Gross verses Net HP - what the heck?

MChalkley · Sep 27, 2000

Moving anything, including air, requires energy. Pulling air through an air filter, for example, requires energy. Thus the claims of greater hp using "free-flow" filters, etc. Likewise, pushing air through the exhaust system, particularly the muffler, requires energy. Turning the radiator fan requires energy, as does the alternator, etc. They all add up to fairly substantial amounts, which is why the "net" hp is considerably lower than the "gross" hp. If all this leads you to believe that "gross" hp is a pretty useless figure, congratulations. It's straight out of the marketing department, and you know what that means. Now, if all you want to use it for is comparing engines, using the same measurement standard, of course, then it's reasonably useful. Just don't think you've got that much hp to use for any work you're planning to do.

Net hp is measured at the flywheel.

PTO hp is measured at the rear PTO.

Kubota obviously derives their tractor model numbers from the net hp. My L4310, for example, has 43 net hp. Does anyone know if Kubota has ever derived the model number from gross hp? Do any other manufacturers do so, or have they ever done so?

MarkC

TomG · Sep 27, 2000

The HP rating thing does get tricky. I think there are different definitions even for flywheel HP that are used for different purposes--at least I seem to recall something termed 'break hp.'

The things you and Mark mention of course detract from usable power. It also takes power to accelerate rotating components. However, that power is stored as inertia and can be recovered under different load conditions. It also takes power to move reciprocating components (pistons, push rods etc.), and that power cannot be recovered. I've heard the loss of inertia in reciprocating components described as part of the idea of parasitic loss. If true, I imagine that gross HP would be very much of a theoretical number, rather than something that could be measured--the sort of thing only a marketer or a physicist could make use of.

glennmac · Sep 27, 2000

MChalkley, the B series uses gross hp. That's what started this thread over on the B2910 vs.2710 topic. The BX1800, BX2200, B2400 and B2700 all have a gross hp corresponding to their first two digits. The B2910, however, is 30 gross hp. The Kubota B-series brochure does not list net hp, although it lists pto hp for each. The B7400 and B7500 designations seem to have no relationship to hp. (What do those numbers mean?). The Grand L series, as you point out, seem to be numbered close to net hp.

This is worse than physicists and marketeers. Only cabalists and numeromancers need apply.

Harv · Sep 27, 2000

Glenn -

Not so sure that the B series has always used the gross hp / model number association. Certainly the new ones seem to -- like the B21 is rated at 21 gross hp, but I have the B2150 listed at 24 gross hp. I think they (the marketeers) switched tactics some years ago, undoubtedly with the intent of making them sound bigger, to increase sales, yatta, yatta, yatta.

I got pretty swept up in all the hp numbers when I was shopping, too, but I eventually started focusing on the PTO hp (how big an implement can I power) and the overall weight of the tractor. My impression is (and I am definitely a novice here) that a good heavy tractor will have all the horsepower you need if you deal with the traction issue so as to get all the available horsepower into your work. I was hoping for something a little heftier than the L2500DT I wound up with, but I'm already impressed with what it can do. Even with filled tires and a box scraper on the back I seem to spin the rear wheels from time to time (probably due to my total lack of tractorin' skills), so I'm already planning to add about 400 pounds of wheel weights.

Of course if your primary task is finish mowing, you might need a different set of guidelines. /w3tcompact/icons/smile.gif

If I'm way off track here, maybe some of you "pros" could straighten me out,

gordon · Sep 27, 2000

Actually my L-3750 numbers are as follows
Gross hp 45
Pto hp---36.96
The gross hp is the manufacturer's estimate
The pto hp is from the Nebraska test
There are no numbers for the net hp in my operators manual. Its best to throw out the gross and go with the net or pto hp if you want an accurate reading as to the hp of the tractor.
Gordon

Mike H · Sep 27, 2000

Sorry guys, I just can't resist trying to totally muck up this thread /w3tcompact/icons/laugh.gif. If you're tractors over 40 hp and been sold in Nebraska after 1919, you're in luck, the Nebraska Tractor Tests have the data you want. For the rest of us, we need to decipher marketing literature.

