How does horsepower affect performance?

WileyKioti · Jan 8, 2006

When I looked at tractors and settled on a brand, I had the choice between the 25 hp, and the 30 hp. They both had the same chassis (weight). I chose the 30hp, because I was going to use it to brush hog in a hilly area. More PTO left over after climbing.

My wife thought it was because it had more HP..... /forums/images/graemlins/grin.gif

God Bless,
Dereck

CK30 HST

CT_Tree_Guy · Jan 8, 2006

If you are going to use your tractor only for the box blade, then, forget about PTO because box blade does not use PTO. So, it is meaningless to compare their PTO powers/torques. 

Just wanted to say something about what might be a misconception about why tractors are rated by PTO HP. (And again, I too wish it were "PTO Torque"). I think it is a useful rating, whether or not you ever use your PTO (which I don't).

It seems to me that the term "PTO HP" can be looked at very much the same way the "BHP" (Brake horsepower) is used to rate automtive power. BHP relates to how much power actually gets to the ground, after all power losses have been accounted for. Starting with engine gross power, subtract power losses from belts, pulleys, alternator, water pump, fuel pump etc. to arrive at engine net power. Then subtract drive train power losses to get BHP.

Same with PTO HP. It just seems that this is roughly the same amount of power that the tractor will be able to put to the ground, or maybe more accurately, how much power is available to rotate the wheels, since both the PTO and the wheels will experience roughly the same amount of power loss due to gearing.

Does this make sense? As always, if I'm "full of it", I want to be the first to know. /forums/images/graemlins/smile.gif

Oh, and Kinetic - as for:

 My wife thought it was because it had more HP..... 

Where DO women get the idea that men like power?? /forums/images/graemlins/laugh.gif

Steve_Taylor · Jan 8, 2006

PTO horse power is used because it is measured at a common PTO speed of 540 RPM. Therefore the HP figure relates directly to torque and can be compared between tractors of various sizes and brands.

Brake HP was generally the output of just the engine as measured on a friction-brake dynamometer. The car manufacturers used this in the power race days because they could advertise a higher number than if the HP were measured at the wheels.

See this http://en.wikipedia.org/wiki/Horsepower for all you would ever want to know about HP.

CT_Tree_Guy · Jan 8, 2006

Hiya Steve -

Man I love the internet. Since your post I've been researching PTO HP, tractor ratings (lots of Nebraska Tractor test stuff), and various cooperative extension service publications. Too much to go into at this hour, but a couple of highlights: 1) I guess drawbar HP is more closely related to what I was "wanting" PTO HP to equate to; I should have left it at my basic sentiment that PTO HP can be useful info even if you don't use the PTO, especially since Drawbar HP data can be a little hard to come by (The only Kubota that the NE labs have tested is the 5700?); 2) although some sources that I found referred to PTO HP being the average HP at varying engine speeds, I think I'll have to defer to you and the Nebraska labs on that one that it's done at 540 PTO rpm (although this seems a little odd since that engine speed is not necessarily where maximum torque OR HP will occur in a given engine); and lastly, 3) GREAT news for horsepower junkies!!!!! (Courtesy of the Cooperative Extension service of the University of Virginia - thanks guys!)

More to come, John

*****

Heck, can't leave y'all hangin' like THAT, especially this close to Christmas - check out the part of this thread where a farmer has to choose between a 165 drawbar HP tractor, and a 314 drawbar HP tractor, for an operation that requires 165 HP. Guess which one they say to go with??!! WOOHOO, I love these guys!!! Go get 'em!! Dang, with that kind of bad-a$$ attitude, the South WILL rise again!! lol

Gear up and throttle down - Univ. of VA Cooperative Extension Service

Gimmegimmegimme more POWERRRRRRRRRRRR!!!!!!!! /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

Reg · Jan 12, 2006

For FIELD work:
Is there an ideal power/weight ratio ?
e.g. the point at which more HP results in more wheelspin, less horse power results in bogging ?
Any rules of thumb ?
I guess this would be in lbs/hp,
would 100 lbs/hp be about right ?
4000 lb 40 hp tractor - seems heavy.
Sand vs mud vs clay vs loam probably affects this (-:

JerryG · Jan 12, 2006

For field work you need 110 to 130 pounds per horse, some say to go as high as 140. I would say that if you go 110 you will get a lot to the ground and it won't be sluggish for other operations.

daTeacha · Jan 12, 2006

( For FIELD work:
Is there an ideal power/weight ratio ?
Sand vs mud vs clay vs loam probably affects this (-:
)

Get in touch with people in your area who have your type of soil. Tires make a difference, too. Not only radial vs. bias ply, but the angle of the bars on R1s, long bar/long bar vs. long bar/short bar, and so on, plus what you plan to do the most. So much depends on your local variables that you really need to see what works best for folks in your area with your conditions. Then it also depends on how wet the ground is when you want to be out on it, too. Around here there are about as many opinions as there are farms. Most anything can be made to work, and most anything will be "best" in some situations but not in others.

tglass · Jan 18, 2006

This sure stirred up a hornets nest. I had the same questions when I bought my last tractor, but the best advice I got was to try all the different brands and see which one I liked best. Some were built alot better, heavier and even do more than the one I bought, but this is the one that felt good to me and did what I wanted at the time. I think all the manufacturers have a good product out and it comes down to preferences, customer satisfaction from the dealer and if you enjoy it when you use it.