Horsepower vs. torque

[brain55]

Instead of just a simple 2 axis graph, with the x-axis (horizontal) being RPM, and the vertical being HP and torque, they probabally have 2 y-axisis. Probabally one to the left being torque and one on the right being HP. If they are not to the same scale, that is where they make the curves intersect wherever they want. BUT if they are to the same scale, they will cross @ 5252

That is exactly what I was thinking. If you look at the actual HP and torque numbers where they intersect the torque will likely be double the HP at approx. 2600 RPM

brian

 

[LD1]

It makes it a bit decieving, but no matter how they play with the graphs and scales, the one and only time HP is the same as Torque is at 5252rpm. ANYTIME under that RPM, torque is greater. ANYTIME over it, HP is greater.

 

[ken4831]

Is this why a 3 h-p or 5 h-p Briggs engine on lawn mower cuts the grass as well as the new 13 h-p to 20 h-p engines. When looking a piston almost interchangeable and stroke same length the only difference I have noticed is years of service. Almost forever for the older and maybe 2 years on the new.
I don't have accurate figures just ball park type. But have changed out more new lawn mowers because cost of repair and still use the old mower because it runs year after year.
ken

 

[LD1]

Is this why a 3 h-p or 5 h-p Briggs engine on lawn mower cuts the grass as well as the new 13 h-p to 20 h-p engines. When looking a piston almost interchangeable and stroke same length the only difference I have noticed is years of service. Almost forever for the older and maybe 2 years on the new.
I don't have accurate figures just ball park type. But have changed out more new lawn mowers because cost of repair and still use the old mower because it runs year after year.
ken

:confused2::confused2:
:confused2::confused2:

 

[looknlearn]

So would the mathematic equation be different for a diesel engine because of more torque at a lower rpm or am I not understanding?

 

[LD1]

So would the mathematic equation be different for a diesel engine because of more torque at a lower rpm or am I not understanding?

NO. Math is the same regardless.

Diesels having more torque down low also mean they would have more HP down low. Since HP is calculated using the torque. If you have more torque to start with at a given RPM, that results in more HP at THAT rpm as well.

Diesels dont make a lot of HP simply because they cannot turn the RPM''s that a gasser can.

For example, a ~400HP modern gasser like in current trucks is making that peak power in the ~4000-5000RPM range. Whereas a ~300HP modern diesel is only spinning ~3000RPM at max. The HP for that gasser down in the range of the diesels max power would be much lower IF the diesel has more torque @ that particular RPM.

So basically, If the diesel has more torque than a gas motor at EVERY point up to the diesels redline of ~3000RPM, it will have more HP as well. But since a gas motor can spin RPM's almost double a diesel, that is how they achieve a greater "PEAK" HP number.

 

[Scrambler82]

Do not want to get into this discussion but to simplify it and as stated earlier; as I have alway stayed it torque gets you moving HP keeps you moving.
Since with a tractor you are moving slow most of the time TORQUE is what you want to look at.

 

[looknlearn]

NO. Math is the same regardless.

Diesels having more torque down low also mean they would have more HP down low. Since HP is calculated using the torque. If you have more torque to start with at a given RPM, that results in more HP at THAT rpm as well.

Diesels dont make a lot of HP simply because they cannot turn the RPM''s that a gasser can.

For example, a ~400HP modern gasser like in current trucks is making that peak power in the ~4000-5000RPM range. Whereas a ~300HP modern diesel is only spinning ~3000RPM at max. The HP for that gasser down in the range of the diesels max power would be much lower IF the diesel has more torque @ that particular RPM.

So basically, If the diesel has more torque than a gas motor at EVERY point up to the diesels redline of ~3000RPM, it will have more HP as well. But since a gas motor can spin RPM's almost double a diesel, that is how they achieve a greater "PEAK" HP number.

So does that mean two engines, gas and diesel, same horsepower rating, same rpm, the diesel has more torque (which feels like a few more hp) because at that rpm the gasser hasn't reached it's peak torque range that it's rated at?

 

[Grand_Pooba]

you got that wrong. HP= torque x RPM divided by 5252

You are right. They will ALWAYS cross @ 5252RPM if the engine can rev that fast. Simply because given the equation above, if RPM is 5252 and the constant is 5252, they cancel out. this leaving torque = HP.

Below 5252RPM, torque is ALWAYS higher then HP. And Above that RPM, HP is ALWAYS higher.

That is why if a vehichle has more torque down low in the RPM range, it will also have more HP down their as well.

HP=torque (in lbs) x RPM divided by 63024

HP=torque (ft lbs) x RPM divided by 5252

Not wrong just different units

 

[Johnbro]

This all seems to make sense--thanks for the education. Here's a graph of the Dodge Diesel HP and Torque curves:

At 4000 RPM it makes about 295 lb-ft of torque from this graph. Converting that to HP by the formula above gives us about 225 hp or 165Kw (easier to interpolate on the left side of the graph) which looks about right.

In racing engines, it's a given that torque helps you accelerate and HP increases with RPMs. Until the valves float or the piston goes into orbit, that is

 

[ken4831]

It makes it a bit decieving, but no matter how they play with the graphs and scales, the one and only time HP is the same as Torque is at 5252rpm. ANYTIME under that RPM, torque is greater. ANYTIME over it, HP is greater.

