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- Mar 5, 2017
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Since the end of the break-in period of my recently overhauled O-540, I have run my Dakota according to the 55% 2100RPM power table in the POH. I became familiar with the concept of the red/purple box and red fin after reading articles by Mike Busch and John Deakin. As I understand it, we use the CHT as a surrogate for the internal cylinder pressure because we do not have an instrument to measure the latter. Mr. Busch alleges that one basically has to stay out of 400 degrees CHT and that he shoots for 380 degrees, but we can basically do whatever we want to our engine below that and not cause any damage as long as we are below about 60% power because the red fin disappears.
At 55% power, I observe CHTs around the following
280
280
300
310
320
320
At 65% power, the CHTs may look something like
297
300
310
320
320
325
Whether I lean to peak EGT (stoichiometric mixture) or lean of peak, my first two CHTs remain below 300 degrees at 55% power. The bottom line is that I am no where near 400 degrees or even 380 degrees, even at 65% power, so I feel that (based on what I understand from Busch and Deakin) it is impossible for me to damage my engine no matter how I lean as long as Im somewhere at 65% power or below (and I run at 55% almost all the time in cruise). I ran a search of prior threads and found some of the CHTs other have posted, and while mine seem to be much lower than others on this forum and I do not know why, everyone else seems to be well outside 380 degrees, too. So, in my mind this raises the question, does the J3A5D variation of the O-540 even have a red box/red fin at all at 65% or below? I wonder if it does not have a red fin at our normal cruise configurations (and perhaps at most a thin tall rectangle) because this is a de-rated engine whose big brother can turn up to 2700RPM.
I am even wondering if I might be damaging my engine by running at 55% power because my CHTs are too low to get rid of the tetraethyl lead (TEL) which I read requires about 300 degrees for scavenging agents to work properly.
Are my observations in line with everyone else here that has a Dakota? Any faults with my line of thinking regarding how I am applying Busch/Deakin?
At 55% power, I observe CHTs around the following
280
280
300
310
320
320
At 65% power, the CHTs may look something like
297
300
310
320
320
325
Whether I lean to peak EGT (stoichiometric mixture) or lean of peak, my first two CHTs remain below 300 degrees at 55% power. The bottom line is that I am no where near 400 degrees or even 380 degrees, even at 65% power, so I feel that (based on what I understand from Busch and Deakin) it is impossible for me to damage my engine no matter how I lean as long as Im somewhere at 65% power or below (and I run at 55% almost all the time in cruise). I ran a search of prior threads and found some of the CHTs other have posted, and while mine seem to be much lower than others on this forum and I do not know why, everyone else seems to be well outside 380 degrees, too. So, in my mind this raises the question, does the J3A5D variation of the O-540 even have a red box/red fin at all at 65% or below? I wonder if it does not have a red fin at our normal cruise configurations (and perhaps at most a thin tall rectangle) because this is a de-rated engine whose big brother can turn up to 2700RPM.
I am even wondering if I might be damaging my engine by running at 55% power because my CHTs are too low to get rid of the tetraethyl lead (TEL) which I read requires about 300 degrees for scavenging agents to work properly.
Are my observations in line with everyone else here that has a Dakota? Any faults with my line of thinking regarding how I am applying Busch/Deakin?
