Wow sitting on the sidelines watching this conversation happen is very interesting.
I started building my 12 and am very slowly creeping along due to a major job change and the economy. I have a budget sheet that I built 4 months before I ordered my tools that covers everything down to the screws to put the workshop together, one of the things that hurts is 28K for the FWF.
I sat talking to my wife about that final chunk and how that was going to be a hurdle, then I saw the ad for the Viking engine, I tallied up the expected costs to about 16K complete that is a huge difference. There is the emotional pull.... (oh and fuel injection, an alternator....)
Reality sets in and you see a few have jumped aboard, I mentioned the option to some of my chapter members and was blasted from all sides... "A Car Engine", "wow that's risky", "Jan who? let me tell you about Subaru..."
so for two weeks I have watched this debate rage and I have decided it WILL come down to two things for me, the primary being weight, I see the lightly built 12's coming in around 740, I can live with 760. The secondary being reliability, those that are forging ahead with this will be my guide.
As for Rotax reliability: Personal experience (not meant as bashing but observable phenomenon)
1.) I have read many posts on here about failed carb floats, failed fuel pumps, and loose even missing engine mount bolts (not necessarily engine issue). Net before Viking came along I had my doubts
2.) reliability in other planes with direct experience:
Remos GX - Flown 15 hours -
a. used a quart of oil every 25 (rental)
b. couldn't use radios unless you revved over 2K on ground (generator issue?)
c. unexplained (to me at least) issues with gear box and engine grounded plane more than once while renting it costing me time and money
Evektor Sportstar #1 - flown 30 hours
a. same voltage issue constant low volt alarm while taxiing (not fun to have a garbled radio on the ground at a busy towered airport)
b. sickly performance in the heat (likely prop and adjustment issues but issues none the less)
c. never saw RPM's above 5050, likely contributor to b. though it was in the shop many times and I was told F2F by the mechanic it was adjusted perfectly
Evektor Sportstar #2 - never flown
Why? both times rented it was cancelled last minute due to "maintenance issue - engine"
All of that said it is really little experience but enough to dissuade me greatly from the 912 IF there were another option.
Now for the other emotional side of this, I belong to an organization called the EAA last time I checked the E stood for experimental, this engine is very experimental but also has the ability to change the face of LSA, I am sorry but 12K+ in savings is substantial. This is why the EAA exists, to experiment and take aviation into new directions.
Listening to those argue that it shouldn't be done because it is a car engine and those (or PSRU or whatever) have failed in the past is almost funny considering the type of planes they are building, the whole dang thing is experimental.
Any person on this forum flying anything other than a certified aircraft made the decision to fly something "somewhat" unknown (more or less depending on what you choose) and with potentially more inherent risk than Joe down the street who got handed the keys to his new Cessna, some here (those choosing to install a viking are taking a greater risk than some, that is a choice)
For those building a 12 looking for the "safe" Continental (and maybe soon Lycoming) choice, Cessna is taking orders for the Skycatcher at 120K average price, much less risky...right?
full circle, yes it is experimental but that is the nature of what we do, imagine this conversation:
Passerby December 17th, 1903: "Hey there gents what is that?" Orville: "Why that is an Aeroplane, we are going to fly it!" Passerby: "Are you nuts people can't fly that is too dangerous." Wilbur: " Yeah your right what were we thinking, lets go back to riding bicycles."
I would be stuck riding a train from Phoenix to Toronto tomorrow