As I've posted previously, the figure of interest in any piston engine is the SFC.
One ex car engine at least has been certified by the FAA, and used the Toyota Lexus V8 as the base. See here if you want a max power of 360HP at 5,200RPM,
http://rgl.faa.gov/Regulatory_and_Gu...E/E00052en.pdf
It quotes a maximum best economy cruise SFC of .382 at 4,400RPM, though how much HP that is is not stated. By comparison Lycoming have engines that will make a SFC of .4 at 75%. Found it interesting that it has a dual channel full authority digital engine control, but only one plug per cylinder.
There is always a downside, the FV4000 was tested in a Malibu and found to be heavy, had some exhaust problems and would have been expensive to manufacture, on the order of $100,000 per unit (2002 dollars).
The factory says that
B2N2's DA-42 at 60% power has a SFC of .338 as an example of diesel economy.
To reiterate, the Lycomings etc don't do too bad, given their simplicity.