If thermal efficiency of an engine is higher then sfoc will be low.
Thermal efficiency ,simply for a Diesel engine is heat input to the work done.
Or it is T2/T1 ,
T2 :- max temp in cycle
T1:- min temp in cycle.
If stroke length is more
Higher the peak pressure, more fuel and relatively more temp (T2)
And if T1 ( scavenge temp) is considered same
Then super long stroke have high thermal efficiency and low sfoc.