How does horsepower affect performance?

jcmseven · Dec 15, 2005

Fellow Posters:

I have a question that came up for debate yesterday between myself and a friend. We discuss often about PTO hp and its affect on attachment performance, but what about hp and ground engaging performance. Does a tractor that has more horsepower and torque have more "pulling power" with a box blade, rear blade, loader, etc (pushing there) than one with less? Since I am a John Deere owner I will illustrate with that brand but any brand owner can chime in. For example: a 3320 John Deere (32.5 hp; 25.5 PTO hp and 62 lb-ft/torque, non-turbo) v. a 3720 JD (44 hp; 35.5 PTO hp and 84 lb.ft torque (rated)); would they pull a box blade the same, or one better because it has more power. They weigh the same; they can both be ordered with hydro trannies and similar tires. Assuming that each is similarly ballasted, which would do better? Would a gear drive (sync) make either one do better? This question came about because one of my work colleagues is looking at a new tractor (the two mentioned). Because the 3720 is more expensive he thought the 3320 would be better, but he is strictly using the machine for grading and loader work. It likely will never see a PTO attachment; should he spend the extra money on the more expensive machine, or would they be equivalent in this case?

John M

HGM · Dec 15, 2005

OK,
I may be off base a little bit, but I'll give my opinion based on diesel engines in general.

No, a 32HP/64lb ft tractor will not have the same capability of a 44HP 84lb ft tractor. As you stated, the PTO HP will be irrelevent if the PTO is not used. However, the engine produces 12HP 20lb ft more than the smaller one. This translates in more ability to do work.

Tq= turning force
HP= ability to do work

I'm not sure if I helped. But if the question is, will a larger HP tractor has more capability than a smaller one. The answer is definitly.

browns40 · Dec 15, 2005

Hi jcm,

For sure more HP(torque)=more work potential...

What is he loading & grading? The 3720 should be able to "size up" in most implements over the 3320 (6ft cutter, Box, BB, tiller(?) instead of 5 foot)

If he goes bigger he needs to get more traction to actually use the extra power, so more ballast... otherwise the machines are equal... both spin the tires with the blade scarifiers trapped, or both spin at the same point in the pile.

Need more weight & traction for more horsepower to use it effectively.

Happy tractoring! /forums/images/graemlins/cool.gif

IslandTractor · Dec 15, 2005

As I understand it, the calculus is one of horsepower/torque and traction. Traction is affected by factors such as tire type, size and very importantly by weight of the tractor. I suppose in the example you gave, if the tractors are otherwise identical, that the higher powered tractor could pull a box blade in extreme conditions that the lower powered tractor could not, so long as traction was not the limiting factor for both.

It would be possible to have a higher powered tractor with less pulling power than a lower powered unit by, for example, putting turf tires on a lightweight tractor (eg 24hp Kubota BX23) and comparing the pulling power to a heavier tractor with Ag tires (eg 20hp Kioti CK20). Though I don't know that anyone has done the test, you would predict that for pulling a box blade or plow that the Kioti rig would still be plowing in conditions that caused the Kubota rig to stop due to loss of traction. However, in theory, if traction was not the issue then the higher horsepower/torque rig would win.

jinman · Dec 15, 2005

I agree with HGM's post; however, if the 32 hp tractor has enough power to break traction on a given load, the same weight tractor at 44 hp can do no better.

I like to use an analogy of a 12 lb test fishing line being used to catch a 25 lb fish. If an 80 lb, 12 year old boy has enough power to break the line, then a 6' 6", 250 lb man can do no better. He will also break the line if he pulls the same as the 12 year old boy. It's the person with the best technique who will eat fish. /forums/images/graemlins/cool.gif

Farmwithjunk · Dec 15, 2005

We could very well be opening Pandora's box with this topic. (BTDT)

Horsepower is almost an abstract term. Torque is PROBABLY a better measurement of an engines capabilities. But the wonderful world of power is related by horsepower numbers for most people. In simple terms, horsepower is (as I understand it) actually a measurement of work performed, more so than work POTENTIAL. I'm sure the "engineer lingo" will start flying soon.

But since most NON engineers compare trucks/cars/lawn mowers/tractors/trains/boats/power tools/ect. in terms of "How many horsepower", that magic formula has a relative worth.

