hp ratings ?

Villengineer · Feb 2, 2004

The real question is whether they mean SAE gross and SAE net hp or something else. Both reference the hp at the flywheel. SAE gross hp (which was the common advertised hp for cars before '71 I think) is essentially the engine with only the absolute bare minimum of components to make it run, ie just about the most hp you can squeeze out of it. SAE net is with all accessories, exhaust, etc...that the engine would have attached to in the vehicle minus the actual drivetrain.
Since net hp varies due to differences in accessories, the type of exhaust, etc.. the gross hp is a valuable figure. If for instance you want to change or add accessories, gross would be the baseline you subtracted them from. PTO hp is the hp available at the pto shaft. This obviously changes a little as you change the net hp.
Now like I stated before, I don't know if they are refering to SAE hp or not.

JackIL · Feb 2, 2004

There are 3 horsepower ratings to distinguish. The SAE defines 2 tests--one dealing with "gross" and one dealing with "net." Both are done in a test cell with an engine alone powering a dynomometer. It is not installed in a tractor or other vehicle.

In the "gross hp" test, all accessories including the radiator, fan, and other cooling system components and all exhaust components are removed. In addition there are also no pumps, etc. These effects are all supplied externally to the engine so that it's own power is not used to support them. Power is measured by the dynamometer at the flywheel. The resulting "gross horsepower" is a somewhat high number and rather meaningless except for advertising purposes. Almost all tractors below 40 PTO hp are advertised on the basis of Gross HP.

SAE "net HP" is the same type of test but the essential cooling system and exhaust system components are in place. Still no hydraulic pumps. Again the dyno measures this power at the flywheel.

When the engine is installed in a tractor the most meaningful measure of power is PTO hp. This is again measured with a dynomometer, but it is now connected to the PTO on the tractor if it has one (some large articulated 4WD tractors do not so their HP is normally quoted as Flywheel HP or Net HP). Note that the tractor is not moving and things like the air conditioner are shut off. As a general rule of thumb, PTO HP is approximately 85% of Net or Flywheel HP.

Now if the transmission is hydrostatic, the PTO HP will be slightly lower than if there is a gear transmission. This is because the hydro pump runs all of the time, so even though the tractor is not moving in the PTO-HP test, some power is consumed by the hydro pump. Usually PTO-HP with a hydrostatic tractor is about 83% of Net HP.

So why do manufacturers use "gross hp?" It is primarily for advertising purposes so that the largest possible legitimate number can be quoted.

The most reasonable way to compare tractors on the basis of horsepower is to look strictly at PTO hp. Keep in mind that this hp is what is measured at the PTO with the tractor stationary.

Sometimes you hear "drawbar" hp mentioned. This can be measured by measuring the horizontal drawbar pull and tractor true ground speed. A tractor pulling on concrete, might get as much as 85% of the net engine hp out at the drawbar. On softer soils it can usually be in the 70% range at best. Therefore, drawbar hp is not usually quoted in advertising.

This may be more than you wanted to know. If so, I apologize.

JackIL

jimg · Feb 2, 2004

Thats great...thanks for all the info. Its as I thought though, gross hp is just a window dressing number which is mostly meaningless.

nomad · Feb 2, 2004

Maybe, Gross HP is being written in user manuals/etc of tractors because the engine MFGs maybe wanting that in their contracts with the tractor MFGs. You know manufacturers of tractors and manufacturers of their engines aren't always same companies.