Fuel Economy

[oecar1 at oec4.orbeng.com.au]

>>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? 

Exactly! at the same RPM

>An engine requires a minimum a/f ratio to overcome frictional losses, 
>keep inertia (accel=0), and move the drivetrain 

not a/f ratio, this is constant at 14.7 or so, but a minimum fuelling 
level, or fuel flow rate (= power)

>The manifold pressure is just a function of the throttle position and the rpm, 
>not of the load, at least I don't think. 

Manifold pressure is the best measure of load (aside from direct airflow 
measurement).  That's why many people in the world use MAP sensors.

>>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?

It's both! (I must apologise for the confusing choice of words) When I say 
manifold pressure, I mean Absolute Pressure, however manifold depression is 
measured relative to atmospheric pressure which is 101.325 kPa Absolute.  
Depression/Vacuum and Absolute pressure are effectively the inverse of each 
other.

>
>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.
>

OK, throttles are notoriously non-linear devices, and the characteristics at
2000 rpm are not the same as at 2720 rpm, therefore maybe you did have your
foot less on the 'gas' but I bet your manifold pressure was higher (or your 
manifold depression/vacuum less!)