PTO horsepower, engine horsepower, drawbar horsepower, and so on, are only usefull when comparing apples to apples. You can't compare (fairly anyway) the PTO horsepower of a 30 PTO HP tractor to the engine horsepower of a competitors model.

PTO horsepower ratings are still usefull when comparing different models. In fact, in the world of big AG tractors, that is USUALLY the comparitive standard.

To me, drawbar horsepower, or even lbs of drawbar PULL would tell me as much or maybe even more about a given tractors potential.

But when you get right down to it, ALL the numbers have some worth.

Then we get into the ability to measure USEABLE power. That has to do with traction (weight, tires, tranny, balance, soil/ground conditions, ect) Not every 30 HP tractor is capable of doing he same amount of work. It would be concieveable that a 25 HP tractor, ballasted correctly, could out work a 45 HP tractor that ISN'T set up correctly.

But hook them up to a PTO-driven implement, and all the wheel weights on earth no longer matter. With that in mind, PTO horsepower is PROBABLY a more useful scale to compare tractors.

All these variables were the motivation behind "The Nebraska Test's" done on farm tractors. They measured every concieveable avenue of power delivery.

But NOTHING is like "real world" "in the field" use to determine who's HP claims actually mean something.

daTeacha · Dec 15, 2005

Modifying HGMs definitions slightly, torque is the twisting force an engine is capable of applying to a shaft. Horsepower is the speed or rate at which this twisting force can be applied. Using cars as a reference since most of those here have more familiarity with cars than tractors, torque will allow you to light up the tires at a stoplight to embarrass that little old lady in the next lane, and horsepower will allow you to blow past the drunk driving 80 on the freeway, should you decide that is the prudent course of action.

When thinking in terms of pulling things with a tractor, a major factor is the surface on which you are working. If you observe carefully, a big farm tractor working hard will actually have the rear tires turning a bit faster than the tractor is moving across the field. Tire slip is a given and controllable factor in working with large tractors. The condition and type of the surface and the interaction of the tires with that surface limit your traction. As has been said, any torque or power delivered in excess of that limit is simply wasted.

All engines have a torque/horsepower curve which will show the rpm ranges for which each of those quantities is highest. The operator looking for maximum efficiency should choose the gear which allows the desired ground speed while keeping the engine running at the torque peak. CVTs (continuously variable transmissions) are the coming thing, allowing the computer to measure the various speeds and forces involved and to pick the most efficient of the infinite tranmission ratios available. For those unfamiliar with the concept of a CVT, think of two pulleys that consist of two moveable sides which can increase or decrease the diameters of the pulleys. As one gets a little bigger, the other gets a little smaller. Connect them with a very strong belt. Bingo! You have an infinite variety of ratios within the maximum and minimum of the pulleys. There are no set ratios, so in sense, it's like a HST. BTW, such engine/tranny combinations usually sound like they're running too fast for the ground speed to those of us accustomed to gear tractors.

For the type of use most posters here are likely to put their tractors to, it's pretty much irrelevant. As long as the thing gets the job done in the amount of time you want, you're golden. I don't think any of us are terribly concerned about fuel costs per job, paying the hired help on an hourly basis, application rate of fertilizers and pesticides, and all the other cost/benefit analysis things a professional farmer needs to factor in when going about their business. Unless someone keeps stalling their engine trying to do a specific job in an allowed time, the tractor is big and strong enough. If time on the job is a concern, get the biggest, strongest machine you can fit in the spaces you need to work. If more seat time is a priority, get a smaller one. /forums/images/graemlins/smile.gif

This will be an interesting thread to follow as the diesel pros get into the act. /forums/images/graemlins/smile.gif

timb · Dec 15, 2005

As has been already stated - traction is likely the limiter. Although you are citing two otherwise identical tractors with different hp engines - it is likely that out of the box (given identical tires) both will spin before they bog.

Now - with maximum ballast - enough so that you can stall the lower hp tractor - then the extra hp of the 3720 *might* start to do some good.

I say *might* - because there is another 'problem' involved since you are specifying hydrostatic transmission equipped tractors.

