Tractor classification — HP vs torque

[mo1]

There would be a performance difference when it comes time to actually do the work. Given proportional sized cooling systems, the larger displacement engine should run cooler. Heat kills engines and bolted-on engine components.
So when the rubber hits the road (or the plow gets dropped into the soil, the bush hog gets pulled through heavy brush) the bigger displacement engine will run cooler and last longer. You can work it harder in the field.

Some things cannot be explained with paper statistics. They just are a certain way a tractor feels when you drive it. In a tractor, I would prefer noticeably more displacement of a bigger engine over a smaller displacement engine, even if the smaller engine had a little more power.

I’m skeptical of these smaller 4-liter-ish, 4 cylinder engines I’m seeing in 100-150HP tractors. I don’t think they are going 10,000 hours. I greatly prefer a 6 cylinder over a 4 cylinder, too. The longer crankshaft and 6 smaller holes firing give me more confidence than a short crank and 4 holes firing.

The cooling system should be sized based on the engine's fueling rate as burning fuel generates heat, more fuel = more heat. The two examples I gave have nearly identical fueling rates, so the cooling systems should be very similar. Whether or not they actually are is a different story and has to do with a bunch of factors not associated with engine displacement such as price point, space constraints, etc.

The smaller engines have similar power figures and torque rises as the larger engines as they either run at a higher speed with the same turbocharger boost, run more boost but have similar operating speeds, or some combination of the two. Either way allows for the smaller engine to suck in as much air per minute as the larger engine, so it can combust the same amount of fuel per minute, and thus make the same power. But, a four-cylinder crank of the same crank diameter and journal size is going to be stronger than a six-cylinder crank as it has less torsional stress due to its shorter length. The reason you see huge sixes but not threes or fours is because engine balance on sixes is excellent, and not so great on threes and fours. Above a certain size, it's cheaper to use a straight six than mess with balance shafts and such on a huge three or four to keep it from shaking itself apart.

A larger engine making the same power as a smaller one turning the same speed but with more boost pressure will last somewhat longer as it has less cylinder pressure to contain. A larger engine making the same power as a smaller engine with the same boost pressure but a higher speed will also last somewhat longer due to less wear on the rings and bearings. How much longer remains to be seen, although I would expect that since the fours and sixes are usually modular designs with a lot of shared parts, the difference would not be massive. Bring in MBA "value engineering" though and all bets are off.

Regarding longevity, I'd be most suspect of the small sixes in the smaller rowcrop units as some of those are the most highly stressed engines in their product portfolios. CNH and Deere run their 6.7 L and 6.8 L sixes up to about 300 HP before going to a ~9 L six. This would be like running the mid-four-liter fours at 200 HP. They don't run the fours nearly that hard, they only run those up to about 150 HP, which would be like cutting off the 6.7/6.8 sixes at 225 HP.

Is it fair to say you dont like the ~5 - 8ish liter 2-cylinders of 6-7 decades ago?

You are questioning the 4-ish liter 4-cylinders......the deere 4.5 is a pretty tried and true engine and runs from 85ish HP up to 135 or 140. And I wouldnt consider a 4.5L small for a 4-cylinder. Probably one of the larger modern ones before they jump to 6+ cylinders?

Yes, most makers go from a four-cylinder engine in the mid-four-liter range to a six-cylinder in the upper-six-liter range, as the four and six are often a modular design that share the same bore and stroke. For Deere this is exactly true, the 6.8 L six (6068) is nothing more than the 4045 (4.5 L) four with two more cylinders (the 4045 came first as the 276 four in the mid-'70s, the 6068 came in the 1990s to replace the 359 and 466 sixes.) Besides Kubota's 6.1 L four, fours rarely go much above five liters.

 

[Hay Dude]

The cooling system should be sized based on the engine's fueling rate as burning fuel generates heat, more fuel = more heat. The two examples I gave have nearly identical fueling rates, so the cooling systems should be very similar. Whether or not they actually are is a different story and has to do with a bunch of factors not associated with engine displacement such as price point, space constraints, etc.

