Ram Pentastar towing

[supton]

Just for fun yesterday I perused Ram's website for the V6 quad cab truck in regards to towing. I was kinda surprised: if you order with 3.21 rear gearing (4x4, quad not crew, 6'4" bed) it is about 4,400 lb rated. But with the slightly deeper gearing of 3.55 it jumps to 7,400 lb. Seems a rather large jump for something that has relatively close spaced gear ratios in the 8AT. I mean, takeoff ratio is better (11%), but that's just off the line, and after that it shouldn't matter. Transmission picks the gear required for the rpm to make enough hp to accomplish what is being asked of it.

Watched a TFL vid the other night where they ran the Ecodiesel up the Ike. They seemed a bit taken back that it wouldn't do more than 55mph. 420ft-lb torque is nice, but in the end, once moving, 240 hp is still only 240 hp. At least it was still (mostly) 240hp & not suffering from altitude power loss like a NA engine would.

 

[dlundblad]

3.21 for towing does seem useless.

 

[supton]

Originally Posted By: dlundblad
3.21 for towing does seem useless.


it does, but... 4.71:1 first gear.
https://en.wikipedia.org/wiki/ZF_8HP_transmission

4.71x3.21 is an impressive 15.1:1 takeoff ratio. 4.71x3.55 is 16.7:1. I wonder if at that point torque management is required with the Pentastar.

My truck by comparison is 3.52x4.1 or 14.4:1. The mighty 5.7 version is 3.33x4.3 or 14.3:1. But my 4.6 and the 5.7 do have a torque advantage, at 2,000 rpm (plus whatever the torque convertor adds for multiplication).

 

[WyrTwister]

Tire diameter also plays a part .

 

[KrisZ]

HP do doesn't tow, it determines the rate of acceleration and top speed, torque is what does the towing. Lowering the final drive ratio increases the torque at the wheels, hence a bump in the towing capacity.

 

[supton]

Torque is applied force while hp is how fast work is (or can be) done. On an engine with a flat torque curve, downshifting does two things: it increases applied force (through gear multiplication), and it also gets the engine spinning to a higher hp level. The vehicle (usually) accelerates when doing this.

I'm not sure you can separate tq from hp. You can't use gear multiplication to make more hp, but with the right gearing you can get the engine to sit at a certain hp.

The difference between 3.21 and 3.55 is 11%. 11% more torque if you'd like, in a given gear, or 11% more hp at a given vehicle speed. But that's way more than an 11% bump in towing. Clicking down through the gears, 8th->7th is 26%, 7->6 is 19%, 6->5 is 29%, 5->4 is 30%, 4->3 is 26%, 3->2 is 49% and 2->1 is 50%. So, what I'm thinking is, once moving, running one gear lower is more of a tq bump than what dropping from 3.21 to 3.55 is.

 

[dlundblad]

Originally Posted By: supton
Originally Posted By: dlundblad
3.21 for towing does seem useless.


it does, but... 4.71:1 first gear.
https://en.wikipedia.org/wiki/ZF_8HP_transmission

4.71x3.21 is an impressive 15.1:1 takeoff ratio. 4.71x3.55 is 16.7:1. I wonder if at that point torque management is required with the Pentastar.

My truck by comparison is 3.52x4.1 or 14.4:1. The mighty 5.7 version is 3.33x4.3 or 14.3:1. But my 4.6 and the 5.7 do have a torque advantage, at 2,000 rpm (plus whatever the torque convertor adds for multiplication).


How would this handle at highway speeds though?

 

[IndyIan]

Originally Posted By: KrisZ
HP do doesn't tow, it determines the rate of acceleration and top speed, torque is what does the towing. Lowering the final drive ratio increases the torque at the wheels, hence a bump in the towing capacity.

Well, torque multiplied by gearing is what tows. I doubt either rear end ratio will hold 8th with a 26' camper trailer behind them into any kind of headwind, so what does it matter if the truck runs in 7th with the 3.55 or 6th with the 3.21? Torque to the wheel will be the same, engine rpm will be close to the same. I suppose higher ratio rear ends run a bit cooler, but I doubt that is an issue.
Perhaps how the transmission is designed, running in certain gears for hours on end isn't good? or maybe the higher ratio diff comes with bigger rear brakes?

 

[WyrTwister]

Originally Posted By: supton
Torque is applied force while hp is how fast work is (or can be) done. On an engine with a flat torque curve, downshifting does two things: it increases applied force (through gear multiplication), and it also gets the engine spinning to a higher hp level. The vehicle (usually) accelerates when doing this.

I'm not sure you can separate tq from hp. You can't use gear multiplication to make more hp, but with the right gearing you can get the engine to sit at a certain hp.