Some definitions that can be beaten to death:

Brake Power = output of the engine crankshaft. The engine may be stripped of accessories.

PTO Power = power delivered through the PTO

Drawbar Power = power measured at the end of the drawbar.

Friction Power = power required to run the engine at a given speed without doing useful work (this is losses)

Indicated Power = power available at the piston (this can be calculated) = brake power + friction power

Observed Power = power obtained at the dynamometer without correcting for atmospheric conditions

Corrected Power = observed power correct to sea-level pressure, temperature, and zero vapor pressure.

Now, doesn't that help!

scruffy · Sep 28, 2000

Gordon, Mike and any and all others, confusion on my part here. Gordon, you stated "Gross hp 45
Pto hp---36.96"
The book on my L175 states "Horsepower at PTO Shaft 15"
"Horsepower 17"
(Somewhere else I found a rating of 17.5 horsepower on motor)
The point I puzzled about is how can the PTO hp be rated @ 15 by the Nebraska test, and the engine only have a rated hp of 17? Somehow, this does not equate, especially considering the losses mentioned above.

gordon · Sep 28, 2000

The Nebraska test I believe anyway to be a little more realistic in the numbers for the pto ratings. Its possible that your tractor has 17.5 gross 17 net and 15pto. It's also possible that there is a different rating for a couple of different transmissions? One having a net of 17.5 and the other having a net of 17 just a guess. A gross rating is a dreamland rating. A pto rating is real world rating.

I do believe that mine has what the ratings say for pto but the 45hp is before the double air cleaner and exhaust and the pulleys and the trans. Oh my currnet tractor at the house is a L-3750 ole tugger.

glennmac · Sep 28, 2000

Consistency is more the issue than reality vs. illusion.

To me, a tractor neophyte, neither gross nor net nor pto hp has any intrinsic "real" meaning. That is, I cannot visualize 30 hp, of any kind, as meaning that I can pull an implement of x size up a grade of y degrees at z speed. I can't even do that with automobile hp, and I have been driving for almost 40 years.

The value of a hp rating to a tractor buyer would seem to be merely as a reference point with which to compare the pulling, lifting and pto-spinning power of different tractors--in a relative sense, not an absolute sense. It also should serve as a statistic to guide a buyer in matching an implement to a tractor. For example, so that we know how much tractor hp is recommended to run a brush cutter of a particular size.

Wouldn't any of these hp measurements serve these purposes adequately if it were consistently used. Stated more technically, if you plotted gross, net and pto curves, wouldn't there be a consistent and predictable relationship among them.

The evil here, it seems to me, is that when the tractor manufacturers flip flop between the different hp measurements, or don't reveal one or more of them, probably for marketing reasons, the average buyer has no consistent metric with which to compare relative power among tractor models and to match implements.

Glenn

JimBinMI · Sep 30, 2000

MarkC,

You said,
"Kubota obviously derives their tractor model numbers from the net hp. My L4310, for example, has 43 net hp."

I have before me three brochures from NH,K,JD...they ALL list Gross HP and the NH & K Gross HP's correspond to model numbers. The JD's have non-corresponding model numbers.

The NH brochure lists TC18-TC33, the only one's that Gross HP doesn't exactly match is the Mighty TC18, its Gross HP is 18.5!

The Kubota brochure lists B1700-B2400, all Gross HP's match the model number.

The John Deere brochure lists the 4100-4600, these model numbers don't correspond to HP, but all hp ratings are in Gross HP in the brochure.

I realize these brochures are a couple years old, since I bought in June '99, but this is my sampling. I would imagine they all HAD to go to Gross listing for Marketing purposes.

JimBinMI

We boys and our toys!