Anyone have firsthand experience going from 22.5's to tall rubber tires?

I would say you're right about past models of DD15's, but the current offering is one engine I was told, no internal differences, from 400 to 505 HP and ff you want more power than that you have to order a 16L DD.
That's what I was told yesterday when an old trucker friend called out of the blue and this thread popped into my head.
He sells WS's these days and I've known him for over 40 years so I trust him, plus I get a free lunch tomorrow!

Seems most engine manufacturers are using one or two base hardware packages with multiple performance curves and power levels to keep manufacturing costs down, (vertical integration), you pay for the extra HP if you want it.

 

I'n not an expert either, but I know for sure my IQ is a little higher than the average bull moose, a solid double digits.

Clear your thoughts young man, you're over complicating a simple performance reference chart.

All modern engines are controlled by a computer according to the variables programmed into it, which the manufacturer considers a safe and economical operating range for that engine and is expected to last at least until after the warranty expires, there's a minimal chance for much to go wrong, aside from how they are driven, and in stock trim they are pretty much driver proof in that respect.

Turbo over speeding, under fuelling, all the tech talk, it's all irrelevant to the conversation, give it no more thought for the moment and focus on the two lines and what they say about the relationship between TQ and HP and economy to you.

Personally, acceleration means nothing to me when talking trucks, I squeeze the pedal a little and the rpms will eventually get to where I want them to be, then I level out or shift gears.
Now if we were talking drag racing...................!

Given the choice, I'm estimating that 90% of truckers would choose the hi TQ engine, which I understand, gear fast drive slow, shift less gears and the perception of quicker acceleration.

The low TQ engine, as I see it, is more of a driver's engine, also the type of
TQ line you would see on a severe service application like a fire truck or off road vehicle, sorta like a marine engine that operates at high revs or WOT.

Theres a little more shifting involved with the low torque engine but it has the upper hand in the 1700 to 1900 RPM range, which for me is desirable for pulling heavy weights, climbing long steep grades, bucking heavy winds or extended periods under heavy load, and I dare say, maybe the more economical of the two, if driven correctly.

At the higher RPM there's more air and coolant flowing through the engine which helps to keep the temps down and less cylinder pressures and stresses on the bottom end than at lower RPM's, and doesn't necessarily mean that higher RPM equals more fuel used.
Drop a gear and the RPM's go up but the engine isn't working as hard and no guarantee that you're using more fuel.

Both engines will lope along all day at 1350 RPM pulling 80,000 lbs or less at 65 to 70 mph, but step it up to 100,000 lbs where the manufacturers recommend gearing to cruise at higher RPM's, typically around 1500 RPM.

The hi TQ engine will be running on the upper edge of it's potential economy range and have no place to go but down from peak torque where the lo TQ engine is good for another couple hundred RPM before the potential economy range falls off at 1700 RPM.

If you need more torque or more HP, just drop a gear and you're one up on the hi TQ engine, that's what them transmission thingies are for, keep that engine in it's power band, just flip that button on the handle and you have instant gear reduction, therefore more grunt.

Are ya feelin me dog?

I pulled A and B trains for years and later on tri axles where the weights were typically from 100,000 to 135,000 lbs and you would be hard pressed to convince me that grabbing a gear on the upside of a hill at lower RPM's is easier, quicker or more beneficial than grabbing a gear at higher RPM's.

Most engines I ever drove seem to come down quicker from a higher RPM than low or middle RPM and I suspect physics and gravity has something to do with that, along with the Jakes that supply more engine braking power on the high end than the low end of the RPM spectrum.

Now I may be rambling out in left field, would appreciate a heads up if I am, stop me before I dig a deeper hole!

Gonna take a break, need more coffee.

 

Take a closer look at the graphs. From 1800 on up, maybe 1700, they look identical to me.

I'm with you on the rest. I'm the guy that'll drop a gear just before the bottom & ride the governor to climb a hill instead of shifting 1/2 way up & then shifting again because of the time to shift & momentum lost during the shift.

I also downshift for better jakeing too. I try to stay off the brake pedal.

I also like to run a lower gear than most in bad conditions too. I find the motor easier to control at higher RPMs than low. Like when you roll in the pedal at low RPM, boost finally builds a bit & blows the tires off bad & take a while to get traction back. The lower gear though has more gearing to help the motor, plus better response from the engine anyways, so if you do give it too much, just back out if it & the tires grab again pretty quick.

I'm working on getting some more coffee too lol.

 

Actually, no.
As I understood, my friend said there were "NO" differences.
I said internal because that would be a safer statement for me to make in case I was misunderstanding him.
I'm making the assumption he knows what he's talking about since probably 80 or 90% of the trucks he sells have 15L DD engines in them.

I'm going to ask him again for curiosity sake, maybe make a call to DD if there's any uncertainty.
Or........., we could all call DD and compare notes!

 

You're probably right about the graph.
I've been staring at them, and every time, I either lose focus or see something different than the last time I looked.

Kudos to you for having a positive attitude, being enthusiastic about the fine points of driving and having a solid understanding of the relationship between engine, transmission and gearing, and pointing out any mistakes I might have made.
Sometimes my brain is firing on all 8 cylinders, sometimes only 3 or 4.
That's what too many concussions, long time medical weed use, near death by cancer and heavy chemo drugs will do to you.
Good thing is, I don't remember a lot of my old girlfriends and stupid things I did in my youth!