Horsepower comparison

[Staghead]

I have kind of a basic question regarding horsepower. What is the difference between the "horsepower" in a big lawn and garden tractor and a SCUT and CUT?
In other words how can a CUT with 25 HP do so much more than a garden tractor with a comparable rating. I've been looking at a lot of brands and it seems that 25-30 HP can do a lot, but it still sounds "small" to me.

Also can someone compare/contrast what a "newer" model 50 horse can do compared to an "older" 50 HP tractor (that probably weighs twice as much). At least this I can somewhat relate to in regards to new technologies.

 

[gordon21]

Someone will probably have an exact answer, but the CUT you see are usually diesel. Assume a 1.5x or 2x factor converting from gas to diesel. A 30HP diesel is comparable to a 45-60 HP gas engine. This same thing baffled me a year ago when I was shopping. My little JD 790 has only about 28 HP and it does amazing things. It drags a 4000# outfit up and down steep hills, can run a backhoe and yank huge rocks out of the ground. I have seldom had to run it above 2000 RPM, even though it is rated to run all day at 2600 RPM.

On the other hand, for three years I had a 16HP gasoline lawn mower/tractor that was used on flat land only. All it hauled around was the mower itself and my butt. Going from 16HP-gasoline to 28HP diesel seems like a 10 fold increase in power.

Some of the really big farm tractors have only 120-150HP diesels. Your average new pickup has 250-350. Which motor can do the most work out in a field???? Gearing and low end torque are the difference.

 

[jbrumberg]

Staghead:

Welcome to TBN ! I agree with everything that gordon21 has stated. You should also consider the fact that many SCUT's and CUT's are 4WD which also increases the apparent power (for some strange reason 17-20% figure keeps flooding up from my old memory banks ). You will find a huge repository of collective real tractor experience here at TBN. Keep the questions coming. Jay

 

[morningwood]

I have often wondered the same thing. You see a high end lawn tractor with 27HP and you have a compact utility with 25HP and it is much bigger machine.

HP is HP but the difference between the two is in weight, gearing, torque and component weight / durability. I doubt you could put a 5 or 6k hours on a 27HP gasoline engine in a lawn tractor or expect it to be able to take the abuse of 5 or 6k hours of hard use.

My personal opinion on your second question is that you cannot compare a "new" 50HP tractor to a older one. The older ones were over built and the newer ones are built like everything else these days. They are built good enough to do the job and not much else and you throw them away after a few years. If I was in the market for a 50HP tractor I don't think I would get anything newer than a 1975 model, plus who can afford to spend 30k on a new 50 HP tractor. They were much simpler machines then and alot easier to work on. The newer ones do have there advantages though: easier starting, power shift, quieter, electronic fuel injection for better fuel management, AC / CD player if you get a cab and more than likely 4WD.

Scott

 

[sandman2234]

I agree with morningwood's discription of horsepower. A hp is a hp, pure and simple. Torque and gearing make up the rest, along with traction.
I listened to a friend complain about tearing up his 12hp Snapper mower trying to clear a lot next to his trailer to build a house. I told him I had a 13hp Ford (He drives a Chevy) that I would bring up to give it a try. He said don't bother, it will just tear up a small mower like that. Took me a little bit to talk him into letting me try, since it was my mower.
That Ford 1100 4x4 with a 4 foot bush hog cleared the lot in about 4 hours.
Made a believer out of him about diesel hp!
David from jax

 

[Volfandt]

All are goods answers. This topic has been bandied about before.

Some recaps:
The majority of small air cooled gasoline engines have their HP rated without any load on the engine, i.e. no alternator, straight pipes and no restriction on the air intake. Their HP isn't measured with it installed on the machine it powers.

The majority of diesel powered tractors (SCUT's, CUT's, farm) have their HP rated at the HST and/or gear tranny's input and at the PTO output. They have every component installed as it's installed on the tractor. Most use the U of Nebraska test to prove their usable HP.

Diesel engines have a longer stroke which allows it to make it's rated RPM at lower RPM than the gas engine. This gives the diesel a wider power band at all RPM.

Due to the higher compression required by the diesel, it is constructed with heavier material which increases longevity.

Liquid cooling also increases an engines longevity.

Theres more in several threads scattered about

Volfandt

 

[VistanTN]

Everyone has pretty well covered it -- HP or (work output) = (Force) x (Distance) / (Time). The garden tractor/lawn mower gets its HP mostly from speed (larger distance in a shorter time). The tractor (SCUT/CUT/Ag) is mostly about (Force). They can't come close to the blade tip speed (or often even the travel speed) of a mower, but they put power into the machine and tractive effort into the ground through weight and big wheels. A HP is an HP ... but a HP at the crankshaft on a bare engine is far less than a HP at the PTO or where the rubber meets the road. Betcha if they rated lawn mowers by HP at the mower drive or "drawbar" you'd see a LOT lower numbers.
VistanTN

 

[jeffghs]

As mentioned a CUT is direct drive, meaning the tranny is directly bolted to the engine more or less, but with a lawn and garden tractor, most are belt driven which will sap an engine of quite a bit of power. I don't know the numbers but I would estimate a 30%, possibly more, power loss by the belt systems alone.

 

[Brad_Blazer]

I thing a significant issue is that the horsepower number is posted on the side of the lawn tractor and shoppers select the product based in part on that number. The manufacturing cost of a 20hp engine vs. a 16hp engine with the same basic design is probably very similar. Just because a lawn tractor with a 42" deck has a 20hp engine doesn't mean it is cabable of using all 20hp. 20 years ago a similar machine would have had a 10 or 12hp engine and done the same job.