The difference between 3.21 and 3.55 is 11%. 11% more torque if you'd like, in a given gear, or 11% more hp at a given vehicle speed. But that's way more than an 11% bump in towing. Clicking down through the gears, 8th->7th is 26%, 7->6 is 19%, 6->5 is 29%, 5->4 is 30%, 4->3 is 26%, 3->2 is 49% and 2->1 is 50%. So, what I'm thinking is, once moving, running one gear lower is more of a tq bump than what dropping from 3.21 to 3.55 is.


I drove a delivery truck for several years , in my youth . All were gasoline powered , none with an engine larger than 366 ci , which many would consider way under powered . With enough gears , RPM's and realistic expectations , it did the job . 3250 RPM at 60 - 70 MPG was about where things ran . MPG was about 5 .

 

[supton]

Originally Posted By: dlundblad
Originally Posted By: supton
Originally Posted By: dlundblad
3.21 for towing does seem useless.


it does, but... 4.71:1 first gear.
https://en.wikipedia.org/wiki/ZF_8HP_transmission

4.71x3.21 is an impressive 15.1:1 takeoff ratio. 4.71x3.55 is 16.7:1. I wonder if at that point torque management is required with the Pentastar.

My truck by comparison is 3.52x4.1 or 14.4:1. The mighty 5.7 version is 3.33x4.3 or 14.3:1. But my 4.6 and the 5.7 do have a torque advantage, at 2,000 rpm (plus whatever the torque convertor adds for multiplication).


How would this handle at highway speeds though?


Well, I doubt anyone would want to be running around cones at highway speeds... might be fun on a closed track, perhaps. Maybe you mean, how fast will it accelerate, 0-60 or 40-60? That'll depend upon the grade, wind resistance and one's tolerance of horns honking behind you.

 

[dlundblad]

Originally Posted By: supton
Originally Posted By: dlundblad
Originally Posted By: supton
Originally Posted By: dlundblad
3.21 for towing does seem useless.


it does, but... 4.71:1 first gear.
https://en.wikipedia.org/wiki/ZF_8HP_transmission

4.71x3.21 is an impressive 15.1:1 takeoff ratio. 4.71x3.55 is 16.7:1. I wonder if at that point torque management is required with the Pentastar.

My truck by comparison is 3.52x4.1 or 14.4:1. The mighty 5.7 version is 3.33x4.3 or 14.3:1. But my 4.6 and the 5.7 do have a torque advantage, at 2,000 rpm (plus whatever the torque convertor adds for multiplication).


How would this handle at highway speeds though?


Well, I doubt anyone would want to be running around cones at highway speeds... might be fun on a closed track, perhaps. Maybe you mean, how fast will it accelerate, 0-60 or 40-60? That'll depend upon the grade, wind resistance and one's tolerance of horns honking behind you.


Im talking just pulling a trailer at highway speeds with 3.21 gearing. How would this behave?

How about a 0 - 60 from a light or ~20 - 75 on an interstate ramp?

 

[supton]

Originally Posted By: WyrTwister
Originally Posted By: supton
Torque is applied force while hp is how fast work is (or can be) done. On an engine with a flat torque curve, downshifting does two things: it increases applied force (through gear multiplication), and it also gets the engine spinning to a higher hp level. The vehicle (usually) accelerates when doing this.

I'm not sure you can separate tq from hp. You can't use gear multiplication to make more hp, but with the right gearing you can get the engine to sit at a certain hp.

The difference between 3.21 and 3.55 is 11%. 11% more torque if you'd like, in a given gear, or 11% more hp at a given vehicle speed. But that's way more than an 11% bump in towing. Clicking down through the gears, 8th->7th is 26%, 7->6 is 19%, 6->5 is 29%, 5->4 is 30%, 4->3 is 26%, 3->2 is 49% and 2->1 is 50%. So, what I'm thinking is, once moving, running one gear lower is more of a tq bump than what dropping from 3.21 to 3.55 is.


I drove a delivery truck for several years , in my youth . All were gasoline powered , none with an engine larger than 366 ci , which many would consider way under powered . With enough gears , RPM's and realistic expectations , it did the job . 3250 RPM at 60 - 70 MPG was about where things ran . MPG was about 5 .


Yep, keeping dropping gears, keep the engine at or near peak hp. In the end, that 366 was what, 200hp? You could only ever do 200hp of work at most.

 

[KrisZ]

Originally Posted By: IndyIan
Originally Posted By: KrisZ
HP do doesn't tow, it determines the rate of acceleration and top speed, torque is what does the towing. Lowering the final drive ratio increases the torque at the wheels, hence a bump in the towing capacity.

Well, torque multiplied by gearing is what tows. I doubt either rear end ratio will hold 8th with a 26' camper trailer behind them into any kind of headwind, so what does it matter if the truck runs in 7th with the 3.55 or 6th with the 3.21? Torque to the wheel will be the same, engine rpm will be close to the same. I suppose higher ratio rear ends run a bit cooler, but I doubt that is an issue.
Perhaps how the transmission is designed, running in certain gears for hours on end isn't good? or maybe the higher ratio diff comes with bigger rear brakes?


