2nd Gen Non-Engine/Transmission Torque or HP for Towing?

[lpmarkman]

What's best... torque (420 @1600rpm) or HP ( 235 @ 2700rpm) for heavy load uphill?

 

[PC12Driver]

Just past the torque peak, say 2200rpm or so. That way as the truck decelerates on the uphill the engine builds more torque as the engine slows.

 

[Loren]

What's best... torque (420 @1600rpm) or HP ( 235 @ 2700rpm) for heavy load uphill?

Like most things in life, it depends.



If you want to get up the hill in a hurry, you want maximum horsepower. Period.



In order to get up the hill AT ALL you need a (variable) minimum torque AT THE REAR WHEELS. The torque required is a function of the load you are pushing (weight plus friction loads) and the angle of the climb.



Lower gears have a higher "torque multiplier" so they increase the torque at the rear wheels more than higher gears. With some vehicles, you often don't have a choice of which gear to run in. You must choose the highest gear that will still provide a high enough "torque multiplier" to reach the minimum required rear wheel torque.



With our trucks' huge torque over a broad RPM range, we can often pick from several gears for a given climb. The most common advice I've heard is to avoid running at too low an RPM. You cannot damage the Cummins engine by overreving it. The governor (the one on the engine - not Arnold) will prevent the engine from reving too high.



If I am in doubt, I shift down and run the engine faster.

 

[lpmarkman]

Great info and thanks for the input. New to the forum and really enjoying.

 

[AH64ID]

To put it very simply. .

TQ gets a load moving, Hp maintains the speed. .

 

[Vaughn MacKenzie]

Great info and thanks for the input. New to the forum and really enjoying.

Welcome to TDR lpmarkman. Tell us a little about your rig (if you have one).



I think maybe what you were asking is what is the proper RPM for getting a big load up the hill? If you take engine stress into consideration it would be better to run at higher RPM to get a heavy load up the hill. Basically as you increase the amount of power you are using from your engine the higher the RPM should be to utilize it.

 

[EKlem]

Loren gave a pretty good explanation of this. What matters in the end is the torque at the rear wheels. You can think of rear wheel torque as the force being applied to the road by your tires because tire size is constant.

Here is an extremely simplified example but hopefully it is helpful. Say that you are deciding between running a direct drive gear and a 2:1 reduction(I am ignoring the gearing in the differential which is constant). In direct, you will run 1500rpm and produce 500ftlbs of torque. In the reduction, you run 3000rpm and produce 350ftlbs of torque. So the question is what gear should you run? The torque at the wheels in direct will be 500 while it will be 350*2=700 in the 2:1 reduction. Therefore, you want to run in reduction because there is more rear wheel torque.

Just because your engine produces less torque at higher rpm doesn't mean that your rear wheel torque will be lower, you need to factor in the gear ratio. When in doubt, run a lower gear and higher rpms. When I am pulling a big hill, I will often run 2500rpm or so. Play around with it some and you will figure out what speeds, gears and rpms work best.

 

[fox]

What's best... torque (420 @1600rpm) or HP ( 235 @ 2700rpm) for heavy load uphill?

I can build a 350 chevy with 700 hp and it won't pull anything like our trucks!!!!!

It is torque that gets it moving and keeps it moving.

Low rpm torque is efficiency.

 

[AH64ID]

Low rpm torque is efficiency.

Not always... Take a look at this dyno sheet. . Just because the peak tq is at 1600 on my motor, doesn't mean its there on the drivetrain...



2200 rpms has been deemed the sweet spot for CR's, not sure on the other gens. . but you can see thats a bit above peak tq.



#ad

 

[fest3er]

Work is what moves mass. Power is simply how quickly the work can be exerted. If you want to move an 8000# object 1000 feet up a 5% slope, you need to exert at least 400,000 lb-ft of work. If you want to move the load at 100 feet per second, you will need to be able to exert 40,000 lb-ft of work per second for 10 seconds. If you can tolerate moving the load at 50 FPS, you'll need to exert 20,000 lb-ft per second for 20 seconds. If you want to move it at 150 FPS, you'll need 60,000 lb-ft/second for 6. 67 seconds. (BTW, those speeds correspond to 68, 34 and 102 MPH, respsectively. ) Converting rotational torque to linear work is the reader's homework.



As far as diesels are concerned, there's another item to bear in mind: boost.



