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I appreciate Walter's comments.
On the subject of whether the manufacturers established a "design point" to help "idiot-proof" engine operations, or whether they were unduly influenced by the marketing department is certainly a fun topic for debate. I too am skeptical of the marketing guys, and nothing they did would surprise me.
On the other hand, their reasons have nothing to do with the science of this issue, and speculating on their reasons does tend to make the issue more emotionally charged.
I freely admit that I am pretty new to this issue, and am here to learn from those of you that know a lot more about it than I do. I am just trying to make sure I understand the facts and don't tear up my airplane.
Let's examine the premise that is it OK to operate the engines according to the manufacturers recommendations. I am already convinced that this is not optimal from an effeciency standpoint. Alas, right now, I really can't afford to properly instrument my engine and get GAMI injectors, so for some period of time, I am going to have to continue operating the plane with nothing more than the factory supplied instrumentation and the optional single-probe factory EGT. Under those circumstances, I want to answer the following questions:
1. Is it OK to follow the POH recommendations. My POH says:
a. operate at 75 percent power or less in cruise flight.
b. operate 50 degrees rich of peak egt when at 65-75% power.
c. operate at peak cht at 65% power or less.
As far as I can tell, no one is disputing that (c) is OK, so let's focus on (b) at 75% power (If anybody is disputing this, please say so).
Walter indicates that this 50 ROP EGT setting is between peak CHT (40 ROP EGT) and best power (80 ROP EGT) based on the tests that they have done. Based on the GAMI charts that I have seen in George's Braley's articles, the CHT's are within 10 degrees of those found at best power. With the instrumentation that I have, my margin of imprecision is high enough that, for all practical purposes, this setting is equivilant to both best power and peak cht (probably some cylinders will be at one, and other cylinders at another).
In order to REALLY know if this setting is OK, I would need to know if the CHT's for each cylinder are running below 400 degrees. If they are, I'm OK, and if they are not, I am running too hot (again, if there is any disputing this point, please say so).
Unfortunately, this is a catch 22. I can't know this with certainty without an engine monitor. So, we are reduced to asking, is this OK IF the engine is functioning properly.
If the engine is functioning properly, then the cylinder with the CHT probe should be the hottest running cylinder, by design. THIS IS A MAJOR ASSUMPTION. Perhaps those of you with engine monitors on the skymaster can answer this question. If it is not the hottest cylinder, then the temperature limit would have to be adjusted down to some extent. By analyzing the data of those of you have engine monitors, we could answer the question "how much."
2. Irregardless of the answer to question 1, is there a better way to operate these engines IF you don't have an engine analyzer.
3. I think a large percentage of us use no more than 65% power in cruise (perhaps because of economy, or concerns about engine wear, or both). Is there any reason to not do this? Does it add enough of a "margin of error" that it becomes extremely unlikely that we will damage the engine no matter what we do with the mixture control?
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