BMW Turbo- und Kompressor Forum

Specific Fuel Consumption (SFC)
SFC is the measure of how effectively an engine takes advantage of the fuel in its combustion chamber. SFC is usually measured in lb/hr/HP or cc/min/HP, and traditionally good values are 0.40 lb/hr/HP (4.2 cc/min/HP) or lower; newer engines with better airflow numbers, and fuel and spark management have lower numbers. As the number gets bigger, the engine is using more fuel to make less power, so you want this number as small as possible. (Turbo and supercharged engines are notorious fuel gulpers and often have SFC numbers of 0.55-0.60 lb/hr/HP.) Note that improvements in SFC affect power output in a direct linear proportion. If at a given flow and AFR you can improve SFC from 0.50 to 0.49, you have increased power by 2%.

SFC is a function of many factors, not the least of which is static compression ratio. Increasing CR increases thermal efficiency, which directly affects SFC. Lowering the rod to stroke ratio can improve SFC to a point due to combustion pressure being applied when the crankshaft at a more advantageous angle (but it also induces greater frictional losses from piston side forces). Use of cast iron or thermal barrier coated cylinder heads, instead of bare aluminum, keeps heat in the chamber and lowers SFC. Likewise, use of thermal barrier coatings on valve faces and pistons drops SFC. Reducing internal engine friction and pumping losses improves SFC, so port your lubrication and cooling systems.

Combustion chamber design has a huge impact on SFC. A chamber with stratified charge (one where AFR varies at different places in the chamber, typically rich near the plug, but lean away from it) or lots of swirl usually was designed to improve SFC. Small, compact chambers which cause the charge to burn rapidly often have a lower (better) SFC than extended, flat ones. Placement of the spark plug in the chamber is important: a central location allows for a faster more efficient burn than an off center one.

Assume/Estimate

.36 BSFC - Turbocharged Intercooled Diesel
.38 BSFC - Turbocharged Non-Intercooled Diesel
.42 BSFC - Race Engine or Older Diesel Engine *
.47 BSFC - Hi Performance Engine *
.51 BSFC - Modern Stock Engine or Light Modifications Engine *
.55 BSFC - Supercharged / Turbocharged Non intercooled Engine

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