 50d LOP and still work happily producing power with clean exhaust gases and cool cylinders.
It is nothing to do with a tight cowling set up at all.
PEAK EGT is only the starting point of running an engine clean and cool.
If one has never experienced let’s say leaning past 30d LOP at full RPM and WOT, it can be difficult to imagine that an engine can produce good cruise speed with safe (low) oil temperature and low CHT’s.
Peter,
I have found that graph to me more or less accurate (power curve very flat in that area), i.e. i haven’t found 130 ROP to produce any more power than 50-80 ROP. That’s why, when I’m trying to climb near the ceiling of the aircraft I fly 50-80 ROP, just to not waste any additional fuel.
CP,
FWIW, essentially all Cirri coming from the factory have pretty tight spreads and don’t show any weird behaviour around the peak EGT point. One can well put them 100-150 degrees LOP without much roughness (it’s just that operating that far lean does not gain anything).