The smaller engines have similar power figures and torque rises as the larger engines as they either run at a higher speed with the same turbocharger boost, run more boost but have similar operating speeds, or some combination of the two. Either way allows for the smaller engine to suck in as much air per minute as the larger engine, so it can combust the same amount of fuel per minute, and thus make the same power. But, a four-cylinder crank of the same crank diameter and journal size is going to be stronger than a six-cylinder crank as it has less torsional stress due to its shorter length. The reason you see huge sixes but not threes or fours is because engine balance on sixes is excellent, and not so great on threes and fours. Above a certain size, it's cheaper to use a straight six than mess with balance shafts and such on a huge three or four to keep it from shaking itself apart.

A larger engine making the same power as a smaller one turning the same speed but with more boost pressure will last somewhat longer as it has less cylinder pressure to contain. A larger engine making the same power as a smaller engine with the same boost pressure but a higher speed will also last somewhat longer due to less wear on the rings and bearings. How much longer remains to be seen, although I would expect that since the fours and sixes are usually modular designs with a lot of shared parts, the difference would not be massive. Bring in MBA "value engineering" though and all bets are off.

Regarding longevity, I'd be most suspect of the small sixes in the smaller rowcrop units as some of those are the most highly stressed engines in their product portfolios. CNH and Deere run their 6.7 L and 6.8 L sixes up to about 300 HP before going to a ~9 L six. This would be like running the mid-four-liter fours at 200 HP. They don't run the fours nearly that hard, they only run those up to about 150 HP, which would be like cutting off the 6.7/6.8 sixes at 225 HP.



Yes, most makers go from a four-cylinder engine in the mid-four-liter range to a six-cylinder in the upper-six-liter range, as the four and six are often a modular design that share the same bore and stroke. For Deere this is exactly true, the 6.8 L six (6068) is nothing more than the 4045 (4.5 L) four with two more cylinders (the 4045 came first as the 276 four in the mid-'70s, the 6068 came in the 1990s to replace the 359 and 466 sixes.) Besides Kubota's 6.1 L four, fours rarely go much above five liters.

Might have same crank diameter or might not. Then theres bearing support. Bearing size and numbers.
Im running (2) 6.6L 6cyls, one at 135HP and one at 200HP. They will live very long lives there. Trying to buy another ”low powered” 6L/6cyl ~125-150HP tractor.

My 270HP tractor is an 8.3L and it is running near the top of its power limit (300 tractor and 330 truck).

 

[LouNY]

Two 90 PTO HP tractors from the same manufacture with completely different engines.
I much prefer the second one, myself.

 

[Hay Dude]

Two 90 PTO HP tractors from the same manufacture with completely different engines.
I much prefer the second one, myself.

View attachment 791376

View attachment 791377

Me too.
The Genesis (8160) can be a 40+ year tractor. The T4.140 is a fine tractor, but will die from it’s own complex design and a smallish engine pushed too hard.

I would say the Genesis Tractors were second only to the Boxcar Magnums as the most popular tractors ever built since the 80’s.

I’m an AGCO guy, but have a lot of respect for the Genesis and Boxcar Mag’s. Came close to buying a Genesis more than once.
I love my big Magnum primarily because of the big engine. You can turn the PTO hooked to a huge baler and it just takes it.