A hydrostatic transmission ultimately 'tops out' (i.e. stops putting through any more power) when it's relief valve setting is reached. IF both tractors (once ballasted) are capable of 'hitting their relief' before they spin AND those two tractors have the SAME relief valve settings - then it would seem that the 3720 will NOT be able to put out any more rear wheel power than the 3320.

On the other hand - if the 3320 simply doesn't have enough power to put the tranny into relief and the 3720 does - or the 3720 has a higher relief setting than the 3320 - then the 3720 could put more power on the ground.

I don't know if this situation exists for Deere, but NH does (or did) have this happening on their TC35-40-45 series. All have identical trannys so the big hp TC45, while it has more PTO hp - can't deliver any more rear wheel hp than the TC35.

And yes - a gear tranny will technically put more power on the ground period over a hydro - it's slightly more efficient - and it would allow the higher hp otherwise identical tractor to get its power all the way to the ground since the tranny never 'tops out' unless the clutch slips.

But again - that extra power delivered to the tires will not do much unless you've got the ballast to get the traction.

HGM · Dec 15, 2005

Guys,
I just thought I'd add something after reading it again. He stated the guy will beusing the tractor for loading and grading. Traction will be an issue, no doubt, it will need to be balasted (/forums/images/graemlins/grin.gifis that a word?). However, the hydraulics and weight handling would dictate the larger tractor, in my mind. Feel free to correct me, ground speed wont be a factor but shear power and ability will.

3RRL · Dec 15, 2005

( If time on the job is a concern, get the biggest, strongest machine you can fit in the spaces you need to work.)

Good post...Especially this last part.

I believe in more horsepower being better. Traction will, without a doubt, change over the work field as he passes over mud, sand, rocks, grass, pavement, etc. Enough traction allowing...assume he encounters a maximum load that stalls out the lessor hp machine, the greater hp machine would continue on, until it reaches it's maximum load. And a machine with even more hp would continue on etc.
I believe your debator, however, focuses more on the practical than theorhetical. Otherwise he would see this clearly.

rk246 · Dec 15, 2005

As I have always thought about it,

Torque= ability to get something moving
HP= the ability to keep it moving...

I would think the higher hp tractor would give you more ability to pull the box blade, grade etc.

I don't know if the bigger deer has larger tires on it? But I went from a Kubota 3130 to a 3830 which has larger tires and more hp with pretty much everything else being the same. You definetely see the increase in hp while doing just about anything....

hudr · Dec 15, 2005

I have to agree w/ most of these guys here. We are talking about slow moving pieces of machinery. Torque multiplication is how tractors get things done. Power (HP or Torque) is almost abstract. It is the ability to put that power on the ground.
Example: I have a 50HP 2WD Mahindra. It is at the edge of its TRACTION envelope w/ a 6.5' offset disc. However, I was lent a 40hp 4WD Kubota during a warranty issue w/ my tractor. With all 4 tires pulling, the little Kubota pulled the daylights out of that disc. It wouldn't budge it in 2wd.
So, I would say power is important, but not as important as the application of that power.
In direct answer to the question: I would get the larger HP tractor if possible. It is much easier to add weight and ballast than it is HP.

jcmseven · Dec 16, 2005

Thanks for the replies so far. To me, this is an interesting question, and one I do not know the answer to assuredly. It would seem logical that the two tractors would be equivalent in their pulling of implements. There seems to be a difference between PTO horsepower and drawbar pull. If the two machines listed are equal in weight and have similar tires, they should seem to pull about the same, even though one has more engine power (both torque and horsepower). The only area that I personally see a difference would be with regard to speed. For example, if each machine were pulling a box blade that requires 4 mph at a given point (grade, hill, etc.) to maintain enough momentum not to slow the tractor until it bogs or loses traction then it seems there may be a difference. The more powerful machine seemingly being more likely to keep the speed up such that it does not dip below the point at which the tractor is struggling to keep momentum up against a heavy object. This would seem to be of little benefit when moving very slowly, or in tight quarters. The big question is: in real world usage, would that be enough to merit spending the extra money? My friend is very practical and would shoot me (kidding) if he spent more than he needed to for the type work he will be doing.

John M

rk246 · Dec 16, 2005

Just tell him you did some research and all of it showed that bigger is better, it was very scientific.... /forums/images/graemlins/smile.gif

slowrev · Dec 16, 2005

To me it works like this.

Torque = the ability to do work.
HP= How fast it will do the work.

