Chas Cas:
I have the a23 24 with c/s prop and 200 HP engine. From day one I was taught when I was getting my checkout, after buying 6956Q, to run her at cruise settings of 23.5 square and running 50 degrees Rich of peak EGT. What are you guys running at cruise?
Editor:
Chas, the IO360 likes to burn oil and likes to be run at 75% True HP output is determined by the fuel burn, especially when at or lean of peak. My personal preference is to go high enough to run full throttle without exceeding 24″ MP, and to control power output using fuel and RPM. My typical cruise setting is 24″ by 2,500 RPM, leaned 25-50 LOP. If I need more power, I use more RPM and/or add fuel to peak EGT. If I still need more power, I go to at least 100 ROP. I never run in the range of 40-50 ROP, despite Lycoming’s handbook. I am satisfied that GAMI has proven that this power point is the hardest on your engine. I suspect that Lyc recommends it as the best power for the lower fuel burn, but it spikes the power pulse in the cylinder before the rod is at the optimum crank angle. It also typically creates the highest CHTs. Peak EGT will reduce both power and fuel burn, and won’t hurt anything on a non-supercharged/turbocharged plane. 100 ROP will give more power (at a higher fuel burn), while being easier on the engine and with lower CHTs (compared to 40-50 ROP).
I deliberately did not mention a specific fuel flow. You should determine that for your application, using your instrumentation. You set the desired power values (for example, full throttle/24″ and 2,400 or 2,500 RPM). Then you perform leaning depending on your intent. If you want economy cruise (usually for burger runs or tailwind cruising), you go to lean of peak at 2,400 RPM, within your tolerance of roughness (from the uneven power pulses). Usually 25 to 50 LOP is possible. You will be rather surprised how cool your cylinders will become, when lean of peak; most likely in the mid to high 200’s. Fuel flow will also be surprisingly low, especially at altitude. I have frequently cruised at less than 8 GPH while LOP.
If you want high cruise, perhaps while fighting a headwind or at altitudes exceeding 9,000 or 10,000, you might want 2,600 RPM, and even 2,700 while topping mountains. After all, if you are up at 12,000 in the Rockies, full throttle and 2700 RPM may only be providing 50% power. Under those conditions you’ll want to get 100 or so ROP, for max power for the conditions. At low altitudes (meaning 5,000-6,000 feet), full throttle (24″) and 2,600 RPM might pull 11 GPH at 100 ROP, and you’ll need to keep an eye on CHTs. If they start reaching 380F you’ll want to go to 12 GPH. There are variations in the displayed fuel flow, as it is really a pressure indication, so it is better to work it out on your plane. I’m pulling these numbers out of my head; others may recall seeing different values, and I apologize if I’m proven to be off-base on some recollection.
There is a lot more about this on BAC. Just search on engine management, leaning, words like that. If you don’t have good instrumentation that enables this level of precision (and fuel savings), you are stuck with using the POH (or Lycoming handbook) tables.