Lycoming cruise power schedule

Lycoming recommends that for maximum engine service life, to use a cruise power of no more than 65% BHP and lean the mixture for best power (100 deg. F rich of peak EGT). This is recommended even though the maximum cruise power setting is 75% BHP (also this is the maximum percent power for leaning). The exception to this being, during new engine break-in. For the first 50 hours of operation or until oil consumption stabilizes, you should run the engine at high power setting (65 - 75% BHP) in order to properly seat the piston rings. 

But how do we know what combination of engine settings gives a certain %BHP? This information is especially vague for engines having a fixed pitch propeller. You can't simply use RPM to determine what %BHP the engine is producing - it depends on other factors; namely, pressure altitude and temperature. Running the engine at maximum RPM does not damage it provided that the corresponding %BHP is not above limits. A cruise power chart/table is needed to determine this...

Referring to the Lycoming Operator's Manual, O-360 and Associated Models, for the IO-360-M1B (180 hp), there are two engine performance charts found in this book. In Section 3, there is Figure 3-6 and Figure 3-26. 

Starting with Figure 3-26, titled "Sea Level and Altitude Performance - IO-360...-M1B", a complex looking chart used to find actual horsepower from altitude, RPM, manifold pressure, and air inlet temperature. However, it seems that this chart is primarily used when performing run-up of the engine to determine if it meets the promised performance. It is not in an easy to use format to be used in the cockpit or during preflight planning to determine cruise power settings.

In Figure 3-6 titled "Part Throttle Fuel Consumption - IO-360...-M1B", this chart simply plots fuel consumption in pounds/hour versus actual brake horsepower. Lines on the chart show 2200-2700 RPM for both best power fuel mixture and best economy. While I have never used a cruise power chart in this format before, this is the best one that Lycoming provides for our engine. For the cockpit checklists, one of the items on there will be this cruise power table, consisting of data taken from Figure 3-6...

RPM                        75% BHP (GPH)              65% BHP (GPH)               55% BHP (GPH)

2700                        11.6                                    10.5                                    9.3

2600                        11.5                                    10.1                                    9.2

2500                        11.3                                    10.0                                    9.0

2400                        11.0                                    9.6                                      8.8

2300                        10.8                                    9.5                                      8.6

After climbing to cruise altitude (which is not a factor in this table) and setting cruise power with throttle at, say 2500 RPM, the mixture would be leaned until the engine monitor shows the EGT 100 deg. F rich of peak. If the fuel flow gauge shows about 10 GPH, I'd know right away that the engine is producing about 65% BHP. The highest altitude at which 65% of power can be produced, will be when the RPM is 2700 and the leaned fuel flow is about 10.5 GPH.