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Originally Posted by jeff88
So basically when accelerating, I should try to accelerate at around 4k RPMs. This has two benefits: best BSFC as well as get up to speed faster so I can than cruise. My question though is the BSFC is in g/kWh, not necessarily MPG, so the engine may pollute the least at 4k, but not necessarily be the most efficient in fuel used?
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Given the condition of WOT, yes it is the most efficient use of fuel to operate in the range of 3-4K rpm. Accelerating in that rpm range is still more energy efficient because the energy released from the fuel is stored as kinetic or potential energy, not lost.
At WOT the engine management will be adding fuel to protect (cool) engine and catalytic converter so just less than WOT will be more efficient than WOT. To detect if that is occurring look for an indication of open loop on your Ultragauge.
The curves for inlet manifold pressures near WOT will be close enough that the rpm range for greatest efficiency will be similar to that for WOT. The 'dip' will be deeper and the rpm of lowest BSFC will be lower as inlet manifold pressure decreases.
If you can get a display of MAP pressure on the gauge then you want that either in the range of 0.6-0.8 bar (when accelerating) or the engine off (or at least idling). Higher, short of going into open loop, is better.
The reason is that, literally, half the engine (4-stroke) is acting as a vacuum pump, pumping from inlet manifold pressure up to exhaust manifold pressure (actually cylinder pressure on the inlet and exhaust strokes but close enough).
You can appreciate that if the engine wasn't an engine but motored as a vacuum pump it would take power to do that. Opening the throttle and increasing the inlet manifold pressure reduces that pumping work.
If you cruise along in gear at light load the engine is still pulling the vacuum. Higher rpm increases the amount of work being done so idling (with the engine disconnected from the wheels) reduces the pumping work. If you could arrange for the engine to be off (as well as disconnected) it would be even better.
The other effect that reduces efficiency is friction. That is proportional to the square or cube of engine speed (depending on the source). That is why the efficiency falls as rpm increases.
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Could this be done with an Arduino and tapping into the ECM? 
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Normally that would be done on an engine dynamometer but yes. Without modelling it won't be possible to separate out road and aero drag or transmission losses from internal engine losses though. With a fuel consumption rate and kinetic and potential energy measurements it will be possible to determine the equivalent of BSFC based on the power used to drive the car. In the real world that is what matters anyway. Maybe call it Road (Power) Specific Fuel Consumption?