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#1
Old 09-21-2006, 08:58 PM
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Why Do Farm Tractors Have Such Low Horsepower Compared to Cars?

I am the proud owner of a new John Deere lawn tractor. That has always been a dream of mine. The thing has 19hp which can do a lot of work but seems puny compared to even a crappy car.

Ok, it is a lawn tractor and will be used mainly for mowing. However, I have been paying attention to lots of ads for the big tractors since I got it. Real, heavy-duty farm tractors often have something like 30hp. The big ones might have 40hp. My SUV has over 5 times that much horsepower.

Cars have to go fast while tractors do not but cars don't have to power and pull some heavy-duty piece of machinery through a rough field either.

Why is the horsepower comparison between the two so lopsided?
#2
Old 09-21-2006, 09:04 PM
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I will illustrate with pictures. There are some tractors with higher horsepower but it still seems way lopsided compared to a car or truck.

John Deere 50 hp

My SUV has more horsepower than this one.

There has to be some fundamental misunderstanding here.
#3
Old 09-21-2006, 09:06 PM
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Tractors don't need HP, they need torque. Check your torque numbers, they are probably much larger than you would imagine. Even big tractor trailer rigs don't have huge HP numbers, but they have gobs and gobs of torque.

HP is great for accelleration which tractors don't need.
#4
Old 09-21-2006, 09:18 PM
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I'm sure someone who actually knows engines or tractors will chime in, but besides that they don't go fast, I would think they might have heavy-duty diesel engines that are designed to operate continuously at some level, as opposed to car engines which are only used for very short bursts of their maximum rated power. A heavy-duty engine for a generator or something will have a lower power rating for its size, compared to a car engine, because it has to keep up its rated power for hours or days at a time.

I don't think the torque comment is precisely right, that just means you gear it differently. More important would be how many HP it gets at an engine speed it can keep up indefinitely, whatever that may be. But it would tend to correlate.
#5
Old 09-21-2006, 09:22 PM
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For an engine, power is proportional to torque times speed. Tractors have diesel engines which have a lot of torque but don't rev very high. A gasoline engine of similar size might be able to accomplish the same thing, but it would have to rev up higher to do it. Would you want to brushhog your field with the tractor engine running constantly at 4,000 RPM or higher? Cars are usually under a light load, only occasionally having to put down more power to accelerate hard, so the infrequent high revving isn't so bad. Lawn tractors have gasoline engines, but smaller engines endure less stress at high RPM.
#6
Old 09-21-2006, 09:56 PM
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Quote:
Originally Posted by Shagnasty
Real, heavy-duty farm tractors often have something like 30hp. The big ones might have 40hp.
This is a big, heavy-duty farm tractor. 530hp on the largest model. I doubt your SUV has that many.

The reason tractors have less horsepower than one might expect relative to automobiles is because what they're designed to do is operate continuously for extended periods towing extremely heavy loads at very low speeds. The reason your SUV is hopeless at towing things compared to a tractor isn't because it is lacking it power. It is because it is lacking in the ability to turn its power into the sort of work a tractor does. Your SUV engine could pull as much as the tractor if it were geared as low and were connected to very large, high-traction tires - though as noted, it would have to run at much higher rpm than the tractor to generate its rated horsepower, and so would be less efficient and suffer greater wear and tear.
#7
Old 09-21-2006, 10:08 PM
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Just checked the new John Deere 7000 series tractors . These are some serious big tractors -- but not the top of the line (Deere makes 8000 and 9000 tractors)

Anyway, the engines are 6.8 liter, but rated at 140-180 hp.

The horsepower ratings are taken at 2,100 rpm.

These engines are designed to have a LOT of reserve. You don't want to wind them out to see how fast they go, you want to make sure that when you drop a plow into the ground, you don't blow the engine when you start to pull.
#8
Old 09-21-2006, 11:42 PM
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Horsepower is torque X RPM /5252.

The higher the RPM the same torque come in at, the greater the horsepower.
For example if you SUV had 200 Ft lbs of torque at 6000 RPM it would develop 229 HP.

Your tractor does not rev anywhere near that fast. Lets say that your tractor makes 300 Ft lbs of torque at 2000 RPM you only have 114 HP. You have 50% more torque, but due to the lower RPM only about half the horsepower.

Like we say in the car business, torque wins races, HP sells cars.
#9
Old 09-22-2006, 12:40 AM
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Quote:
Originally Posted by Rick
Like we say in the car business, torque wins races, HP sells cars.
Isn't that the other way around?
#10
Old 09-22-2006, 07:29 AM
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There is more than one answer to this:

1. Farm tractor HP ratings are usually listed as HP at the PTO shaft with the shaft at 540 RPM. This helps you do a side-by-side comparison of the tractors and it helps you match the tractor to the PTO-driven implements. The engine HP will be somehwat higher than the PTO rating. For example, my 5420 John Deere is rated as a 65 HP tractor. I can't find the number in the manual, but I remember seeing that this engine puts out 95 HP at the crankshaft.

