Despite kenparry's detailed rebuttal on the subject of torque, I think NGjockey is onto something.
Years ago my mob re-engined six of our DC-8-63's with CFM-56 engines making them -73's, at the same time retaining two aircraft in the -63 configuration with the P&W engines installed.
From then on all of the -73's required one or two units of rudder trim in cruise when the procedure described above was carried out. The -63's did not. I assumed that there was some sort of "torque" effect involved due to the big fan but kenparry pretty much torpedoes that theory. And I agree with his logic and explanation.
So what was going on?