huv
Since you haven't had adequate answers to your questions from people with 0-540/Dakota experience, here's my tuppence worth (from someone with zero experience of either your engine or your aeroplane!).
Without a multi-cylinder engine analyser, your options for safe leaning are very limited. This is the case for any large (235hp+) injected engine, and the more so with a carburetted engine like yours where the mixture difference between cylinders is likely to be substantial.
The single cylinder EGT gauge gives you but one sixth of the picture, which is close to useless! You say your engine runs smoothly LOP, but how do you know? If the EGT probe is on the leanest cylinder (and that would have been the intention), the other five cylinders may well all be running ROP! That's not LOP operation.
With just the one-channel EGT gauge, if the engine were mine I would run it as follows:
1. At the 85% power you quoted, I would strongly advise leaving the mixture in full rich. Any rich setting less than 250 degF ROP could have one or more cylinders in dangerous territory. 50 degF ROP is the worst place to be! A weak setting with all cylinders leaner than 90 degF LOP would be fine, but you have no means of knowing that they are all sufficiently lean ... and it is near certain in your engine that with the richest cylinder leaner than 90 degF LOP, one or more of the leanest cylinders will be producing so little power that the engine will be as rough as old boots if it will run at all!
2. At 75% power, where for many of their engines Lycoming say it is alright to run 50 degF ROP, I would still run full rich. There is a strong body of opinion in the USA based on work at GAMI's engine test facility to suggest 50 degF rich at 75% power is highly inadvisable. If you're comfortable that the EGT probe is on the weakest cylinder (not sure how you achieve that), you could run safely at, say, 150 degF ROP. LOP is not an option since you will have no idea where the richest cylinder is running.
3. At 65% power and below, you can safely set the mixture anywhere. To go fast, I'd set the mixture at 50 degF ROP since some or all of the cylinders should then be around peak HP. To go far or save money (and at these power settings you can ignore Kiltie's recommendation only to run ROP), I'd lean the mixture as far as you can with the engine continuing to run smoothly. As you've observed, you'll probably be able to get at least some of the cylinders running LOP. A tip for LOP operations from some pilots that fly behind O- engines at altitudes where full throttle is giving 65% or less power is to back the throttle slightly off from its fully open position. Apparently the turbulence from having the carburettor butterfly valve at a small angle to the airflow improves mixing and reduces the mixture spread between cylinders. A further tip for LOP operation at these lower power settings is to use partial carb heat, which has also been observed to improve mixing.
If the engine were mine, however, I would unquestionably fit a multi-cylinder engine analyser. I've had an EDM-800 in my IO-550 now for 4.5 years, and I view it as: a) a real potential life saver; b) tremendous for reducing fault diagnosis time and therefore maintenance costs - I've been able to tell the engineers specifically which plug out of the 12 was defective, and which fuel injector was partially blocked); and c) an eye-opener to engine management that has significantly altered my practices, given more flexibility for 'go fast' and 'go far' power settings and I'm sure is extending cylinder life.
Last edited by Islander2; 29th Apr 2007 at 17:46.