How does horsepower affect performance?

[jcmseven]

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]

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]

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]

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]

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]

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]

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]

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]

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]

</font><font color="blue" class="small">( If time on the job is a concern, get the biggest, strongest machine you can fit in the spaces you need to work.)</font>

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.