Re: Torque vs. Horsepower

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.
See posts with no answers
Torque is fairly easy to measure on a dyno or engine test stand and the horse power is calculated from that, per the formula quoted previously here. All engine and vehicle manufacturers quote flywheel torque and horsepower, but after market places like Banks or Penske may measure and quote their improvements at the drive wheels, where it counts.

Torque and hp quotes are quoted at their peak values and the peaks occur at different RPMs, so you can never get the peak of both toque & hp at the same time. And often the peaks are at RPMs you would not want to run at anyway, so they don't mean much. For practical purposes, torque and hp that is available between 800 and about 2500 RPMS is what is useful meaningful.

Torque at the drive wheels can be multiplied by gearing but horsepower cannot.

All this is explained in layman's terms in some of the articles referenced here.
 
davemittan said:
Interesting page.  But who is Mark Quasius?

I'm still trying to understand those two types of power.  I've seen graphic representations of HP and torque curves - I assume they're plotted using a dynamometer.  How does a dyno isolate/measure the two different types?
Click to expand...

The page link to the explanation of horsepower and torque does a pretty good job explaining the difference, however, there are several mistakes on that site, so caveat emptor.  If you want a top-notch explanation of Power, Work, and Torque the following is the best explanation I have seen:  

http://www.pump-zone.com/motors/motors/confusing-units-of-measure-work-power-and-torque.html

The biggest misconception folks have (and the reason I made a comment in another thread that was split off into this thread) is that torque is the only thing that matters with a big rig, and that is simply not true.  Torque is simply a measurement of the amount of rotational force than an engine can generate, in this case we call it torque because we are measuring the force performed by a rotating system.  Torque has no time component.

Power, which does have a time component, is how quickly force is being applied.  It can be though of as work performed per second.  It is the product of the force of the engine and the angular speed (RPM).  The calculation to convert torque to power is:

Power (ft-lbs per minute) = Torque (ft-lbs) x 2(Pi) (radians) x RPM

Because 33,000 ft-lbs per minute equals one horsepower, we can convert the equation by dividing 33,000 by 2Pi (6.2857) to get to our convenient 5250 factor and the equation can be simplified as

Power (horsepower) = Torque (ft-lbs) x RPM / 5250

The reason power is important is that the resistive forces of gravity, air, friction, all increase proportionally or geometrically with velocity.  In other words, the rate at which the external forces on your vehicle are pushing against you, goes up with speed. Again, because we are talking about the RATE of applied force, we are talking about power.  

If you want to know how fast you can climb a given hill towing a specific weight, one needs only to look at the available horsepower that the engine can provide to overcome the aforementioned forces.  The available torque is irrelevant in this circumstance.

HOWEVER, I am not making a claim that torque isn't important.  Torque will dictate the force that can be applied to the wheels, which will also dictate acceleration Force = Mass x Acceleration.  A high torque motor will be able to accelerate a mass more quickly than a low torque motor using a conventional transmission.  This is why a gas engine is simply inadequate for coaches with a GCWR exceeding 26,000 lbs...the real world acceleration is not sufficient.

So, simply put, in real world conditions you need both power and torque.  Just don't be fooled into thinking that your 1200 ft-lbs and 275 HP diesel will out climb a gasser with 350 HP and 450 ft-lbs.  If they're moving the same mass, the gasser will climb faster.








 
regularguy said:
I remember my buddies laughing at me one day ... oh about 40 years ago. Ya see we were talking about the bull-dozer that was redoing the street, and the discussion came to its horsepower. My statement was that horsepower wasn't so important as the torque. I further stated that one could get that dozer ot move with a lawn-mower engine if it were geared properly... that as far as driving force was concerned, torque reigned supreme. I tried to use the example of a "one-lunger" but they had never heard of it and by that time were too hysterical to listen anyway.
Click to expand...

You are, of course, right.. I "Grew up" on a farm tractor, Mostly a Ferguson TO-30.. This 30 horse power tractor pulled two sixteen inch plow points turning a full 32 inches of soil with every pass.. In later years my dad upgraded to an MF (Massey Ferguson) 65 Diesel, it pulled 4 or 5 (forget which)

Of course the top speed of the TO-30, lightly loaded, is 14 MPH, that is in 4th (road) gear. the MF-65 had 10 gears and could hit as I recall 20.  Engines are governed which limits speed somewhat.

My Workhorse's Chevy Vortec 8.1 cranks what over 300 HP. and is limited to 80mph by the computer.

