Ancient History (now fuel economy)

[Arnaldo Echevarria]

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

OK
>
>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.  

At the same RPM? Could you clarify this? An engine requires a minimum a/f
ratio to overcome frictional losses, keep inertia (accel=0), and move the
drivetrain (i'm speaking about a fixed RPM, fixed speed).  The manifold pressure
is just a function of the throttle position and the rpm, not of the load, 
at least I don't think.  Now if you place a heavy load on an engine at low RPMs,
then you have to give it more air and/or gas; well, here's where I confuse
myself.
Is the load a cause or an effect?

>
>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! :)

Wait.  You said that the higher the manifold depresion, the higher the 
power loss.  Now you say that the higher manifold pressure you get less pumping
work (ie better fuel economy)?? Now I'm really confused. Which is it?

OK then you say that dropping your axle ratio gives you better gas mileage,
which
I agree (when I went from a 2.79:1 final drive ratio to a 2.05:1 ratio on a 305 
I went from 16 to 21.5MPG).  But then you say it is because you have your foot 
on the throttle harder? No way.  On that same car I remember barely touching
the gas
when cruising at 75mph (about 2000rpm or so) when before I would have to get
on it 
at least 3/8 pedal.

How low you can go on RPMs is really a function of engine size and
bore/stroke ratio.
I had a friend with a 5.0L mustang try the same thing with very little success
(he lowered the gear ratio so much it would need a higher throttle setting
to keep 
the same mpg.)  To me, as a rule of thumb, gas mileage is proportional to
throttle
position (relative to car, of course).  The more you can stay off the pedal,
the 
better (and of course this goes for carburators only)

Arnaldo

MPG)

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