[Walter Atkinson]

Kyle:

All of you points a very well taken and IF the original assumption were valid concerning the design issues, I would agree 100%. Let's first get one issue attended to , then we can discuss the design issue.

Best Power is as you correctly describe (about 80dF ROP), but max CHT is not at that point; it is a little less rich than best power. CHT's will actually be a tad cooler at Best Power than at 40dF ROP. A minor nit, to be sure, but we do want to be as acurate as possible in a scientific discussion because what follows is important in that discussion.

As for the concept that the manufacturer was setting design limits on their recommendations, the reality will show that that concept might be giving them more credit than they deserve! <g>

Think back to the heyday of GA... the 1960s. Beech, Mooney, Piper, and Cessna were in a serious battle for sales. The competition was intense. Speed and range were the issues that sold airplanes. The engineers made recommendations on where to run the engines. They resulted in either LOP for range (too slow to sell airplanes in a competitive environment) and ROP for speed (but where the engineers wanted the engines run was so rich that range was poor and that didn't sell airplanes). So what happened?

The MARKETING DEPARTMENT wasn't interested in listening to the complaints about the rough running of LOP on their poorly F:A ratio imbalanced engines. Recommneding that was not an option. The MARKETING Department decided that at 75% power and leaned to best power, or even better to 25-50dF ROP, the range and speed numbers would be the most optimal to sell airplanes. The engineers objected. They said that longevity would suffer. The engine makers objected for the same reason. Then the OEM told the engine mamufcaturer they would buy the engines from the competition if they didn't support this notion. It was a competivie market place. They knew that longevity didn't sell airplanes. Speed and range sells airplanes. So the recommendations are what they are. At 25-50dF ROP, the speed is within a knot of Best Power and the range is much better--longevity take the hindmost. The very worst things on metal are heat and pressure and that is the mixture setting where those are the worst.... but they'll make it through the warranty period OK.

Some realy bad things begin to happpen to Aluminum as it reaches about 400 degrees and we know that staying away from that can be quite beneficial where longevity is concerned. These things tended to happen AFTER the warranty period was up on the longevity scale.

That's the long and the short of why Cessna, Beech, Piper, and Mooney recommend what they do. It was NOT based on engineering. It was based on sales concerns. It's the only answer that makes any sense and until someone comes up with an engineering reason, it's the only thing that explains the recommendations.

Best regards,