Aden Rich said:
As a rule of thumb, add the Manifold pressure plus RPM to get 48 and that's close to 75% power. So 24"/2400rpm= 48=75%. Be aware of the prop rpm limitation. The HC-2YK-1BF/7666-4 has limitation from 2000-2250 rpm running cont. in this range. You can as go to Lycoming on the web and they have charts for the engine.
A lot of folks claim that this works, and that a sum of 45 gives 65%, and 42 gives 55%. But these rules only get you within about 10% power, as they don't account for the effect of altitude (if you keep the rpm and MP the same, the power increases with altitude), they don't account for non-standard temperatures, and they don't account for non-linearity in the power vs rpm curve. For example, on the IO-360-A series engines,
Sum of 48:
Sea level, 30 deg C, 2100 rpm and 27" gives 66.7% power
Sea level, 15 deg C, 2100 rpm and 27" gives 68.4% power
Sea level, -15 deg C, 2100 rpm and 27" gives 72.3% power
8000 ft, ISA + 10 deg C, 2500 rpm and 23" gives 75.5% power
8000 ft, std temp, 2500 rpm and 23" gives 76.9% power
8000 ft, ISA - 10 deg C, 2500 rpm and 23" gives 78.4% power
Sum of 45:
Sea level, 15 deg C, 2100 rpm and 24" gives 58.7%
10,000 ft, std temp, 2700 rpm and 18" gives 63.0%
If you care about what power you are setting, you should either create your own power chart using the horrible Lycoming graph, or copy a power chart from the POH of a type-certificated aircraft with this engine, or make a power chart using the Excel
spreadsheet that I created. The spreadsheet isn't perfect, but it matches the Lycoming power chart fairly closely.
Or, if you have a calibrated fuel flow indicator, you can get a pretty good idea of the power if you are leaned. Assume about 0.45 lb/hr per hp. So, if you want 75% (150 hp), that means a fuel flow of 67.5 lb/hr, or about 11.2 USG/hr.
