I can see where you are coming from about the 6.9 seeming sluggish on the grades.
My old 6.9 4-speed made my living long-hauling cattle cross-country for many many years and it NEVER did it to really suit me, hence the 6BT being under the hood now = it is more than up to the task.
I have an old gas-burner 1978 Chevy with HD350 and 4-speed.
It takes gas through it faster than I can put it back in, but it will take the same loads up the same hills with far more authority than any 6.9 I ever drove.
That being said, I didn't start making any real money until I quit the gas-burners and went with the 6.9s.
You can't set the brakes, turn up the A/C or heater, and sleep all night in an idling gas-burner; you might just keep on sleeping forever; besides, a gas-burner will use as much idling as pulling.
Many is the hour that I have spent in the sleeper-bunk of the diesels and the engine set at a fast idle, burning maybe half-a-gallon in six hours.
With the old gas-burners, I was forever under the hood, replacing burnt valves, worn-out cam-shafts, sloppy distributors, etc., etc; whereas, very seldom do any of the diesels require more than a new fuel-filter and an oil-change.
I couldn't get six months out of a clutch in a gas-burner; whereas, I can usually get half-a-million miles on a diesel clutch.
I could go on making comparisons for pages, with the diesel almost always being the best.
I made this statement to a guy in the shop a few days ago that best sums it up :
So long as the diesel was a mechanical engine with NO electronics, if fuel were $12/gallon and gasoline were free, I would still pick the diesel truck and I would still make more money when the day was done, period.
