2021 Cummins 1075 lbft peak torque at 1356 rpm

Roy the more I look at your spread sheet the more I’m impressed. Good job.

I made a mistake on the power required to tow 18000 lbs up a 7% grade. It should take about 400 Hp to climb a 7% grade at 50 mph.

Grit dog wrote:

4x4ord wrote:

RoyJ wrote:
Referring back to those BSFC maps I posted, if we want the absolute best mpg, then there's only a small windows (dark blue), between 1500 and 1950 rpm, and 55% to 90% torque output.

That's where a 10 spd would be beautiful.

Roy I think the 3.31 rear end and 5th gear with the Aisin would be hard to beat when towing heavy and when towing light or running empty 6th with 3.31 gears is tough to beat. To bad they did away with 3.31 gears.

4x, this sounds just like a Kaytegism.
IE: It makes no sense. If what you said is true, you’re suggesting the mfg of said vehicle is completely back asswards in their axle offerings. Well, them and every other person who knows generally deeper gears pull better and offer greater fuel efficiency when towing heavy.

You’re right 4.10 would offer more ratio selections for pulling grades but with 1075 lbft of torque I don’t need more than 3rd and 4th for most hills with 3.31 rear gears. 5th is about perfect for heavy towing on level ground which leaves 6th for light towing and running empty. I don’t understand why Ram dropped the 3.31 rear end especially now with more torque.

I don't think it worth doing the math to see how much faster you'd be able to pull the hills but your chart demonstrates a clear advantage on the steeper hills where you could gain as much as 15 HP over the 6 speed. And as you've pointed out on very tough pulls the drop to 2nd might be a bit of a hole. I think the Cummins can put out good power to at least 3000 rpm though and 3000 rpm in 2nd is 41 mph. A hill that would bring the Cummins to its knees in 3rd gear (down to 40 mph) pulling an 18000 lb trailer would be about a 10% grade.

I also see according to your chart I should have dropped the torque off slightly faster after after 2800 rpm ... (peak HP is supposed to be 420 @ 2800 and your chart is showing 423 HP at 2892rpm.... oops)
(Edit: I checked the data I put in to draw the chart and @ 2900 rpm I have the torque at 752 lbft so 415 HP. I have the curve dropping off a little sharper after 2600 rpm so that peak power of 420 is obtained at 2800 rpm.)

For fuel economy purposes pulling hard at 60 mph is probably a good scenario to run (201 HP or 150 Kw) .... This might be towing into a stiff enough wind to bring the fuel economy down to 6.7 mpg.

I'd be 5th gear 1674 rpm and 856 Nm ... so 192 g/kwhr and 6.7 mpg
You'd be 8th gear/1848 rpm and 775 Nm .... and the same 192 g/kw hr and 6.7 mpg

Then it is probably good to see what happens with a lighter trailer ... say one where we are able to get about 11 mpg at 65 mph.

This would be when 93 KW could carry us along at 65 mph.

We'd be in high gear at very similar rpms

I'd be 1484 @ 598 Nm
You'd be 1489 @ 596 Nm
We'd both be 205 g/kwhr or 11 mpg

4x4ord wrote:
Towing 63 mph I’m at 1750 in 5th and putting out 600 Nm (110 Kw) or 1432 in 6th putting out 733 Nm (110 Kw) and getting the 9 mpg.
70 mph empty we are both at 1594 in 6th getting 18 mpg putting out 333 Nm. (55 Kw)
Pulling a 7% grade I’m able to do 45 mph at 2170 rpm putting out 1277 Nm (290 Kw) and according to the bsfc map I should be using 205 g/kwhr which would put me at 2.44 mpg and again you’re not going to do any better.

We're probably splitting hairs, as this is all theoretical. Using 388 hp as the target, 10 spd:



6 spd:



Both are pretty much 100% throttle.

What's more interesting is if the grade gets steeper, the 10 spd can drop TWO gears, down to 4th, and still maintain 45 mph. It might lose 1 mph in order to bring the rpm from 2892 down to 2800, where it develops the full 430 hp.

