SFC is dependent on the engine design, with differences in the SFC between different engines tending to be quite small. For instance, typical gasoline engines will have a SFC of about 0.5 lb/hp/hr, regardless of the design of a particular engine. One exception to the rule is that the SFC within a particular class of engine will vary based on the compression ratio, an engine with a higher compression ratio will deliver a better SFC because it extracts more power from the fuel. Diesel engines have better SFCs than gasoline largely because they have much higher compression ratios, the way they burn their fuel is actually less efficient.
Modern jet engines actually have much higher compression ratios than piston engines, which was not always the case. Whereas a good Diesel might have a compression ratio of 22:1, Rolls-Royce RB-211 developed for the L-1011 in the 1960s runs at 29:1, and the latest R-R Trent runs at 41:1. Nevertheless jets deliver considerably worse SFC, which is due to their compressors being much less efficient than a piston for most pressure ranges.
| Engine type | SFC |
|---|---|
| Ramjet | 1.0 lb/hp/hr |
| Turbo-prop | 0.8 lb/hp/hr |
| Otto cycle | 0.5 lb/hp/hr |
| Diesel cycle | 0.4 lb/hp/hr |
| Otto-Compound engine | 0.38 lb/hp/hr |
| Turbocharged Diesel | 0.38 lb/hp/hr |
| Turbocharged & Intercooled Diesel | 0.36 lb/hp/hr |
| Diesel-Compound engine | 0.34 lb/hp/hr |