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The BSFC Calculation (in SI)
  To calculate BSFC, use the formula BSFC = Fuel rate / Power

Fuel rate is the fuel consumption in grams per second (g·s-1)

ω is the engine speed in radians per second (rad·s-1)

T is the engine torque in newton meters (N·m)

Power is the power produced in watts where P = Tω

  The resulting units of BSFC are grams per joule (g·J-1)  
  Commonly BSFC is expressed in units of grams per Kilowatt-hour (g/kWh). The conversion factor is as follows:  
BSFC(g/kWh) = BSFC(g/J) * 3.6*106

The conversion between metric and U.S. units is:

BSFC_US(Lbs/(HP*Hr)) * 608.277 = BSFC_METRIC(g/kWh)
BSFC_METRIC(g/kWh) * 0.001644 = BSFC_US(Lbs/(HP*Hr))
The Relationship Between BSFC Numbers and Efficiency
  To calculate the actual efficiency of an engine requires the energy density of the fuel being used.
Different fuels have different energy densities defined by the fuels heating value. The lower heating value LHV is used for internal combustion engines efficiency calculations because the heat at temperatures below 150 °C cannot be put to use.

Some examples of lower heating values for vehicle fuels are:

Certification gasoline = 18640 BTU/lb = 0.01204 kW·h/g
Regular gasoline = 18917 BTU/lb = 0.0122225 kW·h/g
Diesel fuel = 18500 BTU/lb = 0.0119531 kW·h/g