But the Aisin would be forced to 2nd and slow down to 38.6 mph

4x4ord wrote:
I missed a scenario. I think towing into the wind at 55 mph would take us about 150 Kw. I’d be running in 5th @ 1528 rpm so 937 Nm. That should put me about 200 g/kwhr and 5.9 mpg.

You’d run in 8th @ 1687 and be putting out 849 Nm. You should be close to the middle of the sweet spot say 192 g/kwhr ..... 6.1 mpg!

I used a target of 201 hp and picked the gear that falls into the dark blue land on the BSFC.

1st up, the 10 speed:



Now the 6 speed Aisin:



Here's the fun part, with the 10 spd: 1694rpm @ 623 lbft (845 Nm) falls right in the middle of the dark blue region.

With the 6 spd, 1535rpm @ 691 lbft (936 Nm) just falls outside by the old map. So with the 2019 engine, we theoretically lose about 5% fuel efficiency. But I wonder if coolant temp / egt is better at the 10 spd's higher rpm.

4x4ord wrote:
Your 10 speed sounds like a good set up. I’ll take you on with the 6 speed Aisin if I can have 3.31 gears. I’ll bet we will find the extra gears aren’t going to buy you much. How about we pull an 18000 lb toy hauler cruising down the highway a 63 mph and say you get 9 mpg with your rig. Then we’ll go 70 mph empty and say you get 18 mpg with yours. Then we’ll climb a 7% grade towing and see how they compare. Then we can compare how they would do heading into a 20 mph wind towing at 55.

Looks like my excel screenshots disappear, Google image hosting doesn't work.

I took the estimated torque curve you posted, fitted it with a polynomial equation which I then used to calculate the Tq and hp at every speed / gear combination:



You'll notice a "Tq %" input highlighted with a red box. With this, we can play with different throttle setting, and see if we can get into the magical [550 - 1000 lb-ft] + [1500 - 2000 rpm] range.

For example, at 60% throttle, I've highlighted (in darker blue) where torque remains above 550:



I'll let you calculate out the scenarios to see what grades can be pulled at the corresponding hp values.

I missed a scenario. I think towing into the wind at 55 mph would take us about 150 Kw. I’d be running in 5th @ 1528 rpm so 937 Nm. That should put me about 200 g/kwhr and 5.9 mpg.

You’d run in 8th @ 1687 and be putting out 849 Nm. You should be close to the middle of the sweet spot say 192 g/kwhr ..... 6.1 mpg!

Towing 63 mph I’m at 1750 in 5th and putting out 600 Nm (110 Kw) or 1432 in 6th putting out 733 Nm (110 Kw) and getting the 9 mpg.
70 mph empty we are both at 1594 in 6th getting 18 mpg putting out 333 Nm. (55 Kw)
Pulling a 7% grade I’m able to do 45 mph at 2170 rpm putting out 1277 Nm (290 Kw) and according to the bsfc map I should be using 205 g/kwhr which would put me at 2.44 mpg and again you’re not going to do any better.

Edit: I'm barely into the bottom of the dark blue at 63 mph/1750rm and 600 Nm ... so probably 200 g/kwhr 9.22 mpg

just outside the dark blue at 1432 rpm in 6th and 733 Nm
.... so maybe 205 g/kwhr or 9 mpg

At 63 mph your at 1569 rpm in 8th and 669 Nm. So 195 g/kwhr/9.46 mpg

RoyJ wrote:

4x4ord wrote:
^^^^ It would be nice to see the bsfc map for the 2021 Cummins. You are right that keeping the torque high and rpm low saves fuel for the heavy trucks that pull hard all day. How or where are you seeing more gears helping the Ram pickup get better fuel economy? Where on that map would you like to see a pickup truck engine run while towing a 16k - 18k trailer down the highway?

Let's build an imaginary pickup heavy hauler. 2021 Cummins dually, 10 spd Ford transmission. We'll have to assume that bsfc map applies to the new engine, with the same torque curve, just shifted up to 1458 N-m.

