I chatted with the guys at Rotec (maker of the 7 and 9 cyl radials). They said the 9 cylinder engine basically made it a single seat airplane since with 2 real size humans, they only had capacity for about 45 minutes of fuel. That ended the conversation for me... Even the 7 cylinder weighs about 80 pounds more than the 912. As much as I love the look, pretty much squished that dream from a realistic use-case perspective.