J430,
Unlike the big fans, the early engines don't really put out enough bleed air to pressurise the a/c [altho it's available from each engine.] Boeing came up with the installation of the turbo-compressor for the inboard engines to enhance the air for pressurisation. The B720 had 2 t/c's [as they're known], dunno about other B707-100's but QF had 3 and the -300 series all had 3. #1 engine missed out. Takeoff config is to run the t/c's on the inboards for takeoff to give symmetrical power settings [.02EPR reduction for T/c ON], then turn the 3rd ON after power reduction.
Hope that helps?
G'day
