I was curious about the operating speed of those engines, I found this:

"The Wright R-3350 Duplex-Cyclone engine typically operates at around 2,400 rpm for normal power and 2,800 rpm for takeoff."

Also that their normal cruising speed was around 305 mph . Airline travel was expensive in the 50s, definitely an upper crust kind of thing.

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1986 300SDL, 362K
1984 300D, 138K