Horsepower vs. torque

[JDgreen227]

When you buy something with an engine that is rated by both horsepower and torque, which rating do you pay more attention to? For me, in an automobile engine that is only subject to passenger use, horsepower is the factor. For my tractor and truck, I based my choices on the torque rating instead. Not sure if others do it that way themselves.

A guy I used to work with asked me one time (he wasn't very mechanical) to explain the difference in the two ratings. I told him:

"Horsepower is what determines how fast you can be traveling before you hit a brick wall, torque determines how far you will travel through it after the impact".

Have I got that right, or not?

 

[Richard]

I was always told "Torque is what GETS you going and HP is what KEEPS you going"

Given the over kill I have at home (full sized industrial backhoe/loader) I don't worry about either one.

Actually, that's not 100% true... although I don't worry about the HP or torque situation, I DO find myself listening while working, to the wonderful whine of the turbo.

Probably strange, but true.

 

[LD1]

HP is derived from torque. It is nothing more than a calculation using the torque because the HP cant directly be measured.

And the best description that feel makes the most sense is:

Torque is how much work CAN be done.

HP is how fast it can get said work done.

So when it comes to things like towing, torque is the measure that defines how much it can tow. HP defines how fast over a given distance it can tow that load.

For a hypotherical example:

Truck 1 has 200HP and 1000ft-lbs of torque. It can tow a pretty heavy trailer, just be a little sluggish.

Truck two has 500HP but only 500ft-lbs of torque. The 500HP looks great on paper, but cant even move the load.

Of course there are other variables as well, like gearing and traction. But that is a whole other issue.

 

[Grand_Pooba]

Horse power = Torque x RPM divided by 63,025. Which means an engine with less torque can generate the same HP as a higher torque motor by running at higher RPM.

 

[Mickey_Fx]

Torque is the rotational force around an axis. It is something that can be physically measured. On the otherhand, HP is a mathematical calculation that defines how much work can be done over a given time period. (not going into the math here)

You can apply torque to an object without it moving. Without movement there is no work being accomplished thus no HP calc is possible.

 

[SandburRanch]

Torque consumes HP.:laughing: If in doubt torque 96 Waukesha head bolts to 600 lbs. in 96 minutes with an ambient temp. of about 120 and you'll find you don't have much HP remaining with your tongue hanging out.:laughing:

 

[Richard]

I don't remember the number, but I remember reading once that the HP/Torque curves always intersect at a certain point (I think certain RPM) because of how the HP is calculated.

 

[LD1]

Horse power = Torque x RPM divided by 63,025. Which means an engine with less torque can generate the same HP as a higher torque motor by running at higher RPM.

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

I don't remember the number, but I remember reading once that the HP/Torque curves always intersect at a certain point (I think certain RPM) because of how the HP is calculated.

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.

 

[JDgreen227]

I don't remember the number, but I remember reading once that the HP/Torque curves always intersect at a certain point (I think certain RPM) because of how the HP is calculated.

My GMC (300 hp Vortec 6000 V8) horsepower and torque curves intersect at close to 2600 rpm, the 4800 and 5300 V8's (these are all Y2K figures) at very close to the same point...so you must be onto something there. :thumbsup:

 

[LD1]

My GMC (300 hp Vortec 6000 V8) horsepower and torque curves intersect at close to 2600 rpm, the 4800 and 5300 V8's (these are all Y2K figures) at very close to the same point...so you must be onto something there. :thumbsup:

That is IMPOSSIBLE. Plain and simple. The math just doesnt support those figures

 

[slowzuki]

May have plotted the curves in metric units kW and N-m? The curves only intersect if you use the right imperial units and plot both on the same axis.

BTW, the original definition isn't right at all. Torque is how hard the engine can twist. Power is just a function of how fast it can turn while providing that twist. Torque is a force, power is force per time.

Example:
You stand on the end of your lug nut wrench and generate 200 ft lbs of torque but if the nut doesn't break free you are not generating any power.
You turn a little screwdriver at 5 ft lbs of torque and 10 rpm, you generate power!

 

[zzvyb6]

Horsepower is the amount of work the motor can perform under a steady state condition. Getting to that condition takes torque. Torque is the first derivative of the horsepower function. Its the slope of the horsepower vs rpm line at ANY stated rpm. Both functions are nonlinear and decreasing functions. That means they act like softening springs: The more you push, the softer they get. These limits are defined by limits on breathing (intake design), compression ratio, number of cylinders, octane rating, supercharging and turbocharging.

