I keep being struck by how it takes around 1800 rpm or a little more to get the (totally unloaded) truck to accelerate with traffic from a stop. It's pretty obvious just from seat-of-the-pants feeling that the engine would be fine with more coupling to the wheels at less rpm. I bet if the converter really tightened up at about 1400 or 1500 rpm instead of 1800 or 1900, gas mileage would go up, and the truck would respond better to light throttle.

I suspect that aftermarket converters for these trucks are tighter than stock instead of looser. Anybody know for sure?

 

Aftermarket's stall at about 1500-1600. Your right though. The stock converter is WAY to loose on these trucks. If I'm correct, it's the same converter they use on the gas engines which is to tight for those.

 

They use the same converter behind the hemi in the 2500's? Now THAT'S a dumbedit idea. edit , it'll stall HIGHER behind the diesel because of the extra torque-exactly what the diesel does not need! Surely the engineers can see the connection between a too-loose converter and elevated transmission temps! Not to mention the fuel mileage penalty. When my truck shifts from 2nd to 3rd under very light throttle, sometimes you can't tell. That's a sign of a too-loose converter if there ever was one.

 

A tight converter might feel better for driving around empty like my '98 5spd manual did but for towing 10,400 lbs, it's a lot better when I can combine that torque with rpm to make some serious horse power at a relatively low 2100rpm to launch and accelerate up to speed. Under 2100 produced low boost and the high torque isn't realized with a heavy load. My 5spd would fall on it's face with each shift until I got back into it's power band of 2,000-2500rpm. With the 4.10 rear end I was turning 2500rpm at 70mph in O/D. Yes it was loud and sounded like I needed another gear but it was unstopable once up to speed.
We refer to diesels as torque engines but what that means is torque is at a low rpm. Torque in motion is horsepower. The faster you apply that torque the more horsepower you have. My SO 460 makes as much horsepower at 2,000rpm as a Hemi does at 3,000rpm. The Hemi would have to run 5,200rpm to make as much horsepower as the 600 makes at 2,900rpm. It would have to run around 4,200rpm to make as much horsepower as the 600 makes at 2,000rpm. So yes, torque is what moves the diesel at such a low rpm. But that low rpm torque in motion is horsepower. High torque at low rpm also means high horsepower at low rpm. Torque in motion is horsepower. The reason we think of gas engines as horsepower engines is because the relatively low torque requires high rpm to make horsepower.
Torque X rpm / 5252 = horsepower. If you play with the numbers of the Hemi and the 600 Cummins and compare the two, you'll see what I'm trying to say. I believe the engineers did their homework and a 2100rpm stall speed torque converter works best with the Cummins. Take a manual trans Cummins and run in direct drive at 1400rpm peak torque. Floor the throttle and see what happens. Now run it in direct drive at 2500rpm and floor it. It'll about snap your neck at 2500rpm because you've made some serious horsepower with that high torque applied at a higher rpm. Still very low compared to 5200 with the Hemi.

 

Can't argue with your point about the hemi's rpm range vs the diesel's rpm range. However, I think they went too far with the loose converter.

There's something about converters you didn't mention. Under a heavy load, they tend to stall higher. That's why a heavy car will "push" a converter to a higher rpm than the same converter in a light car. So, when towing heavy loads, the converter tends to stall higher, which helps you.

The behavior of your manual is not really applicable in this situation, as you have neither the benefit of torque multiplication in the converter nor the penalty of the slippage it produces.

There's no reason to have the diesel revving as high as it does to get an empty truck moving at a reasonable speed. The aftermarket converters for these trucks stall at a lower speed than stock. There's a reason for that.

 

It really isn't the same as the one used for gas motors. But, it is way too loose. Even a stock motor benefits from a tighter one.
The DTT trans in my truck was the single best improvement to date.

 

You're right with the heavy load senario but what you're seeing with light load hard launch isn't stall speed. It's when engine power satisfies the TC's need for forward acceleration. Stall speed is when the engine cannot produce any higher rpm with WOT. If you hold your brakes on hard for a complete stop no motion and floor the accelerator with the tranny in drive, the rpm that you're able to make is stall speed. If you do this, don't do it for more than 5 seconds. If a clutch slips or the fluid is already hot, you can spike the tranny fluid temp in the TC or clutches and do damage before the tranny temp gauge shows a difference.
I'll agree a tighter TC would feel better but I don't think you'd see an improvement in 1/4 ET at the drag strip. If I floor mine from a stop, it shifts right at 3,000rpm 1-2 and only drops to about 2500rpm keeping boost up and horsepower cranking thanks to the high torque of the Cummins. I'm happy with mine but not in any way trying to say a tighter TC isn't right for the next guy. Especially if he's upped the power to his Cummins.

 

You're dead on with the definition of stall speed, but a loose converter slips more below its stall speed than a tighter converter. That's why you don't run a 5000 rpm stall converter on an everyday driver. Yeah, it may not slip to the stall speed every time you pull away from a light, but it's going to slip a lot more than a 2000 rpm stall speed converter. That's my real point-it slips a bunch more than necessary when you're just driving around without a heavy load. It might be a fairly good stall speed for the cummins on the dragstrip, but not when you're just cruising. My basic problem was with the way the converter acts under LIGHT throttle, when you're just driving around. The motor is capable of making plenty of torque below where the converter allows it to rev to when just easing into it. It's a waste to not use that part of the rpm band, and doubly so because low rpm is where the least power would be wasted on internal friction, if only the converter transmitted a decent amount of power down there instead of slipping.

 

Sounds like a great opportunity for a torque converter mastermind to design and manufacture a TC that's tighter up to it's optimum 2100rpm stall speed! That and even for launching with a heavy load, a 1,000rpm lead over idle would get the rpm up into the power band by 15mph anyhow so I could be just as happy if DC used a lower stall speed converter from the factory. How bout one that's almost nill at idle for better mpg in town sitting at traffic lights (I often slip mine into neutral at long lights) but then tightens up just off idle.
You're making me wish I'd opted for another manual tranny.

 

Yeah, I put mine into neutral too. Actually, I put almost every automatic I drive into neutral. I can't stand the thought of sitting there using extra gas to heat the transmission, even if it's only a little.

 

Many guys who have changed to tighter converters have notice large improvements in ET at the drags. I can't quote them.