As long as the engine is turning fast enough to move enough air (and fuel) through the engine, it'll keep the boost up, which will keep the power up. If you don't quite have the power to keep the engine above your 'minimum boost', you'll have to downshift. Of course, if your EGT gets too high for too long, you should downshift anyway, unless you really want to see what pistons look like with a holes melted through them. Downshifting uses the same amount of fuel but moves more air through the engine, thus keeping it cooler.



Since you are new to diesels, the first thing you need to do is install boost and EGT gauges; they both tell you how hard your engine is working and can give you a clue as to how efficiently it is working. If you often tow heavy, a trans temp gauge is almost de-rigeur. Everything after that comes under the 'play' heading.

 

[PC12Driver]

Not always... Take a look at this dyno sheet. . Just because the peak tq is at 1600 on my motor, doesn't mean its there on the drivetrain...

You've got a curve there that's pretty typical of a boosted engine on an acceleration dyno, which isn't very accurate until full boost is reached. The same truck on a loading dyno (something that can vary the resistance to hold an rpm) would show a very different curve under 2000 rpm - something closer to a real-world towing profile. There's much more torque to be had under 2k than your graph shows.

 

[fox]

And Cummins says each of their 5. 9's used in the Dodge trucks are alway more fuel efficient below 2,000 rpm. This is where they are designed to run, by gearing on these trucks.

 

[AH64ID]

You've got a curve there that's pretty typical of a boosted engine on an acceleration dyno, which isn't very accurate until full boost is reached. The same truck on a loading dyno (something that can vary the resistance to hold an rpm) would show a very different curve under 2000 rpm - something closer to a real-world towing profile. There's much more torque to be had under 2k than your graph shows.

Yeah I know... I tried to build it as low as possible, but it didn't work... I have heard there is a pull down dyno here in town I want to try,

 

[Loren]

Work is what moves mass. Power is simply how quickly the work can be exerted. If you want to move an 8000# object 1000 feet up a 5% slope, you need to exert at least 400,000 lb-ft of work. If you want to move the load at 100 feet per second, you will need to be able to exert 40,000 lb-ft of work per second for 10 seconds. If you can tolerate moving the load at 50 FPS, you'll need to exert 20,000 lb-ft per second for 20 seconds. If you want to move it at 150 FPS, you'll need 60,000 lb-ft/second for 6. 67 seconds. (BTW, those speeds correspond to 68, 34 and 102 MPH, respsectively. ) Converting rotational torque to linear work is the reader's homework.



As far as diesels are concerned, there's another item to bear in mind: boost.



As long as the engine is turning fast enough to move enough air (and fuel) through the engine, it'll keep the boost up, which will keep the power up. If you don't quite have the power to keep the engine above your 'minimum boost', you'll have to downshift. Of course, if your EGT gets too high for too long, you should downshift anyway, unless you really want to see what pistons look like with a holes melted through them. Downshifting uses the same amount of fuel but moves more air through the engine, thus keeping it cooler.



Since you are new to diesels, the first thing you need to do is install boost and EGT gauges; they both tell you how hard your engine is working and can give you a clue as to how efficiently it is working. If you often tow heavy, a trans temp gauge is almost de-rigeur. Everything after that comes under the 'play' heading.

Neal,



Don't scare the new guy like that. It is possible to melt a piston if the cylinder temperature gets too high, but that is almost impossible with a stock engine. The general rule is that you can't hurt a stock engine no matter what you do.



That is a very good explanation of the physics.

 

[Loren]

When in doubt, run a lower gear and higher rpms.

That's the best general rule here.

 

[AH64ID]

As long as the engine is turning fast enough to move enough air (and fuel) through the engine, it'll keep the boost up, which will keep the power up. If you don't quite have the power to keep the engine above your 'minimum boost', you'll have to downshift. Of course, if your EGT gets too high for too long, you should downshift anyway, unless you really want to see what pistons look like with a holes melted through them. Downshifting uses the same amount of fuel but moves more air through the engine, thus keeping it cooler.

Not so true on 3rd gens. . Rpm's above 2500-2600 and the EGT's climb sky-high based on the emissions junk... Yes you increase air, thou boost stays the same, but the injection events and timing are messed with for emissions.

 

[lpmarkman]

You guys (and gals) are terrific. This is a lot of information and I find it of real value. This is a stock 5. 9 we just picked up and we're just getting familiar with it. Pulling a 3500 lb pontoon boat or a 1700 lb camper so nothing too awfully heavy. Thanks to all. Might be looking at a programmer for better mpg one of these days but that's a different thread.