: Are you assuming that a carburated, air cooled engine with a fixed :
advance magneto ignition has the same fuel efficiancy as a water : cooled
engine with electronic fuel injection and ignition?

Actually, for an airplane application, even a carb'd gasoline
engine can obtain very good fuel economy, since it can be manually leaned
for its constant operation. The benefits of a more modern engine you
describe a

1. Fuel-injected: Aside from poorer fuel/air distribution in a carb'd
engine, fuel injection doesn't buy you much in an airplane. Even with
computer-controlled injection, all that'll give you is better transient
performance. At cruise (where most fuel is burned), computer-control
doesn't buy you anything more than the red knob does.

2. Water-cooling: This is a double-edged issue that's a bit loaded.
Everything else being equal, a water-cooled engine doesn't give you
anymore power than an air-cooled engine. What it does buy you is the
ability to run higher compression ratios and/or lower octane fuel
(much lower CHTs). A higher CR will give you more thermodynamic
efficiency. Also, a water-cooled engine allows for more flexible (read:
efficient) cooling, but then again that's not a BSFC engine argument so
much as an airframe issue.

3. Timing: Having adaptive timing doesn't buy you much in cruise,
since that's where the fixed-timing is set to be optimal. It will allow
you to possibly run lower octane fuel, but again that doesn't directly
affect BSFC.

While I agree with the idea that having liquid-cooled,
fuel-injected (*perhaps* digitally controlled) high-compression gasoline
engines are good from an aircraft *system* performance, they do not
inherently increase an airplane engine's already excellent cruise fuel
economy. I routinely get 0.42 lbs/hp*hr from my carb'd Lycoming O-360.

-Cory
++++++++++++++++++++++++++++++++++++++++++++++++++

BINGO.

Thanx, Cory


Barnyard BOb -- over 50 years of successful flight