Tractor classification — HP vs torque

[Torvy]

High torque and low weight... How does that work out on a tractor?

 

[sd455dan]

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These four tractors are from the "25 HP" class... obviously of different physical size... Model and engine size I listed in RED... all the specs (rated HP, rated RPM) came from TractorData

Again, I don't get why Kubota doesn't tout their torque figures...

my 2 cents

When testing/dynoing engines Torque is measured. It can be "calculated" as above, but starts out as a measurement.
The amount of force applied at a distance- the twisting force.

1lb foot is 1lb of force applied 1 foot away from the axis of the shaft.

Some tractor owners (All else being equal) will chose an engine with the lowest RPM for the HP desired /rated.
That way the torque will be higher because the HP is just a mathematical derivative of both torque and RPM .
This choice does result in limiting the engines idle to top RPM "width"
But also seems to give a torque rise as RPMs are reduced.
These engines are generally of larger displacement and may be physically larger with more mass as well.
Not a bad thing since the engine is also a stressed part in many tractors.

Some brands like to advertise gross HP... which is pretty worthless.

The PTO HP is much more telling. Always impressed when the advertised HP is closer to the actual PTO power.
There does seem to be an overall trend to increase the RPM and reduce the Cubic displacement in newer CUT's YMMV

 

[finn1]

Back in the mid seventies, one of my first jobs out of college was developing master pump settings for various engine applications on the dyno. What I recall is that a relatively high power turbocharged tractor would have a torque rise of 15% at the time. NA engines had less torque rise, as they would quickly run out of air and be smoke limited as to the shape of the torque curve.Torque rise is the difference between torque at rated engine speed and torque peak. Fifteen percent allows the all speed governors to maintain constant engine speed when the plow or implement encounters difficult going in the field.

combines had no need for much torque rise between rated and peak torque , just based on the nature of operation, ie running on what is called the overrun curve between high idle and rated.
Some fuel systems didn’t have much sontrol of delivery at different engine speed, so you were essentially hardware limited as to the shape of the curve. The drivetrain and cooling system guys were also pretty much in control of what the final shape of the power curve looked like. Putting out a tractor that wouldn’t cool, or blew up geartrain wasn’t a financially viable option from a warranty or customer satisfaction perspective.

 

[Wagtail]

I understand the OP's question, however this is a forum for SCUT/CUT and our uses for these tractors go beyond 'simply' tractoring/pulling field implements... we're looking for a multitool for our smaller 'operations'.

To that, HP is the best benchmark for advertising the capabilities of a tractor to what you're attempting to perform.

My 4105 (41hp) was sold to me (quite rightly) as 'just over the minimum' to handle round bales. And this has proven to be true, in my experience... especially 'in the field'. Thankfully my Dealership/Salesman (a 'farmer' himself) asked me what my tractor needs were and sold me the tractor that I needed.

 

[Anakist]

Because engine torque doesn't really matter.

As you say, it's all multiplied through the gearboxes, final drives and tyre size before it gets to the ground. If you have an engine spinning at 3200rpm and it's gear reduced to be going 5mph pulling an implement there is no meaningful difference to an engine spinning at 2400rpm and geared appropriately to travel at 5mph pulling the same implement.

James

 

[mikester]

At the end of the day vendors sell what the customers are looking for.

Commercial customers buy a machine to do a job and ask questions like "how much dirt can this move per hour and gallons of fuel are used per hour" and "how many acres per hour can I till and how many gallons of fuel are used per acre".

Residential buyers ask questions like "What is the HP" and "where is the cup holder".

 

[Hay Dude]

Because engine torque doesn't really matter.

As you say, it's all multiplied through the gearboxes, final drives and tyre size before it gets to the ground. If you have an engine spinning at 3200rpm and it's gear reduced to be going 5mph pulling an implement there is no meaningful difference to an engine spinning at 2400rpm and geared appropriately to travel at 5mph pulling the same implement.

James

Yep. And that’s why I look at drawbar power first, since after all, this determines what the tractor will actually pull or drag.
If you are planning on tilling, dragging logs, freeing unstuck vehicles, pulling out stumps, etc. that is your go-to measurement on power.
If PTO implements is your need (mower, snow blower, etc.) then look at PTO HP.

I need both, so I try to select tractors that dyno (PTO dyno) well AND have high drawbar power.

What the engine makes on a engine dyno stand outside of the tractor is pretty irrelevant.

 

[finn1]

Because engine torque doesn't really matter.

As you say, it's all multiplied through the gearboxes, final drives and tyre size before it gets to the ground. If you have an engine spinning at 3200rpm and it's gear reduced to be going 5mph pulling an implement there is no meaningful difference to an engine spinning at 2400rpm and geared appropriately to travel at 5mph pulling the same implement.

James

That’s not true, due to the reason I explained before. An engine set up with zero torque rise, pulling any sort of implement at full power, will die and have to be restarted in a lower gear if it hits heavier soil or encounters any condition that requires more torque to keep moving.

An engine with higher torque rise may slow down, but will continue to pull through the difficult situation without stopping to downshift.

 

[ericm979]

Both the engine rated at 3200 rpm and the one rated at 2100 rpm can have torque rise.

 

[rScotty]

View attachment 787302
View attachment 787304
View attachment 787303
View attachment 787305
These four tractors are from the "25 HP" class... obviously of different physical size... Model and engine size I listed in RED... all the specs (rated HP, rated RPM) came from TractorData

Again, I don't get why Kubota doesn't tout their torque figures...

.
Well, that's a good example of the difference between "Internet lookup" versus "education".

Where did you get those calculators? The first thing that an engineer thinks when looking at the calculators is to wonder how they are defining a value for the torque? That's not an easy or intuitive programming problem. Torque is not flat, nor is it a smooth curve with RPM. It varies all over the place. And there isn't a standard for displaying it. At the minimum, there would have to be a catalogue of specific torque curves and some sort of look- up table for each and every motor at every RPM.

As to your original question, I think the value of HP is in salesmanship and simplicity. If your interest is in engine breathing, compression, gearing, and TORQUE.... you probably already know these things and are taking HP with a grain of salt & going for a look at the torque curve with rpm instead.

But there is no way that salemen or manufacturers are going to change.

rScotty