235/80r17 tires turn 654 rpm @ 60 mph. Let's start with 3.31 gears, here's the rpm in each gear from 45 - 75 mph:



This is where the beauty of the 10 spd shows - at every given speed we can put the engine into the rpm where peak bsfc occurs (~1500 - 2000 rpm). 6th & 7th @ 45mph, 7th or 8th @ 50, 7th & 8th @ 55, 8th & 9th @ 60, 8th & 9th & 10th @ 65, 9th & 10th @ 70, and also 9th & 10th @ 75.

The BEST part is we have 2 to 3 gears to choose and still stay within the optimal RPM. This means we can also pick the gear depending on load to get the engine torque into the optimal range.

Your 10 speed sounds like a good set up. I’ll take you on with the 6 speed Aisin if I can have 3.31 gears. I’ll bet we will find the extra gears aren’t going to buy you much. How about we pull an 18000 lb toy hauler cruising down the highway a 63 mph and say you get 9 mpg with your rig. Then we’ll go 70 mph empty and say you get 18 mpg with yours. Then we’ll climb a 7% grade towing and see how they compare. Then we can compare how they would do heading into a 20 mph wind towing at 55.

rhagfo wrote:

Grit dog wrote:
^ Totally agree. And therein lies the best age old argument for a manual trans that ever existed!
I'm smarter than a computer! FWIW, I still believe some of us are when it comes to shifting, as good as the autos have gotten.
One thing to consider is, one can still play big trucker with the little +/- button on the shifter.
Of course we're splitting hairs now. 4speeds, not enough to use a diesel engine most effectively. 5speeds like the torqshift and Alli was a huge improvement. 6 speeds adds the top empty haul @ss across the prairie bobtail gear. 8 to 10 speeds is having your cake and eating it too and 10 is getting a little busy for even a relatively high revving diesel like a pickup truck. I personally think its a couple too many cogs in a gasser truck or car. 8 is the sweet spot for V8 gassers IMO.

If I was strictly using a pickup for towing (not in the city) and hauling, I would still prefer a manual for the exact reason you just mentioned. BUT, based on years of ownership of the last light truck manual trans made (G56 in a Dodge), it is short at least 1 gear maybe 2, to be ideal for heavy work.

Well Grit Dog the biggest issues is that nowadays automatics are stronger than manual transmissions. I will say there are days I miss rowing the gears in our 2001 Ram 2500, but will add towing with the HO/Aisin combination sure is sweet.
The biggest issue with offering a pickup with a manual today is warranty cost! You would have bad drivers tearing up clutches and drive-trains.
To be able to have a clutch able to handle power it would need to be a dual or triple disk clutch.

While I don’t disagree that newer automatics are markedly better than in the past and do really great towing and are very strong, almost intuitive, your perceived “issue” with manual transmissions is about 100% wrong, IMO.
Warranty issues? Clutches aren’t warrantied anyway. And you make it sound like I’m suggesting that manuals be the only option. Pretty sure, no, confident, that most all people getting a manual trans in a truck actually know how to drive one. If for no other reason than if they don’t, it would be miserable for them, so they wouldn’t buy one.
Tearing up the trans? That’s funny. How many times do you think the same person who has the desire to work harder driving their vehicle will repeatedly screw up shifting enough to actually make that happen.
And manuals being too weak? That’s the silliest reason of all. First of all, it’s pretty easy to pair an adequate manual to the power in front of it. Second, let’s use the last and only pickup manual offered since Bush was president as the example.
I had one for 180k miles, just sold it. Running behind an engine putting out as much or more torque than any current model diesel pickup. Single disk clutch, zero transmission issues. With zero regard as to how far I planted the throttle in any gear. More bomb proof than all but the best current model autos on the market. And likely more bombproof than those too.
Because your preference is now autos for towing doesn’t mean manuals are suddenly junk. But there’s also not the option any longer. So, you win!