4x4ord
Alberta
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RoyJ wrote:4x4ord wrote:Grit dog wrote:Good grief...
Yup, lot's of confusion. I'm not sure what happened. Immediately after posting my torque curve reflecting 430 peak HP I checked Ram's website and saw they are advertising 420 HP, so, I changed my torque graph within about an hour. I don't know if Roy got a copy of my graph before I changed it or if he was thinking 430 HP from other sources. You can find claims anywhere from 400 to 430 HP for the 2021 Cummins.
Edit: I'm almost certain that even Ram's website was initially showing 430 HP for the HO engine but they currently are advertising 420 HP.
Let's me try reshaping the equation for 420 hp. I too was confused, some sites quoted 430 hp, some 420.
I don't believe the official specs are on Ram's site yet, but I do see 420 stated on Cummins' site:
https://www.cummins.com/news/releases/20........dutylegendarypickupenginebetterever
What we really need is the official tq / hp curve. If the curve has flat hp plateau like we see on the marine QSB, then a 6 spd vs 10 spd would make very little difference in flat out performance.
In fact, tonight I'll try it for fun with the QSB curve. Let's call it a lightly modded / chipped truck  i.e. programmer in "tow" setting. I bet it'll make absolutely 0 difference (engine hovers @ constant hp during an upshift)
The Ram site is showing 420 peak HP but doesn't say rpm and the peak torque of 1075 and again doesn't mention the rpm.
I would be willing to bet the torque curve I drew is very close to what the actual torque curve will be. If you want a torque curve that would produce a perfectly flat power curve, it would look like this:
Such a torque curve would generate a power curve like this:
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4x4ord
Alberta
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Here are some rpm points along that torque curve with the corresponding torque outputs:

RoyJ
Vancouver, BC
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4x4ord wrote:Such a torque curve would generate a power curve like this:
I bet you're not far off. Here's the power curve from the 419hp marine QSB 6.7:
If that's true, no calculations needed  even with the Aisin, you'd just have a constant 420 hp at any reasonable towing speed.
I once wrote a HPacceleration model in excel when bored working at Daimler. It takes a dyno curve, fitted with an equation, and then uses a finite element model concept to chop up time into 0.1s segments.
I've checked it against magazine 1/4 mile times and it's accurate within 5%. But for trailers, the tough part is finding coefficient of drag and cross section area. At 37k lbs GVW it may not matter much, but with light trailers and 70 mph speeds it would.
Maybe I'll dust it off for fun. I can try parameters like initial speed = 65 mph, grade 7%, GVW 30k lbs, and see how long / how much distance it takes for the rig to slow down to a constant speed.

4x4ord
Alberta
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^^^ I figure a Ram towing a typical 20,000 lb toy hauler should get around 9 mpg at 62 mph. If you use the BSFC map that we have and figure the Cummins will be running in the 205 g/kwhr area of the bsfc map we can come up with the kw required to tow a 20k trailer at 62 mph. 9 miles per gallon is 22182 grams of fuel per hour. 22182/205 = 108 kw or 145 HP. From there you should be able to come up with what you think are reasonable numbers for surface area, rolling resistance and coefficient of drag.
(Keep in mind that the calculated power requirement is at the crankshaft so rear wheel hp would only be about 123 to 125 hp.)
* This post was
edited 01/13/21 02:03am by 4x4ord *

Me Again
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Cummins12V98
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" 6 spd vs 10 spd would make very little difference in flat out performance."
I have said this for a LONG time.
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Grit dog
Black Diamond, WA
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Cummins12V98 wrote:" 6 spd vs 10 spd would make very little difference in flat out performance."
I have said this for a LONG time.
I hope there are more folks that think this...should boost the value of my 6 speed and reduce the demand/cost for a new 8 or 10speed for the next upgrade.
Not holding my breath though....
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Flashman
Tucson, Aizona, USA
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It a great sales gimmick though.

RoyJ
Vancouver, BC
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4x4ord wrote:^^^ I figure a Ram towing a typical 20,000 lb toy hauler should get around 9 mpg at 62 mph. If you use the BSFC map that we have and figure the Cummins will be running in the 205 g/kwhr area of the bsfc map we can come up with the kw required to tow a 20k trailer at 62 mph. 9 miles per gallon is 22182 grams of fuel per hour. 22182/205 = 108 kw or 145 HP. From there you should be able to come up with what you think are reasonable numbers for surface area, rolling resistance and coefficient of drag.
(Keep in mind that the calculated power requirement is at the crankshaft so rear wheel hp would only be about 123 to 125 hp.)
I'm trying the Powerstroke with 10 spd first, as it's a real combo and we can calibrate against real life performance.
First the curve fitting based on your numbers, a 5th order poly provides near exact fit, with R^2 of 0.9998:
I recalculated hp using the equation in the last column, and numbers are near perfect to the input.
For the model, I used a mass of 43,500 lbs, full GVW of an F450. Cross section area of 75 sq ft, based on J2807's max for travel trailers. I guessed a Cd of 0.80. Tire diameter based on 245/75R17, and tire coefficient of 0.015. I'm starting out full throttle on 7% grade, as a "torture test".
The model uses a column for each gear, each row is 0.1s time step. Based on initial launch rpm (2000), it uses the hp curve to calculate final net thrust: gross thrust minus air drag, tire drag, and gravity. Net thrust calculates instant acceleration, which gets ups the new starting speed for the next 0.1s time step:

RoyJ
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Here's the speed vs time output. My gear shifts are 200ms, during which I assigned an arbitrary 50hp (difficult to estimate engine output during clutch pack switchover):
So @ 60s, our rig reaches 43.4 mph. At 120s, 43.8 mph, which is pretty max our Vmax climbing this grade. This happens at 2881rpm in 4th, hp is 464.
Here we can see the gross thrust, before aero, tire, and gravity drag vs speed. Think of this as "drawbar pull" on a tractor:
17.67 kN, or 3971 lbs. Also in 4th of course.

