F450 vs Ram 3500 DRW tow vehicle
So the wife and I are moving to the full time RV lifestyle. We just purchased a 42' toy hauler this weekend, with a GVWR of 20K#. Now I'm looking at a tow vehicles. I'm considering either purchasing a...
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RoyJ wrote:ShinerBock wrote:
The engine output torque curve of a Cummins 6.7 is MUCH steeper than that at the lower rpm range.
On the left is the curve of a 350hp/800lb-ft Cummins 6.7L from similar to what my truck was stock but with 20 less hp. Max rpm is 3,000.
6th gear at 65 mph in my truck is a 1,370 rpm. 5th gear at 65 mph is 1,790 rpm.
I know the torque curve of a turbo-diesel is much steeper, I was referring to the steepness of the BSFC ringlands (320, 365, 410, etc.) on my graph.
If the curve of the 410 and 365 bends up vertical, so that 6th gear (1370rpm) high load lands above 410, and when you shift to 5th (1790rpm) low load, it falls to the 365 ringland, then you could get better fuel economy.
On my graph, this only happens once, where the 200kW constant power curve intersects the 275 BSFC curve @ ~3250 rpm. So running that engine at 3200 rpm WOT lands you in the 320 BSFC zone, with a downshift to say 4200 rpm, and 70% load, you land just within the 275 zone. But we'll need to see an ISB map like that.
These are best I can get. It is on a new 2019 385 hp/850 lb-ft ISB, but I would wager that my 370 hp/800 lb-ft version is not too far off.
I estimated that it would take about 130 hp to tow what I tow with a mixture of flat land 1% grades at 65 mph. Although this does not count for a lot of wind drag which this route is notorious for having a lot of.
Using this, I calculated how much lb-ft it would take to make this amount of hp at each rpm and then converted that to N-m. Red is 6th gear and white is 5th. Ironically if I had 3.73 gears, I would be very close to the dark blue at 65 mph.