Guess when you quote some one at least do the quote line.
Refering to post #22
ken 4831:ashamed:

 

[LD1]

So does that mean two engines, gas and diesel, same horsepower rating, same rpm, the diesel has more torque (which feels like a few more hp) because at that rpm the gasser hasn't reached it's peak torque range that it's rated at?

Not sure what you are asking.

IF the gas and diesel motors have the same HP rating, lets say 300HP.

BUT the gas motor gets that 300HP @ 4500RPM and the diesel makes it @ 2500RPM.....And you are running bothe motors at a constant 2000RPM, the diesel is going to have more HP and Torque at THAT RPM.

But if they are both rated at 300HP at the SAME RPM, they will have the same torque at THAT rpm.

 

[LD1]

Guess when you quote some one at least do the quote line.
Refering to post #22
ken 4831:ashamed:

I dont know why you quoted me and mentioned something about post #22, But you are making absolutly NO sense to me what-so-ever.:confused2:

 

[looknlearn]

Not sure what you are asking.

IF the gas and diesel motors have the same HP rating, lets say 300HP.

BUT the gas motor gets that 300HP @ 4500RPM and the diesel makes it @ 2500RPM.....And you are running bothe motors at a constant 2000RPM, the diesel is going to have more HP and Torque at THAT RPM.

But if they are both rated at 300HP at the SAME RPM, they will have the same torque at THAT rpm.

I was trying to say that when you use each motor at a typical rpm, the diesel will feel like it has more power cause it's torque range is at a reasonable rpm unlike to gasser which's max torque is at an ungodly rpm which it would only last a few minutes at. Is my thought correct?

Just trying to get my head around why a diesel and gas motor with the same hp rating, but the diesel will pull a lot more.

 

[dodge man]

If a diesel and gas motor have the same HP ratings, the odds are the diesel will have more torque. The diesel will also have the max HP and torque at a lower RPM than the gas engine.

I have a 2010 Challenger that I have done a few things to increase the horsepower. Its probably making about 400hp and 400 ft-lbs at the back of the motor, at probably around 5000 rpms. Thats with a 5.7 litre v8 gas engine. If you found a diesel engine in a over the road truck with the same horsepower, it would probably be something like 8 litres, and make something north of 1000 ft-lbs of torque, all below 3500 rpms. In theory, the gas motor in my Challenger has as much HP as that big rig. If you stuck that 5.7 gasser in a big rig, it wouldn't have enough torque to pull up into its power band of 5000 rpms and would be useless, even though it makes as much HP as the big diesel engine.

 

[LD1]

I was trying to say that when you use each motor at a typical rpm, the diesel will feel like it has more power cause it's torque range is at a reasonable rpm unlike to gasser which's max torque is at an ungodly rpm which it would only last a few minutes at. Is my thought correct?

Just trying to get my head around why a diesel and gas motor with the same hp rating, but the diesel will pull a lot more.

That is pretty much it.

If a gas motor and a diesel motor both have the SAME HP rating, the gas motor has to turn a much higher RPM to reach that HP. Therefore the diesel will more than likely have more HP below that RPM.

The thing that has to remain constant is the RPM's. And since HP is a mathematical calculation using ONLY torque and RPM,
IF:
The deisel makes more torque @ 1500RPM vs the gas, it also must make more HP.

If the diesel makes more torque @ 2000RPM vs the gas, it again makes more HP.

So on and so on.

For the gas motors to even get to the same HP rating as a diesel, it needs to turn RPM's in excess of what a diesel can.

Basically Torque and HP are proportional. Two things increase HP. That is torque AND RPM's. So if a diesel and gasser have same HP ratings, the diesel uses gobs of torque and not many RPM's to achieve it. The gasser will use gobs of RPM's but not as much torque to get to the same end result.

The net effect is that the diesel has a power band MUCH lower in the RPM range. That is why they feel like they have more power. And that is why a gas motor will feel more sluggish in comparison because they have to wind up to a much higher RPM to get to the same HP level.

Make sense? Or have I confused you even more:laughing:

 

[slowzuki]

Its a hard concept for people to swallow and the complication that engines only operate at that pretty little curve when at wide open throttle/max fueling also confuses them.

A diesel engine at part load at a good economy rpm, making enough hp to move a vehicle, generally has more torque reserve or room to generate more torque/hp when you mash the pedal without changing rpms.

A gas engine needs to increase its rpm from ideal economy rpm to match the diesels hp so the transmission will need to downshift to allow the engine to generate the power needed. People perceive downshifting as a lack of power and always complain about engines that need to rev out to generate power. For a gas engine thats the way of life.

 

[looknlearn]

Ok, I think I got it,:thumbsup: Thanks for taking the time to explain.

 

[LD1]

Ok, I think I got it,:thumbsup: Thanks for taking the time to explain.

No problem.

 

[Dargo]

Ok, I have a related question. How can someone tell me what they consider a "reasonably accurate" estimation on HP and torque given my 1/4 mile time slip knowing my weight at the line was right at 8200 pounds? Pictured is the time slip. Is it possible for a person to calculate HP and torque assuming that I had minimal tire spin?