I have a ? for you guys who have gone diesel after owning the big gas motors for pulling. I am not looking for a argument just want to understand how HP relates to torque in the real world and so forth for pulling heavy weight.

How much better did you Power Stroke same HP for same HP compared to the Big CUBIC INCH gassers you have had pull a load and get up to speed on hills and so forth with little fuss and less shifting compared to your gas trucks when all was comparably geared correctly and how much easier was it to drive because of this when heavily loaded.

I ask because HP can really be kinda hard to understand and I have no experience with this in the real world pulling heavy loads.

People always throw the HP formula out their and say on paper with the right gearing a RPM spinner gasser can compete with a diesel and it's like you can't tell the difference but something tells me this is really wrong when I see a 300 HP Semi pulling 80 grand and in no way could a 300 HP Gas engine do that no matter the gearing or could it?

Thanks for the info.

 

Actually pretty good question.....

I bet you are going to get a variety of answers...and they all might be correct.

Simply put....a gas engine can do the same work as a diesel engine, but like everything in life, there are trade offs. It might help to think about things from the other end. Which engine wastes more energy?....they both waste quite a bit of the energy that is stored in the fuel but a diesel engine wastes less. In addition, diesel fuel has more energy by volume then gasoline so that is a secondary benefit. Wasted energy=heat and heat is the real problem when a vehicle is trying to get some work done. A diesel engine is more thermodynamically efficient than a gasoline engine therefore it's more practical for heavy work (towing heavy loads).

HP/TQ. That can be a hard to wrap your mind around too. An over-simplified way of looking at it is that torque gets you moving...horsepower keeps you moving (even uphill). Something to think about....for a given slope and a given weight....it takes about the same HP to bring a vehicle going downhill to a complete stop from a certain speed as it takes to maintain that same speed going up hill (not accounting for aero effects at higher speed). This basically means that a truck needs brakes that are just as strong as the engine (HP wise) or it won't be able to stop when going downhill. Because a diesel engine has much higher compression ratio, it makes an ideal engine brake when going downhill. This is super helpful when towing heavy because it means the driver doesn't have to use the service brakes as much to control the vehicle. A diesel engine can run at max hp output for extended durations and not overheat. Gasoline engines are generally not designed to run at peak HP for very long so they can overheat if their cooling system isn't properly designed. Expect to consume a lot more fuel when working a gas engine around the 300 HP level....

When it comes to getting a load moving from a dead stop...
The torque of a diesel engine shows up at very low rpms compared to a gasoline engine. A diesel engine's torque is usually in by 1600 rpm (some much lower) so that makes it more practical to get a load moving. Gasoline engines have to rev a lot higher to reach their peak torque and even then its usually less than half of what a diesel engine can produce.

I hope this helps.

 

The most simple way to figure out the answer to this is by stating one value: Power at the wheels. The engine type/displacement doesn't matter as long as either powerplant compared and the drivetrain used for each puts the same amount of power to the pavement. Directly comparing the torque produced by either engine doesn't matter. If the gearing used allows an even comparison (a realistic comparison helps too), then the only caveat that must be overcome boils down to how hard a person wants to push the engine to get the job done.

BUT.......

Notice I didn't say "torque" at the wheels. The following example will lead you to believe that torque is what matters. Well, in a way, it is ALL that matters but torque alone does NOTHING. Torque is measured as an instantaneous, static value and without horsepower (work over time), it's almost useless.

Realistically, a 300 horsepower tractor (semi) doesn't exist anymore. Few trucks out there that see road-duty are limited to less than 450 these days. However, since that's the number that was given, that's the number I'll use. To accurately answer your question, I'll start with some simple calculations:

The contenders:
L10 Cummins - 300 Horsepower @ 1850 RPM. Maximum torque of almost 900 ft. lbs. @ 1650 RPM (280 horsepower at that RPM).

6.0L Chevrolet gasoline - 300 Horsepower @ 4400 RPM. Maximum torque of 360 ft. lbs. @ 4000 RPM.

These are at-engine numbers that I'll substitute as rear-wheel numbers just for the sake of this discussion.

The vehicle (imagine if you will):
Ford L8000 tandem-axle end-dump.

What do we have in mind? Well, the speed limit here is 65 MPH for a truck of this size.

In top gear (assume 10 speed Eaton with 1.0:1 10th gear) at 65 MPH with the existing gearing and tire size, and an engine speed of 1950 RPM. The governed speed of the L10 is 2250 RPM while the 6.0L gas engine can easily run 5200 RPM. A 6% grade is approaching while traveling 65 MPH on flat ground. We're going to have to use all of that 300 horsepower......

Keep in mind that the L10 makes twice the torque (at the flywheel) as the 6.0L. In many circles, people would have you believe that the contest is over. The gasser may as well give up........

