Originally Posted by
ORAC
The improvement in performance by replacing 13,500lb s.t. JT3Cs with 17,000lb s.t JT3Ds was marked. Conversion of a 707-120 also involved installation of the reprofiled inner wing leading edge, as already introduced on the 720, and greater tailplane span. Re-engining the 720 produced an even greater performance improvement as they were originally fitted with 12,000lb s.t JT3C-7s.
There was no similar imperative to re-engine the 707-320 as they were fitted with 17,500lb s.t JT4As and the fan engines would not have provided a significant thrust increase. The -320B was not just a re-engined -320, they had a wing with redesigned tips and leading edge high lift devices and were certified for higher weights. The 17,500lb s.t. Rolls Royce Conway in the -420 was, of course, a turbofan anyway.
Boeing did propose a re-engined 707 with CFM56s, but it was really too much engine for the airframe and stretching the 707 was not economically viable as it was already a stretch of a stretch. However, many 707 family aircraft were built or re-engined with CFM56s for military purposes, e.g. E-3 Sentrys and KC-135s.