Originally Posted by Tango
My bad...I put the 4BT numbers up. But the "sweet spot" on a diesel is not at or near redline or max HP...it is just slightly above the peak torque number. That puts it around 1800 or so for a 6BT. Running consistently at peak HP will shorten the life of any diesel and also consume way more fuel. That is precisely the reason 18 wheelers have so many gears and you hear them scooting along at 75 at what sounds almost like idle.
Sure, that'll reduce the engine's life if you're constantly producing
peak hp. But running at the rpm where peak hp is made is a different story. Running down the highway at partial throttle, maintaining 2,500 rpm is far different than being under constant heavy load.
The Cummins 5.9 was designed by Case for use in construction/industrial/agricultural equipment where the throttle gets parked wide open for hours on end with varying loads placed on it.
And OTR trucks have a tremendous number of gears and keep their rpm down because they have to. Those engines have tremendously long strokes to create massive amounts of torque, but those big strokes result in very low redlines and hp peaks. An extremely short rev range means you're constantly shifting.