Through the miracle of gearing and given engine power, let's say that the L10 is able to maintain 42 MPH in 9th gear (1.34:1) which puts us at the power peak of 1850 RPM here.

Calculations:
Torque at 1850 RPM = 852 ft. lbs.
852*1.34= 1142 ft. lbs. at the driveshaft
1142*3.89 (ring and pinion ratio)=4450 ft. lbs. at axle shaft.

With the same scenario but the 6.0L gas engine TRYING to maintain the same speed - - -- - -

Reversing the calculations above we know that we must have 1142 ft. lbs. at the output of the transmission to compare to the L10's terminal velocity up the hill so 1142 divided by the torque at the horsepower peak (358 ft.lbs. @4400 RPM) equals 3.19. This happens to be close to 6th gear in this transmission which is 3.32:1.

Given these very simplistic calculations, it's easy to see that the 6.0L gasoline engine can keep up with the 10L diesel engine that makes twice the torque at the flywheel. It would, however, burn a lot more fuel and put more wear on things running 4400 RPM all day long uphill while the diesel engine in this example would stay relatively happy "putting along" at 1800 RPM. In addition, while shifting through the gears, it would stand to reason that the gasoline engine would shift much later in the RPM range and be happier skipping a gear or two on acceleration since it will have a much more broad powerband than the L10.

As we get closer to the newer and more popular 475-600+ horsepower OTR trucks, it gets harder to find a competing gasoline engine that will live RELIABLY making that much power on a constant-duty basis. This is the entire reason for using 15+Liter engines in the "big rigs"....constant-duty power.

Just for argument's sake, a diesel engine has very little engine braking ability. If it weren't for compression brakes (or exhaust brakes in the diesel pickup market), you'd see a lot more runaway truck ramp usage. There is NOTHING restricting airflow through a diesel engine to the point of having measurable engine braking vs. a gasoline engine. With no fuel added to the mix, the same air that gets compressed during the compression stroke is the same compressed air that pushes the piston back down after TDC. Believe it or not, the reason a gasoline engine has much greater engine braking is due to vacuum - the "work" created by the pistons on the intake stroke. Exhaust brakes use the work of the piston trying to push exhaust through a restriction. Compression brakes release the stored energy of the pressure created on the compression stroke so there's nothing to push the piston back down the cylinder.

Also, diesel engines have an Achilles Heel. The power output of a diesel engine is determined by the amount of fuel injected into the combustion chamber. More fuel equals more heat. Adding air density through turbocharging and intercooling helps a lot here by allowing a more efficient burn of the available fuel. However, as a general rule, the "richer" the air/fuel mixture, the hotter things get. With a gasoline engine, since it runs a near-constant air/fuel ratio, the temperature doesn't change much although heat-soaking by having a far greater frequency of combustion events (sustained high-RPM load) can cause overheating if the cooling system isn't up to par. However, this can be the case with any engine regardless of fuel type. Unless there is a reason for the gasoline engine to be running overly-lean under full power, melting pistons isn't that great of a concern like it is with an overly-rich diesel powerplant.

So it all boils down to how hard you want to push things. A blown 521 ci. pulling engine that makes 800 horsepower will pull just the same as an 800 horsepower diesel pickup......but it will burn more fuel at a much higher RPM (read: less efficient). Neither one will like making that much constant-duty power though since glowing exhaust pieces aren't a good sign.

 

Without over complicating things, a general observation I see is that a gas and Diesel engine of the same displacement, the HP will be comparable, where as the torque of the diesel will be about 2X as much. Just generally speaking.

I was told once that torque is what an engine produces, horsepower is how fast it will produce it. Torque is what does the work, so if you have twice as much at the same rate, you will get twice as much done. That makes sense to me.

Look at the 6.2 gas and the 6.7 PSD. About the same displacement, 411 and 400 HP. But the 6.2 had 434Lb-ft where as the PSD had 800. New 6.7 is even better with 425 and 860.

 

The problem with that statement is that torque by itself doesn't do work..... If you're standing on a 4' cheater bar putting 500 ft. lbs. on a bolt that would take 525 ft. lbs. to break loose, you're not doing anything...but there is, in fact, 500 ft. lbs. on that socket.

Also, a diesel will typically make twice the torque, but it's only happening at roughly half of the rate (RPM) so you're doing the SAME amount of work.

 

Rpm being the same, if you have the same horsepower with more torque, you can get more done

 

As far as the OPs question. My answer is this,

It's not that a gas engine won't do it, it's just that a diesel can do it better. The diesel has more grunt at lower RPM, which means less downshifting is necessary. Also can mean that it will pull more, or pull the same more efficiently.

Many people go to diesel because they are extremely easy to modify and get loads of power from compared to a gas. A new diesel truck can be tuned to the level of 500hp and 1,000lb ft by any Joe Schmo on the street. And when you have that much power, you can pull a mountain over mountain. Haha

 

If "x" and "y" are the same, how can "z" be different?

