Quote:
Originally Posted by spacemanspif
Here is the Saturn DOHC map. I just have no idea how to read it  , any insights towards understanding it are much appreciated. |
I assume you realize that it is a 3D map, like a terrain contour map. With the units of BSFC it is a valley/hole, and you want to be near the bottom of the hole at the times when you are operating the engine.
The units used for the BSFC plots may be different to this one. If you converted the BSFC figures into thermal efficiency you would, instead of the hole, have a "mountain" that represented how efficiently the energy contained in the fuel is being converted into mechanical energy. It may be easier to comprehend in that form. You still want to be in the same place on the plot, just near what becomes a peak when plotted with those units.
It is a map of
efficiency, not of the
quantity of energy being converted, so you do want to operate the engine for as little time as possible, period. It's just that, when you are operating the engine, it should, ideally, be near the peak/hole.
That plot doesn't include lines that represent power output (maybe find another one that does for insight). Those lines would represent the
rate of energy conversion. In order to access the higher efficiency load/rpm points you will have to be releasing (and finding a use for) higher power outputs.
That is what you are doing when you accelerate, pulse and glide or climb a hill and coast; you are storing and releasing that energy. Those are quantities of energy. You want to store energy in those forms, energy obtained from the engine while it is operating at high efficiency, and extract it or use it, with minimal loss.
So that's the three concepts required: efficiency, quantity and rate (of energy flows).
Also, that plot uses torque as an indication of load. Engine torque, at a particular rpm, is a reasonably close proxy of manifold pressure but needs some explanation.
If you take the maximum torque value at a particular rpm, and assume that occurs with 0 manifold vacuum, you can use the plot to determine where you want to be in terms of engine load, and hence manifold pressure.
If you feel it would help, maybe replot the data shown with the torque converted to a percentage of the maximum at each rpm, that as an approximation of the percentage of maximum manifold pressure.
Quote:
Originally Posted by PaleMelanesian
You are correct that for cruising you want the most vacuum you can get away with = most closed throttle.
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You still, ideally, want the engine to operate at near peak efficiency. The problem with cruising is there's less opportunity to manage the flows of kinetic (constant speed) and potential energies (on a flat road). That is unless you artificially create the flows with pulse and glide. You don't want to travel too fast either, or you'll lose what you gain in engine efficiency through aero drag.
If you don't want to P&G, then ignore manifold pressure and pick a precise speed-gear-throttle combination based on the lowest instantaneous consumption rate somewhere near the speed you are comfortable travelling at.