Your lawn tractor HP rating is for the engine only and the number is likely at the peak HP RPM rather than at the RPM at which you usually operate.

2. When you get past the PTO needs, tractors are all about traction. The point it to combine as much normal force (weight) with enough friction (big, grippy rubber tires or tracks like a dozer) with torque to translate a load forward. The torque is usually not the limiting factor. Long ago, I used an H model John Deere with the famous old 2-cylinder pony motor (poppin-johnny) to skid some logs. If you loaded the thing down enough, the front end would come up front and it would want to flip over backwards. Too much torque and friction, not enough weight. That motor probably only put out about 20 HP when it was new.

My 5420, in a similar situation, will spin all four tires and start digging its way to China. Enough weight, too much torque, not enough friction.

3. When comparing cars motors to tractor motors, think about the impedance matching. In laymen's terms, that's matching your power to your load. In these type of mechanical systems, matching it done with gears. If you put your 200 HP V8 car engine in a 50 HP tractor, something's going to break. The engine puts out too much power for the rest of the system.

After all of that, the short answer is: The tractor farm has less horsepower than compared to cars because it doesn't need it.

If course, we're not talking about the 390 HP four-wheel drive, twelve-tired beauties.
#11
Old 09-22-2006, 08:56 AM
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Cars need to reach speeds that require powered to be increased exponentially (I forget the formula, but going 70 requires twice as much power as going 50 ..someone will chime in on this).

If your need was just a car to scoot around at speeds of 0-30 mph, you'd have no need for anything more than 30-50 horsepower.

To overcome the significant wind drag, horsepower requirements shoot up rapidly.

Many car engines are also 'oversquare', with a bore that is bigger than the stroke length of the piston. This results in rev happy engines that advertise good peak horsepower ratings at high RPM at the expense of torque produced down low in the RPM band.

So, cars can have alot of torque and HP, but both can peak at high points in the RPM band.

In short, to go fast, the design of the engine will drift away very rapidly of something that just needs to go slow...and to do it for a long long time.
#12
Old 09-22-2006, 09:19 AM
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Quote:
Originally Posted by Philster
Cars need to reach speeds that require powered to be increased exponentially (I forget the formula, but going 70 requires twice as much power as going 50 ..someone will chime in on this).
No the power needs to increase approximatly with the square of the speed. That is not exponentially.
#13
Old 09-22-2006, 11:50 AM
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Ah, that is it. The point in my post was that cars require gobs more power because of the speeds they need to reach.

So...let's see if I can get my own example right: It doesn't take twice as much power to go 40mph versus 20mph....it takes four times as much power?
#14
Old 09-22-2006, 04:41 PM
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Quote:
Originally Posted by gazpacho
No the power needs to increase approximatly with the square of the speed. That is not exponentially.
No, it increases with the cube of the speed. Air resistance is proportional to the square of the speed, but at higher speeds, you're in a higher gear too. It would require eight times as much power to go 70 as to go 35. (That's assuming air resistance is the only drag force. If there's a lot of friction or the car is going uphill, it may vary significantly.)
#15
Old 09-22-2006, 04:54 PM
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Quote:
Originally Posted by Rick
Like we say in the car business, torque wins races, HP sells cars.
I wonder why you would say that. In drag racing, torque is important for the launch and the first 60 feet or so. Once you get to the upper part of first gear and all subsequent gears, horsepower is all that matters. I'm talking average horsepower across all RPM's between gears, not just peak horsepower. Having a nice peak torque at 2,600 RPM (like my car has, higher peak torque than most cars but at lower revs) won't help when you're running at 5,500 RPM.

In other racing where launches are infrequent, torque matters even less. A Formula 1 car produces little torque for a race car, possibly less than my car in fact which puts out around 350 ft lbs. However, an F1 car does it at nearly 20,000 RPM meaning it has a hell of a lot more power. With the same gearing, the F1 car would still be in first gear when I'm shifting to third, maybe even fourth. Probably they have shorter gearing though, so each gear is faster.

Where torque does matter in a car is typical driving. If your engine pushes a lot of torque at low speeds, it'll be happier cruising. It'll also be able to shift to the next gear earlier. Power is good for when you stomp on it, but low-end torque is good for part-throttle acceleration and cruising. So if you're looking for a daily driver car, torque is probably more valuable than power.
#16
Old 09-22-2006, 04:58 PM
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Quote:
Originally Posted by snailboy
Probably they have shorter gearing though, so each gear is faster.
And when I said each gear would be faster, I meant you could accelerate more quickly in each gear. Okay, I'm finished posting for now.
#17
Old 09-22-2006, 05:22 PM
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Quote:
Originally Posted by Philster
Many car engines are also 'oversquare', with a bore that is bigger than the stroke length of the piston. This results in rev happy engines that advertise good peak horsepower ratings at high RPM at the expense of torque produced down low in the RPM band.
And tractors are typically undersquare. Lots of leverage on the crank equals lots of torque.
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