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Tractor Talk Discussion Forum

Horsepower Question

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Ben Shipley

03-07-2007 06:44:32




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I am looking at a new tractor right now, and in my search, I have come across a question that i wanted to try and find the best answer to.

When we farm boys talk tractors, and talk about horsepower, we talk in PTO H.P., correct? In other words, when I say my litte Ford Jubilee has 31 HP, I am talking rated PTO H.P. I have been told that 75-85% of the PTO HP is a close estimation of drawbar H.P. Is this accurate? I have been told this by a few dealers, some use 75 some 85 %. Either way, without a dyno, it is still a guess.

Do you concur that we usually speak in PTO HP most of the time? Is my 75-85% number an accurate figure?

Thanks

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MF Poor

03-07-2007 14:55:26




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 Re: Horsepower Question in reply to Ben Shipley, 03-07-2007 06:44:32  
A few more thoughts. University of Nebraska test's were used to determine ACCURATE PTO and drawbar HP ratings, as well as fuel efficiency. They supplied manufacturer's CLAIMED engine HP ratings, but did nothing to confirm those numbers. (Would have been virtually impossible without removing engines and testing on an engine dyno.) They were after the MEANINGFUL numbers....The power a tractor could deliver to the ground or to a PTO driven implement. For as long as I can recall, FARM TRACTORS are primarily advertised by PTO HP, with engine or drawbar as suplimental information. It seems as if it's been since the compact tractor market started to thrive that we get pages and pages of numbers and statistics to baffle the unfamiliar buyers.

"Magic formula" percentages to convert one rating to another are "guestimates". The percentage isn't constant from engine to engine. They'll get you "in the ballpark", but don't rely on the percentages from a known engine/tractor to "fit" the next tractor in line.

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MF Poor

03-07-2007 11:09:37




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 Re: Horsepower Question in reply to Ben Shipley, 03-07-2007 06:44:32  
For people who are obsessed with statistics and numbers, or those who seem to think quoting a bigger number gives their tractor more power, you'll get a lot of "engine HP" talk. Most SERIOUS end users are only interested in one thing. WHAT WILL IT DO in the field? For several generations, PTO HP numbers have been the standard by which tractors are judged. Drawbar HP is somewhat difficult to translate, but has some meaning. In the end, it doesn't matter if you have a billion ENGINE hp, the only real significant hp number will be measured where the work is done. On the PTO or on the drawbar.

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Red Dave

03-07-2007 08:01:10




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 Re: Horsepower Question in reply to Ben Shipley, 03-07-2007 06:44:32  
If you go back and read the Nebraska tractor tests, you'll see that they rated them one thing at the belt or PTO and another at the drawbar. They aren't the same and never were. A lot of things affect drawbar horsepower besides the engine power. Weight, traction, gearing, groundspeed etc. can all add or subtract from drawbar horsepower.

As I understand it, the Nebraska tests were started to verify or refute overly optimistic horsepower claims by manufacturers.

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souNdguy

03-07-2007 07:03:08




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 Re: Horsepower Question in reply to Ben Shipley, 03-07-2007 06:44:32  
HP is a subjective term. When i speak in terms of HP, I always qualify my statement with belt, pto, or drawbar.

To me, if I ask the generic question, how many hp is your tractor.. I am going to expect a gross eng hp number back from you... as I dind't ask how big a mower you could pull.. etc.

Soundguy



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rrlund

03-07-2007 06:52:39




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 Re: Horsepower Question in reply to Ben Shipley, 03-07-2007 06:44:32  
Don't know about that for sure,but have you noticed that some are advertising engine hp. You have to read the fine print to get PTO hp.



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low budget

03-07-2007 08:36:58




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 Re: Horsepower Question in reply to rrlund, 03-07-2007 06:52:39  
I've noticed that too. Kubota has always done it that way , and now JD does, atleast in the utility and compact models.



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John T

03-07-2007 06:52:42




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 Re: Horsepower Question in reply to Ben Shipley, 03-07-2007 06:44:32  
Ben, I bet whichever rating of all yields the "higher" number is the one used to sell n advertise lol.

One heard of BOTH Drwabar HP and Belt Pulley HP etc on the older tractors and it obviously requires HP to drive the tranny n differential gears n friction yielding less HP on the drawbar versus any belt pulley and the PTO also requires gears n friction yeilding less HP on it versus HP right out of the engine.

Im unsure of your 75% estimate, but experience tells me that may not be too far off due to the friction and gears n tranny n differential losses of engine HP to the time it gets to the ground????? ????? ?????

John T

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Ben Shipley

03-07-2007 07:42:42




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 Re: Horsepower Question in reply to John T, 03-07-2007 06:52:42  
I was a a dealer last week, looking at a NH tractor. I asked about different numbers on the tractors. I was looking at a TN 70. He said if you take 85% of the number on the side of the New Holland tractor, that was its approximate HP.

In my tractor guide, which lists prices, weights trans, and PTO HP, is says the TN 70 has 62 PTO HP. 85% of 70 is 59.5 so its close but not sure if I was being fed a load of crap or what. I shall continue to do more research.

Thanks

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Ken Macfarlane

03-07-2007 08:48:54




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 Re: Horsepower Question in reply to Ben Shipley, 03-07-2007 07:42:42  
PTO hp is the net engine hp after you take your hydraulic pump losses and internal friction losses off. It takes power in an open centre system to keep that oil circulating. SAE has standards for measuring the powers.

Seems to me the engine hp doesn't include many parasitic loads, maybe not even cooling fan, can't remember.

Drawbar hp is completely different, you can have a 250 pto hp machine with 20 drawbar hp.

Tractor pulls dragging the sleds are measuring drawbar hp, a Ford Lightning with what 400 hp isn't gonna pull as well as say a 50 hp, 10,000 lb old piece of iron.

But if you hooked them up to a pto load you'd know the lightning would stomp the old iron.

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in-too-deep

03-07-2007 17:16:34




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 Re: Horsepower Question in reply to Ken Macfarlane, 03-07-2007 08:48:54  
What's a Ford Lighting? Never heard of that.



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IA Roy

03-07-2007 20:31:04




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 Re: Horsepower Question in reply to in-too-deep, 03-07-2007 17:16:34  
Slightly off the subject, but I have seen what they call a Fan Dyno at a old time power show. This looks like a wooden reel off an old combine. The thing is belt driven off a pulley of the tractor. Can anyone tell me how you check the power. I know that horsepower is determined by the torque and the RPM. Is there a variable speed device on the load?



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