Totally unscientific, just my opinion

Ben

IslandTractor · Dec 16, 2005

I suppose you could make an argument that higher horsepower would allow you to pull an implement at higher speed. Not that it would be reasonable use of a tractor but you could pull a box blade at road speed in high gear if the tractor was powerful enough. I think the real point here is balance: does the tractor have sufficient power to get the work done in a reasonable amount of time? Any given task is likely to involve much more than just pulling the implement so doubling horsepower/implement size does not halve the time involved. How often would a few extra HP really change the way you get work done or save significant time to justify higher purchase and operating costs?

3RRL · Dec 16, 2005

It doesn't matter which side of the discussion you're on. In order for something to be "justified" to a person, there are a million factors involved, mostly personal reasons that make sense only, personally, to that individual. Facts are weighed and measured to differing (individual) needs...you know what I mean. "Well, that's nice but I don't need that" or "This one is good enough for what I need" or how about this one "Geez, whish I could afford THAT!" etc...

But as far as your example in the comparison, there is a difference, for sure.

( If the two machines listed are equal in weight and have similar tires, they should seem to pull about the same, even though one has more engine power (both torque and horsepower).)
This not completely true nor does it ( "seem logical")... "about" is closer, though.
Let's exaggerate the example to put this HP and torque thing into perspective, accentuating the difference:
Imagine the two with same weight and tires, as you said, only they are semi-trucks and trailer(80,000lbs). One has a 1500cc VW engine (less hp & torque) and the other a 500HP+ Diesel (WAY more HP & torque)...

( The only area that I personally see a difference would be with regard to speed. For example, if each machine were pulling a box blade that requires 4 mph at a given point (grade, hill, etc.) to maintain enough momentum not to slow the tractor until it bogs or loses traction then it seems there may be a difference.)
The BB being the trailers and the grade being the Grapevine (interstate 5 in Ca)...yes, I would say there'd be a difference.

( The more powerful machine seemingly being more likely to keep the speed up such that it does not dip below the point at which the tractor is struggling to keep momentum up against a heavy object.)
Totally agree there.

( This would seem to be of little benefit when moving very slowly, or in tight quarters. The big question is: in real world usage, would that be enough to merit spending the extra money?)
Just tell your friend to ONLY use his tractor for above description (limit his loads and speeds, stay off ANY grades) and NEVER use for anything else and I'm sure he'll be happy. Tell him he can borrow your tractor to do everything else. /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

Sorry to seem so pushy about this /forums/images/graemlins/confused.gif, but since you brought up the subject, it is important to understand that there really is a difference, and always will be. Now, how much of a difference in the practical, real world...I couldn't put a value on that??? Hope that's alright with you. As I said above, personal prefference and usage...whatever, will play a greater roll in the choice your friend makes anyway.

jcmseven · Dec 16, 2005

Good points on all accounts. I hope this has been a reasonable discussion. I never divulged which side of the discussion I am on, but it has been for more power if the cost is reasonable.In my memory, I can recall no tractor owner, or really owner, who regrets getting a more powerful machine. I have seen many who regretted getting a less powerful machine, and I have been one of those in the past. To me, that difference was worth as much as I could afford, to him maybe not, but we will see.

John M

jmc · Dec 16, 2005

If you only need torque, you can get it by downshifting. You could pull stumps with a squirrel in a rotating cage if you have enough gear reduction, traction, and patience. Two squirrels would pull the stump twice as fast, if they were the of the same ***, straight, and cooperating. You would also need to halve the gear reduction.

John

flwiii · Dec 16, 2005

Horsepower is really a relative term. 1 horsepower, as defined by Mr. Watt is the power required to raise 33,000 pounds 1 foot off the ground in 1 minute. This led to the constant used to measure horsepower (5252). Therefore horsepower is RPM times torque divided by 5252. So horsepower is really an expression of torque at a given RPM.

There are too many variables to determine which tractor would pull better. The biggest variable of couse is putting the power to the ground. This is the same problem that drag racers face. More torque at a given RPM and therefore more horsepower does not necessarily equate to a win. It has always come down to how well you can apply that power to the ground. If you break them loose all the way down the track, you've got too much horsepower. If you can't break them loose at all, then you may well have too little horsepower.