I'm not an expert, but the fuel computers are a one off for this airframe that Cessna did. They take inputs from the power levers, condition levers, RPM sensors, OAT, ITT, and exhaust temps and calculate a "single red line" to help the pilot manage engine power and theoretically limit engine temp exceedance. Supposedly you can just firewall the throttles and the computers will protect the engines although I'm told nobody does that in practice. These computers are one of the "gotchas" with this airframe as there are about 10 shops left that have real expertise in setting them and there is really only one place left that does overhauls on them. You can still legally operate the plane without them, but it is a little higher workload. My understanding is the shop that still does the overhauls is in Australia and plan on $20k - $30k if you need one. So, I believe that is the reason for the placard as there plane can run on other fuels such as 100LL, but you have to have the computers recalibrated.
The 441 is, IIRC, four feet longer, five feet longer wingspan, and about 8" taller. Inside you definitely notice more width in the cabin, but the cockpit feels about the same. I'm really hopeful that SWMBO and the kids appreciate the difference in the climate control system in the 441 vs. the 425. The 441 has a system that is more like the jets with a bleed air driven air cycle machine. The 425 takes bleed air and runs it through an air to air cooler and then directly to the cabin. The air conditioning is a freon system that is marginal and when you're up in the flight levels there is an electric heating element to supplement the bleed air heat. You'd sweat your ass off in Phoenix and pax were always cold at FL250. The ACM takes bleed air, runs it through a air to air cooler, through a turbine(also driven by bleed air) to compress it again, another air to air cooler, then through an expansion valve to remove the moisture and cool it. Then that dry and cold air gets mixed with hot bleed air to go to the cabin. Got that? It is another bit of magic that seems to work, but, again, is another $30k to overhaul when needed. Seems like they go for about 4000 - 5000 hours and mine was just done in 2023.
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