Does HP alone influence to the "power" of a tractor?

[whistlepig]

Horsepower is a 500 pound Sumo wrestler that can bench press 1,500 pounds. Torque is my scrawny old butt with 3/4" breaker bar with a four foot handle and a cheater bar. Give the Sumo wrestler a 3/8" ratchet with a socket and give me the 3/4" breaker bar with the cheater bar. I'll kick some serious Sumo wrestler bootay. How's that for over simplification?

 

[wampum]

Most here probably already know that a Farmall cub did not have a lot of H.P. The one I had only had 9 HP at the draw bar according to the manual. I believe later ones got all the way up to 14 hp. They did and still do a lot of work for a small tractor.

 

[roamerr]

I really believe that HP is just a number with many tractors. It is a good number to look at in a given chassis size but is not fair to compare on different chassis. An example is the new John Deere Turbocharged 60+ hp 4520 and 4720 series. Great tractors but will not pull near as well as the normally aspirated heavy tractors such as an older Massey 245 that is fully loaded. The MF has only 40 or so hp at 1500 or less rpm but can weigh 5500+ lbs. The Deere makes it's HP at 2400 rpm and weighs around 4000 lbs. Hook the tractors together and the MF will pull the Deere in circles. Weight is to the advantage of the MF. I've also seen the older non turbo motors lug incredibly. The newer turbo motors will loose rpm's and then significant power -- do not seem to lug as well.

HP is a good indicator but just one part of the equation.

 

[sld]

I have a good grasp on the hp and torque concept but it is hard to fully understand the differences in my ZTR and tractor.

The ZTR is 21 hp and is made to cut grass at 3600 rpm's. Top speed is around 8 mph. I bet 2 guys could grab the rear bumper and keep it from moving anywhere while it is given full throttle. The unit weighs probably 500 lbs.

The tractor is only 17 hp at around 2800 rpm's. Top speed is around 10 mph. It would easily pull my truck around the yard and does well with a box blade fully dug into the ground. The tractor weighs about 2000 lbs.

The ZTR is gas and the tractor is diesel, but it is still hard to understand the differences in power applied and usable power.

 

[D7E]

I have two identical tractors same wheelbase/tranny/tyres everything and both 320hp yet one has a 10liter and one has a 14liter engine.The little 10 liter is by far the energetic and snappy one but if you drop below 1700rpm it's done whereas the 14liter just plugs away down to almost an idle without dying.
I don't agree about turbo engines losing power..it depends on the size and type of turbo..i've had plenty of 4cyl turbo's that will out pull natural 6cyl's of the same hp and tractor size and fords running at 1850rpm as opposed to deeres at 2400rpm...they are all different..Bore, length of stroke etc..it's finding the one for your job..?

 

[PutnamVictor]

As I expected, lots of good input on the board. Roamerr, you stated "HP is a good indicator but just one part of the equation." Okay then, what are the others? Weight, tire contact patch, ...??? Clutch? Hydraulic pump size? Hydro?

 

[GPintheMitten]

As I expected, lots of good input on the board. Roamerr, you stated "HP is a good indicator but just one part of the equation." Okay then, what are the others? Weight, tire contact patch, ...??? Clutch? Hydraulic pump size? Hydro?

Now I'm having a hard time understanding you asking this second question... or rather the same question again. A lot of factors have been mentioned in response to your original question.

 

[Reg]

As I expected, lots of good input on the board. Roamerr, you stated "HP is a good indicator but just one part of the equation." Okay then, what are the others? Weight, tire contact patch, ...??? Clutch? Hydraulic pump size? Hydro?

It can depend on what you plan to do and whether or not speed matters.

If you have "all the time in the World" and have ground engaging tasks to do then WEIGHT is probably most significant, since TRACTION is what will pull the implement through the soil and just about anything can be geared low enough to break traction.
Tire contact patch can affect the surface, but it is down force that determines traction.
Tires can handle (or not) traction in two ways 1) by biting down like R1s 2) by floating over like turfs or rice paddy tires.

If speed doesn't matter then horse power (per se) doesn't matter.
Torque (at the crank) doesn't matter either.
Drawbar force DOES matter, but that can be achieved with a very small engine, very low gearing and very high weight - as long as you have the TIME (-:

I will guess that if you need to ask this you are probably not dependent on tractors to make your living, e.g. being able to run a 7ft mower is interesting but a 6ft or 5ft would do the job.
Speed is probably not important to you ?
You would probably find that a 20 HP garden tractor would do most of what you want done and do it fast enough.

 

[Hardacrefarm]

I really wouldn't know for certain, but am guessing direct drive, gearing, and actual weight fo better traction on yester-years models, while newer ones "probably" concentrate more on engine torque for snowblowers and mower attachments.

Just a guess, but I'd be interested to know why also.

 

[Dan W]

I did some diesel engine training many years ago, (mostly seminars) and I used to have to stand in front of grumpy truckers and try to inform them about how to operated their new engines which were designed for low RPMs. I found that folks in the audience were proud and generally good operators but they never really understood how engines worked and they needed to understand this before I could venture into something as complicated as lug-down and running at peak torque RPM for best fuel economy.

I'd ask the audience if anyone really knew how torque is produced in a diesel? This was typically followed by a moment of silence.

We all know Rudolph created a compression ignition engine, so that's out of the way

1. In a cylinder, fresh air is compressed by a piston, when the air reaches about 450F, atomized fuel is squeezed into the super heated air and begins to burn rapidly.
2. Expanding combustion gas is a huge expansive force at this point but still has not done any useful work
3. The hot gas drives the piston downward, this is a straight vertical down force
4. The moment we connect the piston to a connecting rod and tie that rod to a crankshaft, the down force of the piston now begins to rotate and that force is now called torque. (a twisting or rotating force)
5. The greater the fuel (high injection pressures of modern fuel systems) and the greater the fresh air charge (thanks to modern turbocharger technology) for any given cylinder displacement, the greater the cylinder pressure become = greater rotating force = greater torque
6. So, fuel/air combusting = linear forces, when tied to a crank = rotating force

Next question: does anyone know how HP is created silence again, but most know there is a speed relationship somewhere in the equation. For ease of explanation, if we replace the engineering phrase "horse power = rate of doing work" with the speed at which the pistons travels up and down which = crankshaft revolutions per minute (RPM) now you begin to get the picture.
HP = RPM x Torque
5252 (a constant)
So now, back to my frist point and this is a Huge questing: why does an engine's torque decrease as speed (or HP) increases?

Dan