 

[Hay Dude]

This is a dyno chart of one of the Cat equipped road tractors we have where I work (heavy construction)

View attachment 790751

I pull double, cement bulk trailers in western Montana… 128,000# give or take…

When I first drove this tractor, and came to “the hill” (6% uphill for a little over a mile), I would start downshifting, four or five gears, to keep the engine between 1,900 - 2,100 rpm which is at the peak of its HP range…

One day I got lazy… ok, I was “experimenting”, I let the engine begin to “lug” … and found the engine would continue to pull hard down to 1,100 rpm… I still needed to downshift a few times, but I didn’t have to keep the engine racing and the tractor/trailer climbed the hill at a greater speed than it would in a lower gear with the engine racing…

Which brings me back to the reason I started this thread… and I STILL can’t wrap my head around why it‘s HP performing the work when, in my experience, I see torque taking over when it comes time to do the heavy lifting…

As was pointed out earlier in the thread, two 25 HP tractors with two different displacements… seems to me the larger of the engines will generate more torque throughout a broader spread…

Another thing larger displacement engines have over small engines I hear commonly talked about by truckers and farmers is “rotating mass”. Once a bigger displacement engine gets going, the rotating mass of the larger pistons, heavier crank, bigger components keep the torque and momentum going.

 

[LD1]

Regarding longevity, I'd be most suspect of the small sixes in the smaller rowcrop units as some of those are the most highly stressed engines in their product portfolios. CNH and Deere run their 6.7 L and 6.8 L sixes up to about 300 HP before going to a ~9 L six. This would be like running the mid-four-liter fours at 200 HP. They don't run the fours nearly that hard, they only run those up to about 150 HP, which would be like cutting off the 6.7/6.8 sixes at 225 HP.

Just a few decades ago....1990's.....deere was only pushing the 4.5L up to about 100hp and the 6.8 was only 100-150hp. Then they jumped to a 7.6 or a 8.1.

Now that they are pushing the 6.8 up to around 275, no doubt they could push the 4.5 harder than 150. But Maybe it has to do with tractor design.....maybe the jump from say a 4.5L 150hp to a 6.8L 160hp machine comes with alot more than just 10hp. Maybe larger frame, wheels, etc.

And no doubt that most people in the market for a 150-200hp tractor would certainly have a preference for the 6-cyl over the 4 cyl.

There is certainly is (or was) some overlap between the 4.5L and the 6.8L. Meaning that some tractors with the 6.8L had LESS HP than some with a 4.5L.

For example, in 2008 you could get a 6140D with 140hp and 4.5L. Or you could get a 7230 with 135hp and the 6.8L

Or the 7130 it was an option for either? Maybe they used that as a case study to see what people prefered in a tractor that size regarding 4cyl vs 6cyl?

But as with anything.....engineering and tech gets better. (at least in terms of HP per liter). Cause it wasnt that long ago 6+ liters was needed just to crack 100hp....and took 8+ to crack 150hp.

We will probably continue to see the same trend in the future IMO. Pushing the 2.9L (which currently stops ~75hp) past the 100hp mark. And the 4.5L past the 200mark. But we will probably see a total engine re-design before then. Because the 2.9, 4.5, and 6.8 have been around for along time

 

[WinterDeere]

I only read p.1 and p.6 of the thread, skipped four pages in between. But torque is unlimited, to within practical gearing losses, just change your gearing to suit. Ten times out of ten, traction becomes the limiting factor, when considering torque. Not the engine.

Horsepower is a legitimate limiting factor of performance, so it makes sense to classify the tractors on this basis. It is the true measure of how much work (by proper definition) a machine can do in a given period of time.

 

[Hay Dude]

I only read p.1 and p.6 of the thread, skipped four pages in between. But torque is unlimited, to within practical gearing losses, just change your gearing to suit. Ten times out of ten, traction becomes the limiting factor, when considering torque. Not the engine.

Horsepower is a legitimate limiting factor of performance, so it makes sense to classify the tractors on this basis. It is the true measure of how much work (by proper definition) a machine can do in a given period of time.

But PTO HP has nothing to do with traction. Spinning up a hay baler or a big 15-20’ bush hog takes a motor with grunt torque at the bottom end. Big engines do that better.

 

[WinterDeere]

But PTO HP has nothing to do with traction. Spinning up a hay baler or a big 15-20’ bush hog takes a motor with grunt torque at the bottom end. Big engines do that better.

