It's sad when I try to get things out of my head but they keep haunting me for days; especially stuff on internet forums.
I'm going to dissect previous post a little bit. Don't take offense at it because it's a personality quirk of mine. I like to teach.
"torque is what pulls 40K lbs up a hill."
Yes. You're right. Torque
WHERE THE RUBBER MEETS THE ROAD is required to pull 40,000 pounds up a hill. However, in it's simplest mechanical form, think about how much torque you'd need to do that on a 6% or 7% grade (typical for most US interstate systems in mountainous country). This HAS to be measured at the drive wheels of the tow vehicle as there isn't an internal combustion engine-equipped vehicle in production that doesn't use a transmission for torque multiplication.
A person could use an old steam engine (massive amounts of torque, but very low horsepower because of a very low RPM) to use as propulsion for a vehicle. Would it pull your 40K GCVW up a hill? You bet it would. Do you have all day for it to do it? I know that most people are far too impatient for that! A person could "gear-up" the vehicle in this case to turn the wheels faster and actually get some speed out of the vehicle. Time for some simple math:
A steam engine creates GOBS of torque from nearly 0 (yes, ZERO) RPM up to a maximum 500 RPM or so. Let us assume that our particular steam engine is capable of a maximum 500 ft. lbs. of torque at 300 RPM (thermodynamics and other losses cause a drop in torque at anything much higher than that). Simple math concludes that this amount of torque at this RPM gives us a whopping 29 horsepower. This is at the engine flywheel, mind you. At a 1:1 gear ratio all the way to the drive wheels, this would give us 300 RPM axle speed. Using a normal tire diameter for a modern pickup such as 32 inches, we would get a road speed of 28.5 MPH. That's unacceptable by today's standards and downright dangerous. Keep in mind also that the torque at the road surface of the tire will be less than the 500 ft. lbs. at the axle shaft.
Let's now assume that we want our steam engine to pull 40K lbs. up the same hill but at 60 MPH. That would require a tire RPM of 630. So in essence, we need to put in a gear ratio of 0.5:1 somewhere in the drivetrain. Here's what happens in a perfect world with no drivetrain loss: Our 500 ft. lbs. of initial torque suddenly becomes 250 ft. lbs. at the axle shaft. That will end up being FAR LESS than the 314 ft. lbs.
at the wheels than what was given in the example of the stock 7.3L truck. Horsepower is very real and very necessary to get a job done in a timely manner. When dealing with horsepower,
TIME is the key word.
That's a very simple rundown of what I'm explaining. People always fall back on the "diesel makes low-end torque" mantra. While it's true that the mechanics and fuel characteristics of the diesel engine lend themselves to making peak torque at a lower RPM than a gasoline engine, bear in mind that the things that limit a diesel engine are also the things that a gasoline engine capitalizes on.
The first of these is RPM. From a general standpoint (and I'll use the 7.3L Powerstroke in this example because it is what we all know and love), we are limited to a window of 800 to 3300 RPM to make our torque. As a general rule of thumb, a stock 99-03 7.3L truck will make peak torque between 2400 and 2600 RPM and fall off pretty sharply after that. Maximum torque starts at around 2000 RPM. As a whole, if you ask all of the "torque bandwagon" folks, we have a 600 RPM window where we're actually doing something. However, with the dyno sheet sitting within plain sight of where I am sitting (and having spent a heck of a lot of time with heavy trailers in tow up long mountain grades), there's no way that I would try pulling a heavy load up a 6% grade at 2200 RPM (70 MPH), nor would I have any luck with it. Heck, 2600 RPM (peak torque) is still going to be a far cry from ideal for a trailer weight of 20K+ up some of the hills around here. Why is it necessary to downshift and get the RPM
HIGHER than the torque peak of the engine to maintain speed up a hill?
In the above text, I mentioned torque peak and maximum torque range. Now I will get into horsepower. Remember, we haven't gotten into the gearing aspect of this yet. Take a 4R100 transmission's 4th gear ratio of 0.71:1 and apply that to the engine's rated torque of 525 ft. lbs. The driveshaft torque becomes 373 ft. lbs. After the final drive (ring and pinion) ratio of 3.73:1 we end up with an axle torque value of 1390 ft. lbs. This is at 2600 RPM (peak engine torque) which would be 91 MPH; not exactly a speed that I'd want to be pulling 20K+ lbs.
Conversely, 2600 RPM
IS feasible in 3rd gear while maintaining a reasonable and prudent speed (65 MPH). Let us figure out what kind of numbers we're putting to the ground here.....525 ft. lbs. into and out of the transmission (1:1 gear ratio) and then multiplied by 3.73:1; 1958 ft. lbs. at the axle.
By what I've been writing, it seems as though I believe more in torque than I do in horsepower. However, like I said at the beginning of this text, "Torque
WHERE THE RUBBER MEETS THE ROAD is required to pull 40,000 pounds up a hill."
Here's where things get interesting. I'm going to interject a bit of gasoline engine information here (since I need to prove my point
) because typically we know gasoline engines to be good at making torque at higher RPM and
in smaller displacement engines making more peak horsepower than torque.
Engine A: 7.3L Powerstroke with modifications to make 920 Ft. lbs.
Engine B: 8.5L 521 c.i. gasoline engine that makes 920 Ft. lbs. (built in desktop dyno with realistic parts...yes, it's turbocharged
)
Which one will put the most "power to the ground"? I assume that we all agree that the truck that puts the most power to the ground will accelerate the fastest and also maintain speed better. The example will put two identical trucks equipped/loaded the same against each other with the engine being different.......Here are some more particulars (both equal at 920 ft. lbs. although I didn't want to get too unrealistic with the gas engine, so it's ONLY 905):
Powerstroke engine:
450 HP @ 2800 RPM
920 Ft. Lbs. @ 2400 RPM
4R100/3.73 gears
Gasoline engine (built on desktop dyno):
835 HP @ 5800 RPM
905 Ft. Lbs. @ 4600 RPM
E4OD/3.73 gears
Don't get too excited over the horsepower difference and the gasoline engine being "bigger"....the 920 Ft. Lbs. is all we're worried about here because as most people are convinced, that's all that matters.
The next posts will include numerous screenshots of results from the above data. I purposely used 10,000 lbs. as a weight because anything more than that just drug out the acceleration times so badly that it wasn't even worth looking at.....