Yes... ...rear-wheel torque, at a given speed. And if they have the same rear-wheel torque, and are moving the same speed, they have the same HP, and thus can pull the same load. The gasser might be screaming to do it, but if the engine HP is truly the same as the diesel HP, it will move the same load.


Agreed. I bought my 7.3 because I thought it would tow better than my GMC. I was disappointed. Don't get me wrong, I'd still MUCH rather tow with the Ford, it is a much more pleasant experience overall. But I was surprised that the GMC would tow the same load up the same hill at a higher speed, that is, I was suprised until I really took a close look at the HP vs. torque relationship. Then I understood. The math clearly supported the difference in performance between the two trucks.

 

a diesel will get a bigger load moving than a gasser,if you have an equal hp gasser it will hold speed just as good as a diesle would,although the diesel will get a much bigger load moving that a gasser couldnt handle,after you are cruisin thats where the hp takes over

 

Generalizations like that are not supported by the physics. IF the gas motor has the same HP as the diesel, and IF the gearing is correct to get the gas motor to its HP peak, then the gas motor will be able to generate the same rear-wheel torque as the diesel, and it is the rear-wheel torque that causes the load to move.

In practice, it might SEEM like diesels have the edge, because they do have more low-end grunt. It feels like they're not working as hard, because they are not turning as fast. People don't like to wind out gassers the way they would have to to get equivalent rear-wheel torque; they're afraid they're overworking the motor, or they just don't like the sound of it. But if they did, they'd find that the 250 HP gasser is just as capable as the 250 HP diesel.

Same power, different RPM, but geared differently to result in the same rear-wheel torque. That's all there is to it. Go back and look at the formula I posted before. There is no way around that. You can't get something for nothing.

 

My favorite quote I saw was....HP is how fast you hit the wall.....Torque is how far you push the wall after you hit it!

 

A 300HP gasser with 250 ft-lbs, and a 300HP diesel with 800 ft-lbs will get the same load moving at the same speed at the same time, IF the gearing on each engine is setup properly

 

i get what your saying, but a diesel engine is a better choice because they do put out more torque....stock,if you hopped up a gasser to match the torque of a diesel,the gasser would probably pull more because it would most likely have more hp,but you cant find a gasser that puts out the torque of a diesel from a dealer,and most people wouldnt want to spend all of that money for mods on their gasser,which is why people will most likely buy the diesel

 

So if you gear the gasser right then you will be able to do the same work as the Diesel??? Right..

 

Well, to be fair, diesels cost more to start with, but I do agree with what you are saying. Besides, even though the gasser might be as capable, it would never be as econmical, or as nice to tow with.

 

If the HP is matched, they can both pull the same amount. But you will have to wind out the gasser alot more then the diesel, making it more pleasant to tow with the diesel

 

You guys are really missing the physics on the whole HP vs. Torque debate. Torque is a measure of rotational force. HP is a measurement of work performed--think in terms of torque applied over a period of time or energy used. In other words, HP has an additional factor of time included in its units.

A vehicle at a constant speed on the highway is at an equilibrium--the torque produced by the engine is matching the sum of the frictional driveline losses, wind resistance plus rolling resistance--which are all forces as well. HP is an indicator of how quickly the engine can apply its torque.

Two engines of the same max torque will be able to move the same maximum load. The one with the higher max horsepower will do it more quickly.

Diesels tend to be better at pulling because of the way the fuel burns--more slowly, so the force on the piston is applied longer, allowing for more overlap (or less gap) with the next cylinder that fires. Gasoline burns quickly, which is why it does better at higher engine speeds.

Jason

 

The HP calculation does not include time, only torque and velocity. HP is not a measure of energy, it is a measure of instantaneous power.


Yes, but in a typical gas vs. diesel comparison, for matching loads and vehicle speeds, the engine torque being generated by a gas engine is much less than that of a diesel, BUT, the engine speed is a lot higher. So you can't just use engine torque alone for a comparison. You have to look at both engine torque and engine speed together, if you want a useful indicator of capability.


That is not necessarily true, unless they happen to make that same torque at the same speed (i.e., they have the same HP).

That is true.

 

1 HP = 33,000 ft-lbs/min. Ft-lbs is a measure of torque (rotational force, which includes distance). 1/min is a time factor. It doesn't get any simpler than that. Read up here: Horsepower - Wikipedia, the free encyclopedia and Torque - Wikipedia, the free encyclopedia "Power is the work per unit time."

Sorry, but you're way off on this one. Take the example of a breaker bar vs. an impact wrench. With a long enough breaker bar, I can put 500 ft-lbs of torque on a nut. So can a decent impact wrench. But it will spin it much faster--because it has more HP. That is why I emphasized that HP includes the time factor.


At least we agree on something.

Jason

 

Oh, right. Time is taken into account with the RPM. But HP is still not a measure of energy, it is a measure of power.

And when you say, "Ft-lbs is a measure of torque (rotational force, which includes distance)", what do you mean by "which includes distance"?

Re-read the above three posts, and you may see that I misunderstood you initially. I thought you were speaking in general terms, saying that two trucks with the same torque rating would be capable of towing the same load at the same rate. I believe now that you were talking about the engines alone, and how much force they could generate.

And based on your breaker bar vs. impact wrench analogy, it's clear that you understand that it takes more horsepower to apply a given force faster.

 

I probably shouldn't have said, "which includes distance", because now that I re-read it, one could think that I was implying a travel distance, when what I was referring to was the radius of the torque.

And, yes, when I was speaking of getting the same maximum load moving with two identical-torque/different-HP engines, I mean all else being equal, the higher HP engine will move it faster.

I think we're pretty close to being on the same page here...

Jason

 

Excellent, I think we are. Thanks for clarifying, and for making me think about it.