Awhile back I wrote about the religious debate about operating airplane engines. I finally got a chance to run lean of peak on my recent Texas trip. Compared to rich of peak, LOP was 20% more fuel efficient, 5% slower, and about 20° cooler. But I'm still running ROP.

The plane is only now capable of running lean of peak thanks to our engine monitor, fuel totalizer, and GAMIjectors. The stock injectors had one cylinder peaking 1.0 to 1.5gph sooner than others; now our "GAMI spread" is 0.4gph, a win whether we ever run lean of peak or not.

On my recent flight I was at 9500', 20.5" manifold pressure, 2400 RPM. Flying at my typical 50 ROP I was getting 140kts TAS, burning 10.3gph, with EGTs at 1357/1378/1352/1324 and CHTs at 303/332/337/290. Leaning out to 40 LOP I was getting 132kts TAS, burning 8.3GPH, with EGTs at 1401/1441/1416/1352 and CHTs at 281/316/311/271. So the difference there is running lean of peak I saved 2gph, ran my engine 20°C cooler, and gave up about 8kts to do it.

Saving gas but going slower: is LOP really a win? Cessna didn't publish performance tables for lean of peak, but they did publish tables for saving gas at the recommended 0.5gph ROP. At 10,000' Cessna says full power gives 145kts TAS at 9.2gph. If you want to save 2 gph they have you dial way back on power to get 126kts TAS at 7.1gph, for a 19kt slowdown. So roughly speaking, LOP saves me just as much gas as going slow ROP, but makes me 11kts faster.

Despite the good results I still flew over the hard, unforgiving mountains of the Southwest running ROP. The engine feels wrong in the transition to LOP, you can feel the sluggishness. I've also had various salt-of-the-earth mechanics all warn me against running LOP, and while their reasoning doesn't make sense it still makes me nervous. And of course LOP really does make you slower and with four hours of West Texas ahead of you, you want every knot. The cooler operation seems like a real benefit though.

(A caveat: those EGT numbers are a bit wonky. The actual measured peaks with some experimenting were 1462/1494/1466/1410 at 8.8/9.1/8.9/9.2 gph, so it's possible I was running more like 90 ROP than 50 ROP and could have saved a bit of gas while ROP. There's some lag in the EGT response time, I was doing stuff too quickly.)

  2011-05-08 00:17 Z