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kane66,
It seems to me there's something key that you're missing from the relationship between engine design, BSFC, and vehicle fuel economy. The throttle plate in a gasoline engine doesn't "waste fuel" per se, but it reduces the engine's overall efficiency by increasing the power required to pump air through the engine. This restriction becomes less of a factor near wide-open-throttle, or at the last 15% or so of throttle opening before WOT.
BSFC does not correlate with vehicle fuel economy. The 1.9L TDI may have a better peak BSFC than the 1.6L TDI but remember what BSFC stands for: Brake-specific fuel consumption. Units are lbs fuel per horsepower-hour or grams fuel per kilowatt-hour. When you put an engine in a vehicle at steady-state highway cruise on flat ground the vehicle determines the power required (the brake portion of BSFC). The gearing and the cruise speed determine the engine RPM. Looking at the BSFC chart for that particular RPM you must find the load point, which would be roughly the horsepower your vehicle requires to maintain speed divided by the peak horsepower that engine can produce at that particular RPM (% load).
That load percentage would be higher on a less powerful engine like the 1.6L than it would be on the 1.9L. Higher loads usually yield lower BSFC. Depending on what efficiency island that places you on you can not necessarily say that the 1.9L engine would give you better efficiency because it has a lower peak BSFC.
Your statement about comparing diesels to gasoline engines is false. Gasoline engines can be measured in the same manner as diesels relative to BSFC. True, the graphs look different due to the efficiency loss associated with throttling at partial load and the expanded peak efficiencies afforded by diesels' increased compression ratio and turbocharging. Neither design of engine "wastes" fuel, they both convert fuel into rotational energy in different ways at different efficiencies.
I'm somewhat of the opinion that there is far more vehicle fuel efficiency to be gained in properly engineering an engine's application than most realize. Many think that plucking a more efficient engine from another vehicle and dropping it in a less efficient vehicle will improve FE, and it may, but actually *matching* the engine to the application properly will yield much improved results.
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