sorry, it is very difficult for me to understand that your has write and in the same manner, it is very difficult for me write to you that one I think...in Italian no problem, but an English I can only try
For an correctly reading of BSFC, the map it must be without the torque value, but with the % of PME max(pressure, average real).
The map is created put the engine in the braked dyno.
the BSFC it is a consguence of vary factors: engine respiation, friction and AFR.
The engine respiration it is defined by cams phases, valves diameters, intake and exhaust tubes lengs and diameters, resonace and pulsation phenomena.
When the respiration is at the optimum, the respiration friction losses it is at the minimum, obviously at open full throttle.
The engine friction loss increase with the engine revolution, and the AFR it is an other trouble factor.
When we interpolate all factor then obtain the BSFC map.
For to go the engine in the optimal fuel economy (in this case by 250 gHP/H), it is necessry the needed load.
In normal operation, in flat road, in the last gear and without acceleration, it is impossible go to optimal area of BSFC.
Only way, it is to decrease the engine torque available the same rpm and car load throoughout AFR modification.
This, does not have effect on the friction loss and on the volumetric efficient, but only on the PME
So, we can free-moving the engine load (point in the BSFC map) with the same engine torque output
I am understandable?