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TestDrive (and others),
The dark black dots and dark line across the top of the chart you posted is the WOT torque curve for the engine. The vertical axis is torque (i.e. engine load) and the horizontal axis is RPM. That's your typical full-load power pull from a dyno.
The rest of the chart is a 2D projection of a 3D map where BSFC in g/kW-hr is the "z" axis normal to the x-y of RPM-torque. In full 3D (which Excel or even better Matlab could do, but it would be hard to read) this would look something like a funnel cloud tornado with high values along the 0-load RPM range and a deep valley centered at 2500 RPM/ 125 Nm. The circles drawn on the chart are incremental elevations like a topographical map of this 3D chart and the lines would then be isoconsumption (?) lines.
To create such a chart you would have to perform multiple pulls at various loads. You would have to measure steady-state fuel flow at fixed speed/torque points. Every 500-1000 RPM would be good enough to start and it would take nearly forever. It looks like the chart posted has values every 250 rpm and 3 Nm or so.
Using the same data you could also turn this into an isoefficiency plot by calculating the indicated horsepower (IHP) from the heating values of the fuel used and comparing that with the measured power (calculated from torque and RPM) to determine the percentage efficiency. It would look similar to the BSFC islands.
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