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 01-13-2003, 20:02 Post: 47464
perplexed



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 H P verus RPM

I've read that bigger is better when it comes to tractors. But when I talked to my Kubota dealer he stated that in order for the tractor to run at peak efficiency and for it to last longer It should be worked at around 2600 rpm. If this is the case would to much hp cause a reduction in the r's? As you can see, I'm a novice trying to learn.
thanks again, Rude
Whoops, should be versus.






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 01-13-2003, 20:23 Post: 47465
Billy

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 H P verus RPM

perplexed,

Most implements need to be run at PTO speed. Since PTO speed is predetermined, The only thing about HP is if you have enough to keep the RPMs where they should be.

For example a brush cutter, 4ft on a 16hp tractor. You set the RPMs so the PTO runs at 540. You go through some TALL grass or heavy brush. The motor pulls down so you have to go slower so the RPMs stay up.

Same cutter on a 20hp tractor. The RPMs is set the same, to keep the PTO at 540. You go through some TALL grass or heavy brush but the tractor just chugs right along. This is where the extra HP comes in.


Billy






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 01-13-2003, 21:50 Post: 47467
slowrev



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 H P verus RPM

Unlike automotive and small gas engines Tractor engines are designed for more torque at lower RPM's. This also translates to longer life for the engines. Check out the HP specs for your car engine vs a tractor engine.






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 01-14-2003, 07:21 Post: 47485
TomG

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 H P verus RPM

It's possible to go round in circles on this torque vs HP thing, and there's a lot of reading on the subject in the archives. One thing that simplifies the idea is that HP and torque are highly related. A HP is defined as 550 ft.lbs./second. Torque usually is expressed in ft. lbs., so one can be calculated from the other and rpm (There's probably a fudge factor for multiple cylinder engines). Although the measures are opposite sides of the same coin, torque is a measure of force and HP of work. A HP rating is what a tractor is capable of not what is actually delivered in an operation. From physics, no work is done is nothing is moved.

One bang gives torque and HP. A bigger band gives more torque and HP, but more smaller bangs during the same period may give less torque but more HP. Increasing an engine's rpm increases the HP delivered provided the torque stays constant or increases. However, torque doesn't stay constant as rpm changes.

An engine has a torque curve across rpm that produces a peak HP at a certain rpm. Tractor engines, both diesel and gas, are designed so torque increases rapidly at low rpm's and then falls off, which produces peak HP at fairly low rpm's and usually right around pto rpm. Automotive engines are designed to produce higher torque at higher rpm. Therefore, a smaller engine can have a higher peak HP rating. However, the characteristics of such engines aren't too good for tractoring work. They also travel further to do the same work so they wear out faster.

Anyway, I think this is a decent set of basic ideas in a reasonable space for why the dealer would say the best life and efficiency is achieved at 2600 rpm. People here who have engineering training may add something or differ with parts of what I said.






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 01-14-2003, 09:02 Post: 47488
DRankin



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 H P verus RPM

Most small diesel engines designed for tractors seem to have max rated RPM's in the 2600-2750 range.

This maximum RPM is preset and controlled by a mechanical governor. You cannot, under normal circumstances, exceed this RPM level, even if you wanted too.
These are probably the types of engines your dealer is referring to.

There are however, other small diesels that are set up to run most efficiently at 3200 (+/-) RPM. Kubota's BX series and the new Deere 2210 come to mind. These higher speed engines should be operated at that RPM level, especially during PTO operations.

My Deere 4100 spins the rear PTO AT 540 revs with 2650 engine RPM. The BX gets 540 PTO revs at 3068 engine RPM.
Hope this helps.






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 01-14-2003, 21:04 Post: 47522
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 H P verus RPM

Old diesel technology produced torque at low speeds. Some of the larger engines would only turn maybe 800 rpm, red line was at 1200 rpm. In general these engines had long strokes and would fly apart at the higher rpms.
Piston speed equals wear and the older engines were designed to last as long as possible. The long strokes were needed to provide high compression, larger diameter pistions, needed for shorter strokes would not have proper breathing due to the then current valve technology.
Are the newer high rpm diesels going to last as long as the older ones? I am not sure, but I still like to see the PTO rpm nearer to 2 than to 3. I guess you can call me old school.






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 01-15-2003, 19:37 Post: 47578
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 H P verus RPM

Perplexed, believe your dealer, the new small diesels are designed to run at high RPM's. Luging them at low RPM's is much harder on them. If they still used the low RPM/ big cube theory, the engines would simply be too big and too heavy for small tractor use. Be sure to use only the grade of oil your owners manual calls for, and don't be afraid to change it a little oftener than the book says if you are using it only for short jobs that don't fully warm the engine. Oil and filters are free compared to parts.






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 01-17-2003, 08:01 Post: 47658
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 H P verus RPM

New or old, more RPM's mean more wear. There are several small or compact tractors on the market today that turn only around 1800-2200 rpms. The Kubota dealer was right but only about kubotas. Kubota engines bore is usally equal to its stroke. Or some actually have shorter strokes than the diameter of the bore. This necessitates the RPM to produce the torque and HP needed. Engines with strokes longer than the bore don't need to turn very fast to produce massive amounts of torque. In reality, looking at the HP only is an injustice to a diesel engine. You should look at the lb/ft of torque that it produces. You will be quite suprised at the wide range of torque and at what rpm the engine must turn to do it. A lot of manufacturers hide this rating but it can be found in most shop manuals.






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 01-17-2003, 08:07 Post: 47659
slowrev



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 H P verus RPM

Yes Torque is really what counts in a tractor engine, but is almost never published. I would like to see the ft/lbs of torque produced while the engine is running at PTO speed. This information would help me to make a better choice.






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 01-17-2003, 08:13 Post: 47661
marklugo



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 H P verus RPM

HINT: the PTO RPM=torque peak on many tractors.






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