Yes, FADEC $$$$! But, that's the way that the engine is certified. I too appreciate the simplicity of a purely mechanical diesel engine (my excavator is so equipped). But, FADEC allows optimizing power and fuel economy, as well as making the engine operationally simple. The FADEC does everything, after you tell it how much power you want - single lever. Though I'm used to airplanes with many levers, I see the world going toward single lever simplicity. This is actually creating a little too much simplicity for the Beaver, and we're designing around this total simplicity to optimize a few things (like being able to command coarse propeller pitch to make the glide better - sort of feathering, but not totally). That's the next phase of my testing.
When I see the data stream from the FADEC, and how an emailed file can further optimize the engine operation, based upon my observations while testing it, there's something to it! I don't see us ever going back to complete mechanical diesel engines for airplanes (nor most other machines for that matter). FADEC is just becoming too mainstream. My first few film cameras did not require a battery (other than maybe for the light meter). Can you think of a camera which does not need electricity now? Things change....
Yes, I have flown -40 a few times. The lowly Lycoming did what I needed it too. The RED diesel will too - as long as it is preheated to the minimum start temperature. We'll be taking SBN somewhere very cold this winter to test this!