Torque multiplied by gear ratio is still torque.

 

[supton]

Originally Posted By: dlundblad
Im talking just pulling a trailer at highway speeds with 3.21 gearing. How would this behave?

How about a 0 - 60 from a light or ~20 - 75 on an interstate ramp?


0-60, 11% worse than if 3.55's were used. 20-75, might be the same, or between same and 11% worse, as the transmission has larger gear splits than 11%.

 

[supton]

Originally Posted By: IndyIan
Perhaps how the transmission is designed, running in certain gears for hours on end isn't good? or maybe the higher ratio diff comes with bigger rear brakes?
I haven't check on brakes, that is possible--but didn't think about bad gears that wouldn't take to long pulls. Maybe that's it, maybe they worry 1st would get used for too long.

Originally Posted By: KrisZ
Torque multiplied by gear ratio is still torque.

Sure, but but you just made more or less of it.

 

[WyrTwister]

[/quote]

Yep, keeping dropping gears, keep the engine at or near peak hp. In the end, that 366 was what, 200hp? You could only ever do 200hp of work at most. [/quote]

Really no idea what the hp of the 366 was / is . The trucks were either full box bobtails or semi's pulling a box trailer . Wind resistance was a BIG issue at highway speed . Hook a flat bed trailer to it or a blat bed bobtail ( stake bed ) and that improved a lot .

Wind resistance was not an issue at in town speeds .

Important thing engine wise , was keeping the engine RPM well up into the power / torque curve at your anticipated high way speed . And again , the proper expectations . You have to accept you are driving a truck , not a car .

 

[KrisZ]

Originally Posted By: supton


Originally Posted By: KrisZ
Torque multiplied by gear ratio is still torque.

Sure, but but you just made more or less of it.


Isn't that what I said in my original reply?

Also, that 11% increase in the torque multiplication may not sound like much, but it probably puts the engine RPM closer to the peak TQ, especially during shifts, for that engine, so the actual TQ available at the wheels while driving is higher than 11%.

 

[IndyIan]

Originally Posted By: KrisZ
Originally Posted By: supton


Originally Posted By: KrisZ
Torque multiplied by gear ratio is still torque.

Sure, but but you just made more or less of it.


Isn't that what I said in my original reply?

Also, that 11% increase in the torque multiplication may not sound like much, but it probably puts the engine RPM closer to the peak TQ, especially during shifts, for that engine, so the actual TQ available at the wheels while driving is higher than 11%.

But with 8 speeds there's always more than 11% more torque available by shifting down a gear. With the old 4spd autos, the rear end ratio was a big deal in matching engine rpms to torque demands, as shifting down a gear was bigger step. And running in 3rd instead of 4th on the highway cost you alot in gas, and noise.

 

[supton]

Originally Posted By: KrisZ
Originally Posted By: supton


Originally Posted By: KrisZ
Torque multiplied by gear ratio is still torque.

Sure, but but you just made more or less of it.


Isn't that what I said in my original reply?

Also, that 11% increase in the torque multiplication may not sound like much, but it probably puts the engine RPM closer to the peak TQ, especially during shifts, for that engine, so the actual TQ available at the wheels while driving is higher than 11%.


I must have missed it / went over my head. Don't know about peak tq, as the torque curve is flat on the Pentastar, so I don't think that is a big factor.

Maybe you're right--the gear splits aren't as close as I thought. Those are closer to 30% for most gears. Maybe just dropping gears doesn't fine-tune rpm enough? I was thinking the 8AT would have less of an rpm drop between gears, but at 6k you can't go down another gear, and going up one is a big drop in tq. 11% is close-ish to half a gear, so perhaps it just optimizes the system that much better.

 

[KrisZ]

Originally Posted By: IndyIan
Originally Posted By: KrisZ
Originally Posted By: supton


Originally Posted By: KrisZ
Torque multiplied by gear ratio is still torque.

Sure, but but you just made more or less of it.


Isn't that what I said in my original reply?

Also, that 11% increase in the torque multiplication may not sound like much, but it probably puts the engine RPM closer to the peak TQ, especially during shifts, for that engine, so the actual TQ available at the wheels while driving is higher than 11%.

But with 8 speeds there's always more than 11% more torque available by shifting down a gear. With the old 4spd autos, the rear end ratio was a big deal in matching engine rpms to torque demands, as shifting down a gear was bigger step. And running in 3rd instead of 4th on the highway cost you alot in gas, and noise.


I don't think you can compare the new automatics with the old ones or the manual transmissions as far as shifting is concerned. The new ones can lock/unlock the TC in pretty much all gears except 1st, some can skip gears, so it all probably plays a part in the final towing rating.