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...
Forum Discussion
RoyJ wrote:4x4ord wrote:
This is very useful info but it needs to be interpreted properly. So the 5.9 was capable of achieving its best fuel economy at 2000 rpm BUT that is under heavy load. Under heavy load it was able to make 175 hp at 2000 rpm. If its fuel consumption was measured at 2000 rpm and it was only loaded to 50% of its capability it certainly would have been using more fuel than .334 lbs per hphr. I'm talking about slowing an engine down under light load conditions to improve fuel economy.
If that old Cummins had a demand on it of 80 hp, I would call that a light load, It could easily produce 80 hp at 1500 rpm and I would be willing to bet it would achieve better fuel economy producing 80 hp while running at 1500 rpm than it would running 2000 rpm with a 80 hp demand on it.
What I'd really like to see is the following BSFC map (key work being map, not curve) for an ISB or any other modern diesel:
This shows for a given power, a heavily load 5th gear, vs lighter loaded 4th gear, at constant road speed, constant power output (110 kW), you always land on a better BSFC map.
That absolute best for this NA engine is between 4000 - 4500 rpm, and between ~80% - 95% of max torque output at that given rpm range.
Unless a Cummins' curve is MUCH steeper at the lower rpm range, so that 6th gear lands on a lower BSFC "ring" than 5th at a higher rpm, Shiner's case of better fuel economy (in 5th @ lower load) is unlikely. But then again I haven't seen a Cummins BSFC map.
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.
Here is an actual dyno of my truck. Note that the rpms on the bottom are not exact because it is hard to get the torque converter to lock at such low rpms. The curve should start a few hundreds rpms less than it does here.
