BSFC is just engineer-speak for how much fuel is consumed by the engine per unit of energy output¹.

This webpage 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      |
+------------------------------------------+----------+----------------+-----------+

Observations

• 4-strokes are more BSFC-efficient than 2-strokes

  The main reason is that a single power stroke actuates two complete revolutions (720° crank), vs a single revolution for 2-stroke designs.

  Of course, there are other inherent differences that impact efficiency, such as compression ratio.

• 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.

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