# How to compare two SFCs, one in thrust, the other in power (both turbofans)?

I want to compare some recent turbofan engines in terms of fuel efficiency, particularly the PW535A and FJ44-4.

I found the SFC for the PW535A to be 0.44 lb/h/lbf. I couldn't find anything on the FJ44-4, except for an earlier model. The FJ44-3A has a SFC of 0.456 measured in kg/kW*h.

Firstly, how can I effectively compare these? I tried rearranging the units so they could immediately be compared but kept ending up with laughably different values.

Furthermore, where might I be able to find data on the FJ44-4?

• The most important information that is missing from your data is the speed at which both values were measured. Thrust ($T$) and power ($P$) are connected by speed ($v$):$$P = T\cdot v$$ – Peter Kämpf Jun 6 '18 at 22:06
• I think your reference for the FJ44-3A contains a typo for units, and the 0.456 is actually in lb/h/lbf. It is in a table comparing piston, turboprop and turbofan aircraft and was likely just copied over into the third table. Kg/Kwh is common measure for shaft output and an aberration for a fan output. – Pilothead Jul 6 '18 at 19:55

For comparing turboprops vs turbofans, you would normally divide the turboprop's SFC by the prop's thrust coefficient at comparison airspeed.

For two turbofans, always compare TSFC. It's a function of airspeed and altitude too, but the curves can be found in the paperwork, and for common engines online.

Jet engine SFC is rarely mentioned and has few practical applications outside of engine design and engineering. All the customer (pilot, etc) will see is thrust.

The FJ-44's TSFC is right there on the wiki page you've linked. The entry for 44-4 is n/a, but you can generally assume it about the same among the variants. The actual vatiance is within a couple percent.

• Both engines are turbofans – Pilothead Jul 6 '18 at 20:00

The FJ44-3A figure specifies engine power. The PW353A specifies thrust, so it includes the propeller. The propeller isn't 100% efficient, so that's the factor you're missing.

• Both engines are turbofans. – Pilothead Jul 6 '18 at 19:56