*BSFC is just engineer-speak for how much fuel is consumed by the engine per unit of energy output*¹.
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[This webpage](http://www.dynomitedynamometer.com/dyno-tech-talk/using_bsfc.htm) provides a very concise comparison for several different engines:

    +------------------------------------------+----------+----------------+-----------+
    |                  Engine                  | @ 1K RPM | @ Peak Torque  | @ Peak HP |
    +------------------------------------------+----------+----------------+-----------+
    | 4-stroke (low compression, carburetor)   | 0.62     | 0.47           | 0.52      |
    | 4-stroke (high compression, carburetor)  | 0.60     | 0.42           | 0.47      |
    | 4-stroke (high compression, closed loop) | 0.52     | 0.35           | 0.42      |
    | 4-stroke (supercharged, carburetor)      | 0.75     | 0.50           | 0.55      |
    | 4-stroke (turbocharged, closed loop)     | 0.57     | 0.45           | 0.50      |
    | 2-stroke (low compression, carburetor)   | 0.85     | 0.55           | 0.60      |
    | 2-stroke (high compression, carburetor)  | 0.80     | 0.50           | 0.55      |
    | Diesel 4-stroke                          | 0.35     | 0.25           | 0.30      |
    | Diesel 2-stroke                          | 0.40     | 0.29           | 0.34      |
    +------------------------------------------+----------+----------------+-----------+

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Observations
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- ***4-strokes are more BSFC-efficient than 2-strokes***

- ***best BSFC occurs at peak torque***

  Because that's where the engine is most efficient

- ***less diesel needed per unit work compared to gasoline***

  Now you know why diesels are the [go-to choice for heavy trucks](http://mechanics.stackexchange.com/q/23865/675).

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¹ - Alternatively, BSFC = fuel flow rate per unit power