i guess another common factor
when the fuel economy goes up
but torque and max HP goes down
is when the
air to fuel ratio
is leaned out.

Somewhere between
18 and 28
air to fuel ratio
missfiring begins to increase
and fuel economy and torque both
go down

Very lean a/f burn engines
are similar to Atkinson cycle engines
in that their expansion ratio
per lb of fuel
is high.

Also very lean mixtures 20 to 28 a/f
can be more highly compressed
without detonation
than the "worst to ping"
14.7 to 17 a/f ratios can

Half throttle Atkinson Cycle gasoline engines,
Lean Burn gasoline engines,
and quarter rack position loaded diesel engines,
are all operating in
somewhat similar conditions
inside the cylinder
come to think of it