But I hooked that 65 HP diesel to a F-350, (around 250HP) which was stuck in mud and put it in compound low (low gear low range) when the chain came tight, the engine did not even change note.. the truck, however, was no longer stuck in the mud.

Sixty Five HP
 
Mc2guy, thanks for posting that explanation and the link. Takes me back too many years to my school days, but a lot of it came back as I was reading. Might be time for an update/clarification to our library file.
 
Mc2guy said:
So, simply put, in real world conditions you need both power and torque.  Just don't be fooled into thinking that your 1200 ft-lbs and 275 HP diesel will out climb a gasser with 350 HP and 450 ft-lbs.  If they're moving the same mass, the gasser will climb faster.
Click to expand...

OK, you lost me here. If this is correct, then why can't my 20,000 gvw m/h with a Ford V-10 keep up with my friends 26,000+ gvw m/h with diesel when on a long grade. On a flat there is no problem, but when we hit a long grade, he just cruises up while I bog down. I have more hp, but he has the torque.

It seems that the real world shows that when moving heavy loads like a motorhome, torque is what is important. Race cars (especially Formula 1) have enormous hp because they are light and want to go fast. Big rigs, busses, etc. use high torque engines because they want to keep their load moving, although not necessarily fast.

You said torque is required to accelerate a heavy load. Maintaining a set speed on a grade is essentially the same as accelerating, that is, continuing to provide sufficient force to offset the various drags. Can you maybe clarify this?
 
Just don't be fooled into thinking that your 1200 ft-lbs and 275 HP diesel will out climb a gasser with 350 HP and 450 ft-lbs.  If they're moving the same mass, the gasser will climb faster.
Click to expand...

Having owned coaches of both types, I can assure you that is not so unless you also state that both engines deliver their torque & hp at the same RPMs and have similar transmissions. Or that you are willing to climb that hill with the gasser screaming at 4200 RPMs or more.

Quotes of torque or hp out of context with RPMs, gearing, etc. don't have much practical value.

If this is correct, then why can't my 20,000 gvw m/h with a Ford V-10 keep up with my friends 26,000+ gvw m/h with diesel when on a long grade. On a flat there is no problem, but when we hit a long grade, he just cruises up while I bog down. I have more hp, but he has the torque.
Click to expand...
It because he has plenty of low end torque (and therefore low end horsepower too), while you do not. If you wind that V10 up towards it red line, you will keep up with him.
 
jrshall said:
Race cars (especially Formula 1) have enormous hp because they are light and want to go fast.
Click to expand...

Those engines can also turn at up to 20,000 RPM.  Remember, HP=torque X RPM / 5250.  

With most gas engines, you get both the torque peak and HP peak at high RPMs.  So in order to get the power, you need to downshift to get the RPMs up, but sometimes you can't get the RPMs up, which means you can't get the power.

I was going up a medium incline on Saturday with my Xterra.  My powerband is above 4000RPM.  I was going 60MPH and I had the pedal to the floor.  The transmission downshifted from OD to 3rd gear, I was going too fast for 2nd gear.  In third gear at that speed, I was stuck around 3300 RPM, too low to get power.  I was not loosing any speed, but I could not get any either.  If I had approached the hill at 75MPH, I would have had power to use.  I was hauling an empty utility trailer with 20" plywood sides and no cover.  So the 300 lbs was negligible, but the I could feel the wind resistance.

If I would have had more low end torque, I would also had more HP at those lower RPMs.

So with very high torque at low RPM, you can produce more HP at lower RPM.  So yes HP is very important, but you need torque to get that HP, and depending on the engine, you get your torque at various RPM, therefore also get your HP at various RPM.
 
RV Roamer said:
Having owned coaches of both types, I can assure you that is not so unless you also state that both engines deliver their torque & hp at the same RPMs and have similar transmissions. Or that you are willing to climb that hill with the gasser screaming at 4200 RPMs or more.

Quotes of torque or hp out of context with RPMs, gearing, etc. don't have much practical value.
It because he has plenty of low end torque (and therefore low end horsepower too), while you do not. If you wind that V10 up towards it red line, you will keep up with him.
Click to expand...

Again, there are many variables at play, transmission included, but ultimately, assuming the transmission allows for it, my arguement stands...horsepower is what is required to overcome the gravity, wind, and drive line losses.

Yes, you will have to rev up a gas engine to do it (that's what it is designed for), but you can and will outpace a lower HP rig assuming you 1) can get to the useable rpm with your transmission, and 2) are carrying the same weight.

