Ancient History (now fuel economy)

[RABBITT_Andrew at mv8.orbeng.com.au]

>Granted, the change of fuel makes a big difference, but the point was 
>that 28:1 AFR is Very Lean, even for methane. 

The biggest influence on fuel consumption is in pumping work (ie 
manifold pressure, or lack thereof).

If you choose to run a 400+cid engine, then to produce the sort of 
power necessary for cruising (10-20 hp), you will be running 
relatively low manifold pressures compared to a smaller engine 
producing the same power.  Remember, the manifold depression is 
un-recovered, therfore represents a power loss.  

The slower you spin the engine, then the higher the manifold pressure 
is for the same airflow (power), hence your pumping work (losses) is 
less.  Dropping the axle ratio (numerically) will give you benefits in 
fuel consumption at the same (road) speed simply because you have your 
foot on the throttle harder (strange but true! :)

Running lean does effectively the same thing, by running the same 
fuelling level, but higher airflow, the manifold depression is 
reduced, saving power.