I don't think it is the constand 4k rpm that made your father
car's engine smoother..but rather it's the fact that there is a
constantly similar rpm value that made it run smoother now if you
get what I mean..
What I mean is as long as the engine is left to run constantly at a
similar RPM, regardless of whether it's 2k, 3k, or 4k, it will help
clear the carbon deposit in it..it's kinda true..but it has to be
kept constant and long enough..a drive up North will be long
enough.. 