Torque Understanding

[Stallion]

I have been currently reading about the fundamentals on torque and trying to understand them as best as possible, but I still have some things to clear up about it.

As I understand it, there is a simple equation relation HP, RPMs, and Torque. It is as follows (according to Richard Newton):
HP = (RPM x Torque) / 5,250

So, HorsePower (HP) is a constant then? Or does it change with the RPM changing (as, of course, the RPM must change)? Or is it Torque that changes? From my understanding, Torque is the variable in engines and maybe HP is the constant?

If this is so, then with higher RPMs comes lower Torque. But Newton said that you want higher Torque because Torque is power. Right? But in racing, don't you want the higher RPMs? If so, wouldn't that result in lower Torque which would be less power?

And if HP is so desireable, wouldn't more HP mean less Torque?

I'm afraid I don't understand this. Could somebody help me clear up my confusion?

Thanks!

TR

 

[JohnZ]

The explanation can get pretty complicated if you get in deep enough, but in basic terms, TORQUE is FORCE applied; think in terms of a wrench on a bolt - if the wrench is one foot long and you hang a ten-pound weight on it, the force applied to the head of the bolt is ten foot-pounds.

HORSEPOWER is WORK - the rate at which a given force is applied, which implies time; in the torque example above, if you take one minute to turn the wrench one full turn while applying enough force to move it, that doesn't take much effort. However, if you exert the effort to move it that same one full turn in only five seconds, you've accomplished the same result, but in 1/12th the amount of time, which took more WORK.

Looking at engines, torque is what gets the car moving quickly from rest, and plants you back in the seat; horsepower is what makes you go fast (in simple terms).

Formula 1 engines vs. the Viper V-10 are a good example of how the design characteristics of engines are biased one way or the other. Formula 1 engines are 180 cubic inches, and develop around 800 horsepower, but their power peak is around 17,000 rpm in order to develop that power, and they have VERY poor low-speed torque (about 250 ft-lbs.), and their power band is very narrow, almost useless below 12,000 rpm, and as a result the're extremely difficult to drive, requiring seven-speed transmissions in cars that hardly ever run below 100 mph. The Viper engine is 505 cubic inches, almost three times the piston displacement of a Formula 1 engine, and makes 520 horsepower at 5600 rpm; however, it has over 400 ft-lbs. of torque at only 1600 rpm, 530 ft-lbs. at 4000 rpm, and still has over 400 ft-lbs. at its 5600 rpm power peak. These characteristics make it easy to drive, the very high torque launches it effortlessly, over 80% of the torque is still available at the power peak, and even though the Viper is three times the weight of the Formula 1 car and has three times the aerodynamic drag, it will go 192mph.

In 1/4-mile terms, torque = elapsed time, and horsepower = speed at the end.

There are 400-page Engineering textbooks that explain this; I've just tried to illustrate the basics.

 

[Ken]

John, can you help clarify why sometimes it is referred to in pounds/feet, and sometimes foot/pounds? I thought I had the basics down pat a while ago, but we muddied the waters a bit in another thread some time ago. :eyerole

_ken

 

[Stallion]

Thanks for that explanation, John. And I guarantee you, I'm to read it about 12 times fold while I'm sitting in this chair (I've already read over it about 7 times ) in hope that I truly understand it.

So, if TORQUE is force, then should torque only count when you are accelerating? But, when you acceleration is 0 and your velocity is, say, 192mph then that's when HORSEPOWER matters?

That's my summed up understanding of what you say, John. Is this correct? Or am I still missing the torque basics?

Thanks again!

TR

 

[Ken]

"Torque gets you moving, horsepower keeps you moving." Heard that one somewhere.

_ken

 

[Stallion]

Hehe, thanks, Ken.

So to have a truly "fast" car, you have to have a good combination of both? With the assumption that you want fast acceleration. Otherwise you would want less torque and more hp like a Formula. Right?

Thanks again!

TR

 

[Geof180]

ken, i will give it a shot.... torque is just like an applied moment that causes a twisting force...

a moment is a force multipled by the perpendicular distance from the applied force to a point of interest...
i think the reason people switch them back and forth so much is because one person might be accustomed to using the metric system, hence N-m (Newton * meters).. force times distance..

therefore the english equivalent would be lbs-ft..

they are just the force, in pounds... multiplied by a distance, in feet... the units of ft-lbs or lbs-ft are the same thing...

did i clear anything up?? or shall i try again...??

have a good evening

Geof

 

[Ken]

No Geof, that is perfectly clear, as long as it's correct. :L

Let me see if I can find the thread to which I refer, where we discussed this at length...

