Question About Horsepower

[NoDakInMN]

When we look at horsepower ratings of our old two cylinder John Deeres and compare them to modern horsepower ratings, the phrase I often hear is that "they measured things differently back then". This seems to be the go-to response for most people to explain, for example, why a 27 HP, or so, 44 A can pull a two bottom plow but when you hook the same plow to a modern 3 cylinder diesel of a higher horsepower, it struggles with it, if it can even can pull it at all.

Now I realize that the A has a lot more inertia with all those heavy pistons, rods, crankshaft and flywheel parts spinning around, but was a horsepower back then measured differently than now or what exactly is it that allows a smaller horsepower tractor from years ago to do more effective work at the drawbar than a higher horsepower modern tractor?
 

[Loosehandle]

A modern 27 hp tractor would need a lot of
weight added to it in order to pull 2-16
in my clay, even if it was 4wd.
 

[dan_41jdh]

Horsepower is a term that can be misused. Technically, it is a measure of the rate of doing work. James Watt, the steam engine guy back in the 1700s, established a definition of one horsepower being the rate of work done (comfortably) by one horse, and that was the ability to pull a load that required a force of 180 LB at a speed of about 181 feet per minute (a little over two miles per hour). Although you usually think about horses pulling in a straight line, such as with plowing, horses also created torque by walking in circles pulling on the arms of a horse wheel, but the method of measuring that power, straight line or circular, was the same - force x distance divided by time.

Anyway, the Nebraska Tractor Test Laboratory measured horsepower of tractors from the drawbar and horsepower from the belt (or PTO) during their testing. But what they measured from the drawbar dynamometer or the belt-driven dyno was only accurate for the exact air temperature and barometric pressure when the test was made. Doing the same dyno tests on a hot day or at much higher elevatioons would result in lower HP readings. So those test measurements were then corrected to standard conditions of 60 degrees Fahrenheit and sea level barometric pressure.

The Nebraska tests also determined what was defined as rated horsepower which was 75% of the maximum corrected drawbar horsepower and 85% of the maximum corrected belt horsepower. This was done to account for other variables and to allow a certain amount of leeway for variable working conditions. Those horsepower ratings were determined throughout most of the two-cylinder era.

But some ratings you see are of engine horsepower (such as with garden tractors and lawn mowers) and don't allow for geartrain and other losses, so one really needs to know what the manufacturer's definition of horsepower means.

A few years ago, Shop-Vac (and maybe others) got into trouble by advertising their vacuums with what they called Peak Horsepower ratings, each trying to claim a higher value than their competitor. But it was all a bunch of advertising hooey dreamed up by the marketing geniuses. Shop-Vac indicated they could produce 6.5 Peak Horsepower on a 120 volt vacuum. Really? A single-phase electric motor would require an absolute minimum of 40.5 amps at 120 volts to develop 6.5 HP. Kinda hard to get that much amperage out of a 15 amp outlet.

So when people talk about horsepower, they really need to be specific about what they are describing.
 

[Brendon-KS]

It is easy to think of drawbar pulling force and drawbar power as the same thing but they are not. The number of plow bottoms that a tractor can pull is a factor of its pulling force
which is a direct result of the tractor's weight, traction, and gearing. But, if we think about how many acres can a tractor plow in a given time period we're talking about drawbar power.
The difference is that pulling force doesn't take time into account. The large steam tractors and early prairie tractors could pull very large plows but did it at a snail's pace so the
actual drawbar power wasn't very high. A lighter weight, more modern tractor might pull a third the number of bottoms but do it three times as fast to give the same drawbar power output.
Manufacturers know that today's smaller tractors are rarely used for high-draft work and are much more often used for mowers, loaders, tillers, snowblowers, and other such equipment. Jobs
like this need good PTO power but lugging around the size and weight necessary to provide a high drawbar pulling force is unnecessary (if not even detrimental) for the way most of these
tractors are used.
 

[buickanddeere]

It is an indication of the sad state of the education system .
When kids graduate without knowing what force ,
Distance and time is .
Put those tractors on a PTO generator and tell us how that works out .
 