Jrshall, As for why your friend is passing you on a hill, if you expect a gasser to pull up the hill at 2500 RPM, it isn't going to happen.  The physical characteristics of a gas engine design allow it to spin faster (shorter stroke, wider bore, less rotating mass).  Don't be afraid to let it rev up for the power you are looking for.

To be clear, I am NOT making a gas vs. diesel argument.  When all variables are considered (efficiency, durability, longevity, and performance), diesel engines win hands down when it comes to pulling heavy loads over long distances.  That's why they are used in these applications.  I am merely clarifying why "power" is equally if not more import when considering engine performance specs for a given application.




 
I'm convinced, and will be replacing the big CATs (combined 1320 hp) on the boat (50,000 lbs) with a 10hp gas outboard motor. Might take longer to get out of the hole but, once it gets up there, it'll be screaming along. OTOH maybe I could use the 42 lbs thrust trolling motor from the fishing boat. No more fuel bills, and the 10 golf cart batteries should last all day  ???
 
I'm with ya tom, I think I'm gonna put my ole 455 olds motor in my coach, heck it's got 500hp to my 350 cat whatta ya think? I guess I'm just a dumb country boy. :eek:
 
kevin said:
I'm with ya tom, I think I'm gonna put my ole 455 olds motor in my coach, heck it's got 500hp to my 350 cat whatta ya think? I guess I'm just a dumb country boy. :eek:
Click to expand...

Sounds like a good idea!  Try a hamster wheel, or better yet a rubber band...it'll work even better. ;)

 
We live in high altitude, and do most of our driving in high altitudes with mountain passes.

We just got back from Durango, CO, and coming up across Wolf Creek Pass (I don't know....over 11K elevation?), our 8.1 gas engine screaming at 4,300 rpm, engine fan roaring, going 30 mph, and this guy I had passed on the flatlands came whizzing up and passed me like I was standing still....I couldn't believe it.  :eek:

He was driving a newer Dodge Dually with a Cummins, pulling a monsterous 5th wheel......
 
I went tru all of the posts (with the exception of the articles refenced), and noticed that no one mentioned the reason why a diesel has more torque than a gas engine despite the gas engine having more horse power. Well, travel displacement of the piston on any engine plays an important role on the torque output of such engine. A diesel engine has more travel displacement than a gas engine of similar characteristics. This gives the diesel more torque per revolution; therefore, it takes the gas engine more revolutions to accomplish the same torque. Most claims of HP on gas engines are calculated at the shaft and at RPM's of 5000 or more (I'm yet to see a winnie going at 5000rpm's). Of course this means that if you do much of your driving between 1500 and 2500 RPM's, chances are your gas engine is outputting about 1/4 the HP of what is capable of (not counting the tranny, drive shaft and differential which will reduce the HP even more). Having said that, when climbing a hill, a diesel engine is going to require less RPM's (not speed) to get to the top than a gas engine. Also weight plays a role on torque after the differential. This last statement is more noticeable on a 1/2 truck with a posi-lock differential, but it goes the same for any other vehicle. The more load you put on a differential, the higher the transfer of torque to the wheels. Now if you ask me which is better torque vs. HP, I would say, " well, it depends whether we are hauling or racing".
 
kevin said:
but which one will have more torque? cuz I like torque, or should I go with more horsepower?
Click to expand...

I don't know, I guess it depends on the size of your hamster!  ;D
 
If anyone seems to think that gas engines are better at pulling loads than diesel then please explain to me why there are no gas engine motorhomes over 38 feet.
 
seilerbird said:
If anyone seems to think that gas engines are better at pulling loads than diesel then please explain to me why there are no gas engine motorhomes over 38 feet.
Click to expand...

That statement was NEVER MADE...go back and re-read the posts.  This is a HP and Torque discussion, NOT as gas vs. diesel discussion. 
 
Mc2guy said:
That statement was NEVER MADE...go back and re-read the posts.  This is a HP and Torque discussion, NOT as gas vs. diesel discussion. 
Click to expand...

Calm down. I never said that statement was made, if anyone would have made that statement I would have quoted the statement. I said "if anyone thinks..."
 
You must log in or register to reply here.

Latest posts

Members online

Total: 1,102 (members: 23, guests: 1,079)

Forum statistics

Threads
131,933
Posts
1,387,733
Members
137,683
Latest member
chaderic
Back
Top Bottom