# Horse Power



## grttractorhunt (Jul 9, 2011)

I have a crazy question, how is the relation of lawn mower HP and tractor HP? are they the same or is there a difference?


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## wjjones (May 28, 2010)

Welcome to the Forum!..grttractorhunt.. Yes there is a difference if its diesel v/s gas.. I know torque, and rpm are 2 factors. I am sure some of the other members can give some more examples of the differnces between the 2..


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## Mickey (Aug 14, 2010)

Not sure I fully understand the question but if it is *is there a difference in the kind of HP between these 2 pieces of equipment*, then the answer is *no*. HP is a mathematical calculation, not something you can measure directly. It is a measurement of a specific amount of work being done over a specific amount of time, i.e. force x distance x time. The problem is, for quite a few yrs, the lawn equipment mfgrs have been playing the horse power race just as Detroit did with cars. HP sells to the uninformed. Lawn tractor mfgrs lost a big suit last yr over the false claims. Today you don't see the wild HP claims being made by some mfgrs that you did previously. Real tractor mfgr are more honest in their power claims.

A major issue here is getting the engine power coupled to the ground. Regardless of engine power, you're not going to be able to get that power coupled to the ground with a 500#-800# piece of equipment as you are with something that weighs 2000# and up.

Today's riding mowers, lawn tractors and Garden tractors are not getting any more work done that yesteryears models with 10-12 HP rated engines. I will add that the larger mower decks you see today do take more power than the smaller decks of old models but certainly not the 20HP ratings you see today.


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## grttractorhunt (Jul 9, 2011)

wjjones, thanks. Yes the tractor is a 20 HP diesel with a y380 engine and the lawn mower is a gas engine. Mickey, thanks also. I was wanting to know if there was any difference in the HP ratings betweeen tractors and lawn mowers. In other words, 20 HP is 20 HP no matter what? LOL. Just wondering before I spend a lot of money on small HP tractor


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## Country Boy (Mar 18, 2010)

HP is really useless in rating tractors and other outdoor power equipment. On a tractor, your engine generally runs at one speed while doing work. HP is a factor of speed (in RPM) x torque (in ft/lbs) divided by a constant number (5250). An engine running at 3600rpm showing 20hp is going to have considerably less power than a 20hp engine running at 2400rpm. Torque is what gets the job done, so the more torque you have, the better. If you are looking at a tractor, look into the torque numbers rather than horsepower.

Horsepower is useful in comparing things like cars where you engine speed changes as you travel different speeds. It lets you know how much power the car has at a particular speed, and how it changes over time. In a fixed RPM setting, however, it is not a good comparison for the reasons stated above. Think about it this way. Stihl makes a 9hp chainsaw (MS880). We sell 9hp snowblowers at work. The original Farmall Cub tractor was around 9hp. Chainsaw can't run the snowblower, and the snowblower can't run the tractor. Chainsaw runs at 15000 rpm, snowblower runs at 3600 rpm, tractor runs at 1570 rpm, yet they all have the same HP rating. See the difference?

Mickey, the reason that larger tractor manufacturers have been more honest is because of the Nebraska Tractor Test. Nebraska passed a law stating that no company could sell a tractor in their state without having it tested by the University of Nebraska. No one wanted to lose an entire state as a market, so the vast majority of the tractors were put through some pretty grueling tests. Dishonest claims were quickly debunked, and consumers could see the actual data from the tests. Lawn tractors don't go through that same amount of testing, so anything goes.


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## Mickey (Aug 14, 2010)

Country Boy said:


> HP is really useless in rating tractors and other outdoor power equipment. On a tractor, your engine generally runs at one speed while doing work. HP is a factor of speed (in RPM) x torque (in ft/lbs) divided by a constant number (5250). An engine running at 3600rpm showing 20hp is going to have considerably less power than a 20hp engine running at 2400rpm. Torque is what gets the job done, so the more torque you have, the better. If you are looking at a tractor, look into the torque numbers rather than horsepower.
> 
> Horsepower is useful in comparing things like cars where you engine speed changes as you travel different speeds. It lets you know how much power the car has at a particular speed, and how it changes over time. In a fixed RPM setting, however, it is not a good comparison for the reasons stated above. Think about it this way. Stihl makes a 9hp chainsaw (MS880). We sell 9hp snowblowers at work. The original Farmall Cub tractor was around 9hp. Chainsaw can't run the snowblower, and the snowblower can't run the tractor. Chainsaw runs at 15000 rpm, snowblower runs at 3600 rpm, tractor runs at 1570 rpm, yet they all have the same HP rating. See the difference?
> 
> Mickey, the reason that larger tractor manufacturers have been more honest is because of the Nebraska Tractor Test. Nebraska passed a law stating that no company could sell a tractor in their state without having it tested by the University of Nebraska. No one wanted to lose an entire state as a market, so the vast majority of the tractors were put through some pretty grueling tests. Dishonest claims were quickly debunked, and consumers could see the actual data from the tests. Lawn tractors don't go through that same amount of testing, so anything goes.


I am aware of the Nebraska test. Here is a good read regarding horse power.

Horsepower - Wikipedia, the free encyclopedia

As one can see there are some minor differences in the standards of measuring the amount of work but the various std are quite close. There are also differences between industries and how work is defined/measured.

I would also prefer the larger engine with an abundence of torque, over a small higher speed engine. But if the rated HP is honest and the same, they can both accomplish the same amount of work as defines by the HP std.


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## Country Boy (Mar 18, 2010)

Mickey, I'll have to read that later. Bookmarked it.

In this situation, I disagree with your assessment that


> if the rated HP is honest and the same, they can both accomplish the same amount of work as defines by the HP std.


Again, I bring you to the example I listed above. According to the mathematical equation used to calculate horsepower for engines (at the crankshaft center), the chainsaw, the snowblower engine, and the tractor engine all are the same horsepower. The chainsaw at 9hp and 15000 RPM would have about 3.15 ft/lbs of torque. The snowblower with 9 hp at 3600 RPM would have 13.125 ft/lbs of torque. The tractor engine at 9 hp and 1570 RPM would have about 30.096 ft/lbs of torque. In a practical application, they are not equal. From a scientific standpoint, they all accomplish the same amount of work in a given amount of time, but that's not applicable in this situation. If you hooked the 9hp chainsaw engine up to the snowblower, it would snub out. If you hooked the 9hp snowblower engine to the tractor, it would snub out. They cannot be swapped like that. So, for practical applications like comparing a tractor or small engines, a torque reading is more accurate than HP.


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## Mickey (Aug 14, 2010)

CountryBoy, what you are overlooking is the fact if that chainsaw engine was "geared" down so it's output was rotating at same speed, the torque (minus frictional losses do to the speed reduction) would be multiplied by same ratio. In your example the chain saw engine is turning 10 X the tractor engine and if that speed is reduced to match the tractor engine by a factor of 10, then the torque at the output after reduction would be multiplied by the same 10X factor and that puts it right there with the tractor engine torque.

We're strictly talking about amount of work being accomplished not the desirability of doing it one way Vs another.


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## Country Boy (Mar 18, 2010)

True about the gear reduction, however if you are looking at two nearly identical lawn tractors and both are rated at say 20 hp with different engines, but one is running at 3600rpm and one at 2800rpm, the slower speed one will have more torque. That's why torque is a better indicator of an engine's output than HP. There's a reason why your smaller engines are now rated by torque or displacement rather than HP. Companies got burned by the lawsuit mentioned earlier, and now are giving you a more accurate way to measure the engine's worth. Comparing two push mowers, I'll take an engine with 10ft/lbs of torque over one that only has 7.


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