[lowell8970]

There is a difference in horsepower ratings nowadays. A tractor was rated at rated engine speed and rated pto speed.Rated engine
speed was generally @ 21 to 2200 engine rpm. PTO speed is 1000 pto rpms with the engine turning around 1900 to 2100 rpm.Now they
rate them with the engine pulled down to maximum ECM horsepower which is the most that engine can ever possibly put out at any rpm,
at the fuel settings installed in the ECM. Most of the time that is theoretical horsepower.
I have an 8970 New Holland that was rated @ 210 PTO horsepower and that's the way I bought it, a 210 hp tractor. Well the way they
rate tractors today that tractor now becomes a 260 to 280 engine horse tractor On the dyno it actually registered 315 hp @ 1400
engine rpm.You are not supposed to lug them down that low for any length of time. But that's basically how they rate the new
tractor.Look at a new New Holland 410 hp tractor and look in the literature and see what the Rated PTO Horsepower is. It drops by
120 hp.They still say it's rated @ 410hp. All the manufactures do it except for Versatile.Very confusing,but all I know is our old
Genesis and old Magnums still pack a punch!
 

[Brendon-KS]

I actually have two tractors that demonstrate this concept quite well. The first is a 1948 Deere M that puts out a dyno-proven 19 hp at the PTO. With its rear wheel ballast it weighs something around 3500 lbs. I also have a modern sub-compact diesel tractor that also is rated at about 19 hp at the PTO so the two are directly comparable. It weighs less than half of the M, about 1500 lbs. Due to its light weight and small tires its drawbar pulling force is nowhere near as much as the M, even with 4WD. However, when I put the same 4-foot rotary mower on the two tractors the performance is essentially the same, if not better with the sub-compact because the ground speed is infinitely variable. This should be no surprise because the two tractors can provide the same amount of power to the mower. Now if the same engine from the sub-compact were to be placed in a larger, heavier chassis (which, incidentally, the manufacturer does offer) the drawbar pulling force would be a lot more comparable to the vintage tractor. But, it wouldn't be as practical for mowing a lawn which is a primary use for the physically smaller model.
 

[Stuart]

I believe they still rate tractors in PTO HP. Where is 120hp being lost?
I have dynoed many brand new tractors that out out 50 or so mor hp than what they were rated for
 

[nlastovi]

[NoDakInMN]

I think Brandon-KS has the best handle on what I was trying to ascertain, that what is it that causes old two cylinder John Deeres outperform somewhat higher horsepower modern tractors when it comes to pulling power. Belt HP ratings on these old tractors that have the belt pulley on the clutch are good indicators of pure engine horsepower and they are powered right off of the crankshaft, and PTO does suffer some loses in transmission of horsepower to the end of the PTO shaft. But drawbar horsepower is is what I'm referring to and it is not so much the ratings as the difference in performance, power to the ground so to speak. That is wear the differences become most obvious.

I don't believe that tractor weight alone is the answer, for while greater weight provides more traction, it also creates more engine load in just moving it around. I did mention the inertia in the moving parts of the engine, pistons, rods, crankshaft and flywheel, and I think that the weight to horsepower ratio of the older JDs to the newer ones is part of the answer as well as how that is transmitted to the drawbar is another part.

So could it be that factors other than horsepower need to be considered in determining the true power delivery capability of a tractor? That comparing horsepower to horsepower might be overlooking the value of torque or something along those lines?
 

[PeterE]

That was the era of the sea level corrected figures. Nebraska estimated what a tractor's performance would be at sea level on a 60 degree day. Many manufacturers used those higher figures in the sales
literature.
 

[nlastovi]

[Brendon-KS]

Your reference to the 9RX 590 illustrates something that is fairly common with the largest sizes of tractors where the engine is electronically derated in certain situations to prevent
damage to the drivetrain. In the case of this tractor the engine power is reduced during stationary PTO operation as well as when driving in transmission gears 1 through 3. This would be
the only situation where a tractor's drawbar power would be greater than PTO power since pulling with the drawbar involves more inefficiency.
 

[buickanddeere]

[Butch(OH)]

Two reasons. First one is that most people
have a very confused state of mind on what
exactly HP is so they make up a version
based upon what they would like to belive.
Torque isn't HP, RPM isn't HP period, get
that out of your mind. Secondly most of
the advertised HP today on small tractors
is not PTO or drawbar but engine HP. Thus
when you compare a 27HP on tbe PTO A to a
25HP compact with hydro trans your
probably looking at less than 20 PTO HP.. Then add to that differance the mechanical vs hydro
transmission losses the drawbar HP differance is even
bigger. With no facts in front of me Id guess that a hydro compact advertised as 25HP has a has between 1/2 to 2/3 the drawbar HP of that 27 PTI HP model A, and there is your answer.