Nah, I just spent ten minutes looking for that thread, but I can't find it. Our search engine isn't the best in the world, hell, I have a hard time finding something even if I know the title and author of it. The search engine is just plain stoopid! :duh

_ken :L

 

[Stallion]

What happens if you have a lot of torque and little horsepower? How would the driving be and act? And if you have little torque and a lot of horsepower, like a F1, then it would be terrible acceleration, but amazing speed. But what about the other way around?

Thanks!

TR

 

[Interceptor430]

a lot of torque and a little horsepower (and many gears), is
what moves an 18 wheeler down the road. yea, ok...throw in the high compression of the diesel engine too, and the turbo...but few rpms. most diesels develop their highest torque at only 1200 rpm.
Torque = acceleration (moving the load)
HP = Speed.

Lots and lots of variables...but that's the general idea I believe.

 

[Rob]

Stallion,

You may also want to check out our article in the Knowledgebase: Horsepower and Torque.

 

[Stallion]

Okay thanks, everybody. I think I pretty much understand the basics of torque vs. horsepower now. Thanks again!

TR

 

[JohnZ]

John, can you help clarify why sometimes it is referred to in pounds/feet, and sometimes foot/pounds? I thought I had the basics down pat a while ago, but we muddied the waters a bit in another thread some time ago. :eyerole

_ken

The general convention for describing torque was "ft-lbs" for decades; somehow it gradually got switched around to "lb-ft" in the late 80's - early 90's, possibly as part of the metric-conversion hysteria prevalent at the time. There are still some domestic monthly automotive industry trade journals (like Ward's Automotive Reports, etc.) that continue to express torque in Newton-Meters (N-M) and horsepower in kilowatts (KW) as they do in Europe, but it hasn't caught on, and it's a PITA to noodle the conversions every time they do it.

The semi-truck analogy is a good one for low horsepower/high torque; another one is a big farm tractor - not much horsepower, but GOBS of torque to pull heavy drawbar loads all day long at relatively low rpm. Here's another example - 12,000 horsepower twin-turbocharged V-20 diesel with 19,000 lb-ft. of torque - made for locomotives and ships that run at constant load all the time at high loads and power settings:

 

[Stallion]

So the difference between a Corvette/Viper and a truck/boat/tractor is that once a Vette/Viper gets accelerated with the torque, we have the horsepower to maintain and have speed. But the truck/boat/tractor on the other hand have to keep up that constant force and extreme torque to do their necessary task.

Is this correct?

And that's a big engine, JohnZ!! :eek

TR

 

[JohnZ]

So the difference between a Corvette/Viper and a truck/boat/tractor is that once a Vette/Viper gets accelerated with the torque, we have the horsepower to maintain and have speed. But the truck/boat/tractor on the other hand have to keep up that constant force and extreme torque to do their necessary task.

Is this correct?

And that's a big engine, JohnZ!! :eek

TR

Yup, that's a pretty good analogy

 

[Stallion]

Just one more question (hopefully )...

In this equation "HP = (RPM x Torque) / 5,250"...RPM is a variable, and so is Torque? Correct? Is HP a constant in a car? So, if you are 30mph or 90mph, the HP is always the same?

Thanks, John!

TR

 

[Interceptor430]

Stallion,
Look at the equation again....HP, the answer, will change depending on the values on the other side of the = sign.
i.e. if RPM changes, the answer(HP) will change.

 

[Stallion]

So then HP is a variable? And Torque is a constant? The equation could easily be changed so that Torque is on the left side alone, and HP in the mess.

TR

 

[Interceptor430]

The only constant (not changing) item in the equation is the
5,250. The others are changeable/variable/and are affected by the values of each individual variable.

basic algebra stuff.
hp = (rpm X torque) / 5,250 can also be written as:

a = (b X c)/ 5,250

ok...solve for a, and replace b by any rpm number
and replace c by any torque number

yes, you can also solve for b by giving a and c a number...etc.

 

[Stallion]

I think I understand. So if you have x amount of torque at y RPMs, then you will have z HP. I think I see now.

Thanks!

TR