American, along with Pan Am and Qantas, converted their original 707-120 (and American their early 720-020) aircraft to fan engines very quickly indeed, some of the later 1960 build were changed over after less than one year of operation. I presume Pratt & Whitney offered some sort of trade-in for this. TWA didn't bother, and stuck with theirs until well into the 1970s - in fact they later picked up some secondhand straight-pipe ones.
The early intercontinental 707s couldn't be converted to fan engines as they had more significant airframe differences, so everybody was stuck with those.
The B47 above is reminiscent of an Airspeed Ambassador (Centaurus sleeve-valve piston engines) on cold engine start after standing for a couple of days .......
Last edited by WHBM; 6th Mar 2013 at 11:21.