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torque and fuel consumption graphs?

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  • torque and fuel consumption graphs?

    In the announcement for the new tune, they included a very low-res torque graph, does anyone know where to find a readable version of this?

    Also, does anyone have a graph of what the fuel consumption per HP would be at different RPMs? the cumminsrepower 'calculator' that shows 'good' fuel economy from 1500-2000 rpm and then 'bad' fuel economy from 2000-3600rpm is pretty worthless.

  • #2
    Welcome!! Feel free to introduce yourself in the welcome room and tell us what you are building.

    I will see if I can get a high res photo of it.

    As for the fuel vs power. There are far too many variable to make a graph for every vehicle. Going from aerodynamics, gear ratio, tire size, transmission, altitude, even AC will change the graph.

    From experience.... thousands of miles on many different chassis. With EGR and a brick (jeep/Land Rover, and oversized tires) at altitude below 1800-1900 rpm will not pull a grade without slowing down or down shifting.
    With something closer to stock tire size or aero you can lower that rpm and still pull. It is possible to go too low! Running around at 1500 rpm is not good for economy or performance.

    2000-3600 is not “bad” rpm. There is a vixen RV running around at 3000-3200 rpm and he gets 28-30 mpg. All because of aero, tire size, but mainly load at Rpm.

    The best reference is load at rpm within the Murphy gauge. High load means using more fuel, less load, Less fuel.


    • #3
      I'm not the one saying 2000-2600 is bad rpm, says it in the gear ratio calculator at the bottom of the page. We both agree that that's nonsense.

      you are misunderstanding what I'm asking for in terms of the fuel economy. I'm not asking what a particular vehicle is going to get under any conditions.

      I'm asking for (the amount of fuel used at wide-open-throttle) / (HP produced) across the RPM range.

      If the RPM is too low, power suffers, and similarly if the RPM is too high power suffers (all while burning the same amount of fuel)

      That doesn't depend on the gearing/vehicle/aerodynamics. It's a way of figuring out that if I can be in three different gears at a given speed (and not accelerating), which gear is going to be the most efficient. You will be at different throttle settings in the three gears but producing the same HP.

      The knee jerk reaction is that the lower the RPM, the better, but that is only true down to some point.

      In reality this is a three dimension graph where you have throttle position and RPM as your X/Y with fuel/HP as Z. but a two dimensional graph that just covers the WoT case is a lot easier to generate.

      I ran across a graph like this in some paper on the 6.2L engine some years ago, surprisingly to me it was pretty flat (only getting significantly more inefficient at the very low and high ends).

      If someone has access to the fuel map to know how much fuel would be used for each cycle, this graph could be calculated fairly easily.If the fuel map is only injector duration it's a bit uglier (but even comparing ms of injector open/HP would highlight the speed that you start getting significantly less efficient)


      • #4
        I see what you are getting at. I will see if I can pull up a fuel table like that, I am not a Cummins engineer so finding it will take me some time.

        There red is a limit to where economy and performance is optimal.
        Ill see what I can do


        • #5
          Thanks, I tried calling Cummins for this, but they said I would need to ask my local dealers for this info, and the local dealers don't know anything about this tiny engine (they are all big-rig and large equipment shops in my area)


          • #6


            • #7
              Thanks, I updated the table on my calculator


              • #8
                is the RPM limiter really set to 3600 rpm? I thought I saw a post here about power dropping off sharply after 4000 rpm.


                • #9
                  No it is not set at 3600.
                  The R2.8 is governed at 4200 rpm. It will rev to this rpm and even past slightly. It cuts fuel delivery at 4200, from my experience. However there is no gain to spin this engine this fast because past 3600 you are not making more power, always better to grab the next gear.


                  • #10
                    depending on how fast the fall off is past 3600, it may make sense to keep it there. In my scout with the T-19 transmission 3.54 gears, 3600 rpm in 3rd gear tops out right at 65, and there's a noticeable drop in at-the-wheel-torque between 3rd and 4th at that point, so if I'm climbing a hill in 3rd, it would be nice to be able to continue a little faster, even if the power is dropping off.

                    but that's the reason for wanting the economy curve, it may be that above 3600 it starts to be significantly less efficient.

                    but more info is always better :-)

                    anyone have any idea how quickly power tapers off between 3600 and 4000 rpm?


                    • #11
                      allowing the engine rpms to continue up to 4200 (assuming power drops off just worse than linearly, to 190 ft pounds at 4200) it ends up with the total HP dropping off above 3600, but if you are trying to squeek out the most power, 3rd gear is still better than 4th up to the rev limiter when power in 3rd drops to match the power in 4th, at 80 (between 40 and 50 there is also what may be a noticeable difference)

                      I'm probably overthinking this, and shouldn't be pushing as hard (too many years driving underpowered vehicles where I need to know what I can do)


                      • #12
                        Here is the the fuel consumption at both governed power and peak torque:
                        Fuel Consumption Governed HP
                        27.6 kg/hr (60.8 lb/hr)@3600RPM
                        Peak TQ
                        15.1 kg/hr (33.3 lb/hr)@1700RPM