Thats because Cessna own Lycoming
Thanks GG, forgot that.
An excellent BSFC (Brake Specific Fuel Consumption) for a well-developed, 4-stroke naturally-aspirated, high-performance liquid-cooled engine at 100% power is in the neighborhood of 0.44 – 0.45. Claims of gasoline engine BSFC values less than 0.42 at max power tend to be suspect. At reduced power settings (in the region of 70% and below) BSFC values of 0.38 have been achieved.
The operator manual for a 300 HP Lycoming IO-540-K, L, or M series engine shows a full power fuel flow of 24 GPH which is a BSFC of 0.474 and a Thermal Efficiency of 28.3%. Those numbers aren't too bad for an air cooled engine which meets the FAR-required detonation margins. However, the turbocharged TIO-540-V2AD requires a MINIMUM of 39.2 GPH at 350 HP for a BSFC of 0.663 and a TE of 20.4%. Avoiding detonation at high power eats fuel, literally.
The most efficient piston engine is a marine diesel of 108,920 horse power with BSFC of .278 at max power, dropping to .26 at max economy, and a thermal efficiency of over 50%. It's advantages are one of low frictional losses (piston speed, windage, bearings) due to an operating speed of 102 RPM at max power, and a low of 7 RPM.