Typically, the brake specific fuel consumption on larger engines is better than small ones. Kind of perverse, but the really small aircraft engines use a lot of gas to make the smaller horsepower.

Also explains why the 550 gets a bit better "mileage" than the 520s.

I've got an orginal IO-720 Operators Manual.

75% power, 2400 RPM, mixture setting "best economy" is 20 GPM for 300 HP.

At 400 HP, 31 GPM. No "best economy" setting at that power level

For the Beech Boys (and girls), you gotta have two engines to do that.

They do have a very distinct rumble that is quite impressive.

The one on the field also had a MT Prop on it for what it is worth. Looks very cool and the owner loves it.

-MO

Apples to apples those numbers make sense. Yes, a bo will use 1/2 of that, it also makes roughly 1/2 of the power.

A IO550 at full power (300-310hp) burns what, 29gph ? Just like no sane person would fly that power setting in cruise, nobody would run a 400 at full steam for anything but climbout.

As pointed out earlier, at lower power (e.g. FL200), the fuel flow will.be the same as a smaller engine making the same power.

Pug Piper didn't believe in turbocharged 'small' engines. The 400 had that engine to maintain power into the flight levels.

Same concept applied to the Mooney Ovation. Instead of the Bravos TC engine, they tried to obtain performance by hanging a big (for the M20 airframe) engine.