For example, it actually only takes 12 -14 hp to make a car go 60 mph. But the torque function for a motor whose max HP is 14 is pretty flat vs rpm under a massive load. So, you need at least a 100 - 200 hp motor in order to get the acceleration (torque) that is satisfactory for you to get a 3000 - 4000 lb car up to speed in a 'reasonable' time or distance.

Diesels are generally known for lower torque for the same power level compared to gas motors. This is due to combustion engineering and the lower rpm a heavy fueled, high rotating inertia motor needs to run at.

Steam and electric motors come to mind as the highest torque at low rpm which rolls off as rpm increases. They're max torque is at stall rpm.

A hybrid vehicle tries to take advantage of both types of motors: Electric to get you going and gas to keep you there at that speed.

 

[JDgreen227]

That is IMPOSSIBLE. Plain and simple. The math just doesnt support those figures

I'm sorry that you disagree, but the information I furnished was directly from the sales brochure for my GMC....for the 6000 V8, peak torque of 355 net lb-ft is at 4000 rpm, peak net horsepower is 300 at 4800 rpm. The graph shown illustrates the horsepower and torque curves intersecting at approx. 2600 rpm. All I did was state the information provided from the factory literature. They are NOT in metric units.

 

[brain55]

I'm sorry that you disagree, but the information I furnished was directly from the sales brochure for my GMC....for the 6000 V8, peak torque of 355 net lb-ft is at 4000 rpm, peak net horsepower is 300 at 4800 rpm. The graph shown illustrates the horsepower and torque curves intersecting at approx. 2600 rpm. All I did was state the information provided from the factory literature. They are NOT in metric units.

It was in the scaling of the graph. Because HP is a mathematically derived number, torque will always be double what HP is at 2626 RPM it is mathematically impossible for it to be anything else, just as it is mathematically impossible for the HP and torque curve to not intersect at 5252 RPM.

Brian

 

[brain55]

Here is the math to back it up.

HP = (torque x RPM)/5252

if you make the RPM 5252 here is the equation

HP = (torque x 5252)/5252
5252 x HP = torque x 5252
(5252 x HP)/5252 = (torque x 5252)/5252
HP = torque

So if you have 500 HP at 5252 RPM you will also have 500 ft lbs of torque at 5252 RPM

at 2626 RPM here is the equation

HP = (torque x 2626)/5252
5252 x HP = torque x 2626
(5252 x HP)/2626 = torque
HP x 2 = torque
HP = torque/2

If you have 500 HP at 2626 RPM you will have 1000 ft lbs of torque at 2626 RPM

If you don't understand algebra this will probably do more damage than good.

Brian

 

[JDgreen227]

I don't understand algebra, and apparently, neither do the engineers who authored the GMC sales brochure I quoted my information from...

 

[brain55]

I don't understand algebra, and apparently, neither do the engineers who authored the GMC sales brochure I quoted my information from...

I'm sure the engineers understood the math, but the marketing department skewed the graph to make it appear as the HP and torque were the same at 2600 RPM. If you scale it so every HP equals 2 ft lbs of torque, you could get the graph that you saw.

Brian

 

[JDgreen227]

I'm sure the engineers understood the math, but the marketing department skewed the graph to make it appear as the HP and torque were the same at 2600 RPM. If you scale it so every HP equals 2 ft lbs of torque, you could get the graph that you saw.

Brian

THAT kind of logic makes sense to me...thanks...:thumbsup:

 

[slowzuki]

Not exactly, yes, the power is what you need to get a car up to speed as 12-14 hp as you say could only move a really really aerodynamic car at 60 mph on flat ground.

You need the extra power to:
1) climb hills, your 12-14 hp goes out the window when you climb a hill. A one ton car needs about 40 hp to climb a 600 ft high hill in 60 s.
2) fight headwinds. No one wants their car to come to a stop when driving into a 60 mph headwind.
3) Get up to speed in a reasonable time, consumers want 0-60 in less than 10 secs now with every kind of vehicle. Need power to do that, heavier the vehicle, more power needed.

For example, it actually only takes 12 -14 hp to make a car go 60 mph. But the torque function for a motor whose max HP is 14 is pretty flat vs rpm under a massive load. So, you need at least a 100 - 200 hp motor in order to get the acceleration (torque) that is satisfactory for you to get a 3000 - 4000 lb car up to speed in a 'reasonable' time or distance.

 

[LD1]

Thanks for backing me up brian:thumbsup:

As stated, it is impossible for the curves to cross at anything other than 5252.

But as brian mentioned, engineers like to play with the numbers to get it to show what they want.

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