 

We have W X Y and Z here. X and Y can be the same, but W is different. Thus, Z is different.

 

Simplification:


While there is great information here, there is no "root cause" of all that diesel torque - at whatever RPM. The OP wants to understand the difference between gas and diesel - I'll trim the fat.


Gas: Air-fuel mixture sucked in, compress, spark-ignite all at once - SLAP!


Diesel: Air blown in, compress more, now force atomized fuel in (much slower process than sparking) - PUSH!

 

Christopher Columbus was told that the Earth was flat. Obviously whoever told him that wasn't getting good information. In our case, a general blanket statement citing a diesel's trait of creating twice the torque than a gasoline engine at the same horsepower level means that a diesel will outpull a gasoline engine is misleading.

I'm sitting here looking for the w you mentioned. RPM same (x), horsepower same (y)......uh, where's the other variable (w)? It can't be torque since torque is mathematically figured from horsepower (x) and RPM (y).

Example:

300 Horsepower (x) at 4400 RPM (y) equals 358 ft. lbs. (z).

As you claim:
Let's work with this.....

RPM is the same (4400)
Horsepower is the same (300)
Well, the math STILL comes out to 358 ft. lbs. I can't find more torque here at this particular RPM. I'm still not getting more done.

Don't give up hope just yet though. There is another facet to this unfair math process that always gives the same answer. No, it's not "common core" wherein 2+2=5 as long as I can show on paper how I got that answer. It's called "powerband". This anomaly of power production may be the saving grace for your AWOL w.

The original post begged the question, In the real world, different engines will produce power differently from one another. The "big rig" diesels (as an extreme case for comparison) create gobs of "low-end torque". The term "low-end" is kind of a misnomer though because the "high-end" of a large diesel's rev range IS the "low end" of smaller engines' rev ranges. Back to the story, this torque is short lived; it comes up quickly from an idle and then free falls like a purposely demolished World Trade Center building after it peaks. By shading-in the "area under the curve" of this imaginary line, we can get an example of this "peaky" torque curve. Keep in mind that the larger diesel engines used in commercial trucks have a very limited RPM range that usually terminates at or under 2200 RPM at the top end. This means that the engine actually only makes any real "usable" power in a very, VERY narrow RPM band. This is exactly the reason that you constantly hear big trucks shifting up and down. For lighter loads, a 9- or 10-speed transmission is sufficient but 13-, 15-, and even 18-speeds are common for heavier-pulling trucks (that can be shifted as a fewer-speed transmission if desired).

Lighter-duty diesel engines in pickups are able to achieve higher RPM than their larger counterparts. I'll pick on a stock 7.3L for a while (since we're all here). Peak horsepower occurs at around 2800 RPM. This is nothing magical, but it just so happens that the mathematical equation shows that the torque is at a level in rev range where the horsepower is the highest due to mathematical equations. If a person were to graph these findings over the rev range (680-3300 RPM), it would be found that the 7.3L, while making less peak torque than the "big rig" example above, actually makes power over a higher percentage of its operational range. This allows less downshifting and nullifies the requirement of needing so many transmission ranges to accomplish pulling duties at each vehicles' maximum GVW. That's not to say that a 7.3L/automatic pickup wouldn't be better with a lower first gear and another intermediate range between third and fourth gear.

 

A friend of mine once put it this way. Horsepower is how hard you hit the wall, torque is how far you go through it.

 

I've heard that countless times over the years, and it makes zero sense. Re-read what Cody posted in this thread and you'll start to understand why that statement doesn't make any sense.

Remember folks, horsepower and torque are not the same. They don't equal each other, and they are not interchangeable mathematical units that can be substituted for each other all ***** nilly.

Comparing what is more important, horsepower or torque, is like asking if air or water is more important for survival. Neither one are the same, yet they are both necessary. You can't substitute one for the other. You can't drink air and breath water.

Now if you want to re-word the statement above to make more sense (which it still doesn't, but it's food for thought), then here it goes: "RPM is how hard you hit the wall, horsepower is how far you go through it."

Think about that for a minute and let it sink in.

 

I think were all spending too much time trying to nit pick what is actually what.

Torque is the rotational force around an axis.
Horsepower, in it's technical term, is the force required to move 550 lbs, over one foot, in one second.

That means, that horsepower is needed to move. But, now we get into power-weight ratios. More power with less weight will accelerate faster.

But torque is what actually pulls the load. You could have a 1hp engine, with 500lb-ft of torque, and a 500hp engine with 500lb-ft of torque. They will both pull the same load, but the 500hp engine will just get it done a lot faster.

 

Sorry I have been gone, thanks for replying. I will read over this. I always had a hard time understanding how something be this HP and do this and something else was rated like this. And then people say well it's all in your gearing ?