I think what he's saying, and I'll let him speak if I'm wrong, is on larger engines, 1800 may not be the most efficient rpm.
I used a ship engine as an extreme example, and yes I realize big ships run heavy bunker fuel, but large yatchs, tugs, often run diesel. Regardless of fuels the ideal rpm drops as engine stroke increases.
But I agree lugging an ISB at 1100 rpm is just as bad as revving an ISX to 2100 rpm. In both cases, BSFC is really poor, because the piston speeds are too far away from ideal diesel burn rate. I think we're saying the same thing, just worded differently.