Definitely. I was really thinking more of my usual tasks of plowing, climbing hills with a big load, snowblowing, etc. Hadn't even considered what you or a commercial farmer might be putting the machine to. But then again, your primary machines are likely way above the 25 hp range discussed in the OP!

 

[mo1]

Just a few decades ago....1990's.....deere was only pushing the 4.5L up to about 100hp and the 6.8 was only 100-150hp. Then they jumped to a 7.6 or a 8.1.

Now that they are pushing the 6.8 up to around 275, no doubt they could push the 4.5 harder than 150. But Maybe it has to do with tractor design.....maybe the jump from say a 4.5L 150hp to a 6.8L 160hp machine comes with alot more than just 10hp. Maybe larger frame, wheels, etc.

And no doubt that most people in the market for a 150-200hp tractor would certainly have a preference for the 6-cyl over the 4 cyl.

There is certainly is (or was) some overlap between the 4.5L and the 6.8L. Meaning that some tractors with the 6.8L had LESS HP than some with a 4.5L.

For example, in 2008 you could get a 6140D with 140hp and 4.5L. Or you could get a 7230 with 135hp and the 6.8L

Or the 7130 it was an option for either? Maybe they used that as a case study to see what people prefered in a tractor that size regarding 4cyl vs 6cyl?

But as with anything.....engineering and tech gets better. (at least in terms of HP per liter). Cause it wasnt that long ago 6+ liters was needed just to crack 100hp....and took 8+ to crack 150hp.

We will probably continue to see the same trend in the future IMO. Pushing the 2.9L (which currently stops ~75hp) past the 100hp mark. And the 4.5L past the 200mark. But we will probably see a total engine re-design before then. Because the 2.9, 4.5, and 6.8 have been around for along time

The 4.5 slowly crept up from no more than 100 HP in the 6400 to the current 145 HP in the 6145M, the 6.8 went from a maximum of 130 engine/112 PTO HP in the 7600 to now up to 270-297 engine/224 PTO HP in the 7R 270. The 8.1 started out as low as at 155 engine/135 PTO HP in the 7710, the 6.8 in the 7610 only went to 140 engine/120 PTO HP.

The 7130 initially had the 6.8 six at 121 HP but in 2010 went to the 4.5 four at 125 HP.

Diesels weren't pushed very hard until recently, it was rare to see a tubocharged one make much more than about a third more power than the naturally-aspirated version did (or likely would have) until relatively recently. The naturally-aspirated 2.9 three made 55 HP in a lot of applications over the years, the 4.5 four made up to 80 HP, the 359 six made 105 HP, the naturally-aspirated 466 made 110 PTO HP (about 130 engine HP), and the 6.8 and 8.1 only ever came turbocharged in tractors, but Deere does sell a naturally-aspirated mechanically controlled 6.8 in non-emissions countries as an industrial engine with a 125 HP rating.

The upper limit of what is currently done appears to be just over 45 HP/L for tractor engines. Picking on Deere's engines, the most powerful version of each of their engines (continuous ratings) are:
- 2.9 three: 74 HP in the 5075E/M, 25.5 HP/L
- 4.5 four, 145 HP in the 6145M, 32.2 HP/L
- 6.8 six: 270 HP in the 7R 270, 39.7 HP/L
- 9.0 six: 410 HP in the 8R 410, 45.6 HP/L
- 13.6 six: 632 HP in the 9R 640, 46.4 HP/L

The 2.9 is a special case as it is kept below 75 HP to not require DEF/SCR, Deere did make a marinized version with 86 HP in the pre-Tier 4 era. If Deere pushed all of their engines as hard as they do the 13.6 in the 9R 640, the 2.9 would make 135 HP, the 4.5 would make 209 HP, and the 6.8 316 HP.

The 2.9 and 4.5 have been around since the later 1970s, the 6.8 was actually developed in the early '90s by adding two cylinders to the 4.5. All of those engines trace their lineage back to 1965's 1020 152 three and 2020 202 four which were the first of the 300 series modular engines.