Brad

 

[AMP762]

Simpley put torque is the work force,torque x rpm is hp.Most air cooled mower engines run about 3600rpm,most of our tractors are run 2000-2500. you can make big hp out of anything if you can twist it fast enough.

 

[BobG_in_VA]

All are goods answers. This topic has been bandied about before.

Some recaps:
The majority of small air cooled gasoline engines have their HP rated without any load on the engine, i.e. no alternator, straight pipes and no restriction on the air intake. Their HP isn't measured with it installed on the machine it powers.

The majority of diesel powered tractors (SCUT's, CUT's, farm) have their HP rated at the HST and/or gear tranny's input and at the PTO output. They have every component installed as it's installed on the tractor. Most use the U of Nebraska test to prove their usable HP.

Diesel engines have a longer stroke which allows it to make it's rated RPM at lower RPM than the gas engine. This gives the diesel a wider power band at all RPM.

Due to the higher compression required by the diesel, it is constructed with heavier material which increases longevity.

Liquid cooling also increases an engines longevity.

Theres more in several threads scattered about

Volfandt

I just read (a few days ago) a thread about a 350 CI Chevy V8 vs. a 1948ish Farmall M. The 350 makes well over 200 HP at something like 4500rpm together with about 300 ft lbs of energy/Torque. A Farmall M had about 245 CI, 4 Cly gas engine that made about 35-38HP. RPM rating on that engine was only about 1650rpm. Guess What? It also made about 300ft lbs of energy at less than half the rpms. Regarding longevity, I don't think anyone can contest that a whole bunch of M's are still out there and running while over 60 years old (M's started manufacture in 1939 and continued thru about 1954). Oh, they were a pretty long stroke engine as well. I don't think I know of any "new" diesels that top out at only 1650rpm and make 35HP and 300 ft lbs of energy. I'm not disputing anything that has been said, however, diesels are "Johnny Come Lately" in the tractor business. The M was available as the MD (d for diesel) as early as about '50 or so (don't quote me on the exact year). They started on gas and once warmed up, the operator switched it over to diesel. They were/are very strong tractors! PS: I have a 60HP diesel 4WD that I wouldn't trade for anything, but I really do enjoy running the old Farmalls. BobG in VA

 

[Volfandt]

Good points BobG, I had a 1948 TE20 Ferguson that was rated at around 21hp at the PTO and it could easily lift 1000lbs at the 3PH. It drove a 5' bushhog, tandem disc and single bottom plow without any problems what so ever and it too was an inline 4 cyl gas engine. It was runnign fine when I sold it alittle over 2 yrs ago.
I also have a 1966 WheelHorse 856 w/a K181s Kohler which is an aircooled gasoline engine that I estimate at over 2k hrs and I work it weekly running a sicklebar mower. Course it's a cast iron engine thats designed to be rebuilt.

But to 'splain my post and what I thought was being asked, the gasoline engines in question are the "air cooled" variety that comes on the majority of modern L&G tractors. They don't compare favorably with the modern diesels nor the liquid and air cooled gasoline engines of old.

Diesels may be Johnny come lately in the tractor world but they've taken it over by storm. More power at nearly half the fuel usage in most cases.

Volfandt

 

[knute_m]

All great answers. Here are just a couple additional points.

In general, gasoline engine horse power is proportional to rpm. If a gasoline engine starts to lug, it also loses horse power, which makes it lug more, etc. As far as I know, the air cooled gasoline engine horse power ratings are derived at maxium rated rpm.

A diesel engine has a much flatter horse power vs. rpm graph. With that said, if a diesel engine starts to lug, it still has a significant percentage of the horse power it had at the higher rpm. The horse power change over low vs. high rpm is a much smaller difference for diesel vs. gas.

Secondly, most of the SCUTs and CUTs have larger rear wheels than lawn and garden tractors, which again results in higher torque where the wheels meet the ground.

Our old 2-cylinder 1948 John Deere H was only rated at 11 hp, but with the 38 inch wheels and lots of weight, it really could pull.

 

[Staghead]

Thanks for all the replies. I searched a little for this info in previous threads, but must have just missed it (probably could have looked harder).
I hope everyone realizes how helpful this site and its various forums are to someone like me. I have learned more from here than I ever would have kicking the tires at the local dealers. I do understand the value in that as well, but for general info you can't beat the information here.

I haven't gotten my CUT yet, but I still can't stay away from this site since I found it. It makes it harder and harder to wait to get a CUT. My credit score just isn't good enough for the State Bank of My Wife to float me a loan

 

[BobG_in_VA]

Diesels may be Johnny come lately in the tractor world but they've taken it over by storm. More power at nearly half the fuel usage in most cases.

Volfandt

No argument here! I especially like the direct inject diesels. 10 or so seconds with the air heater (does not require a glow plug for fuel heat up) in 20 degree weather and it fires right up. BobG in VA

 

[whodat90]

Another thing to consider is that each measurement is PEAK horsepower. Gas engines, particularly those in lawn tractors, are designed to operate at a particular RPM, i.e. 3600. They put out peak hp there, but even a little bit slower and they produce disproportionately less hp. A diesel engine has a very flat torque curve, and in fact often have a 'reverse' torque curve. This means that the slower they spin, the more torque they produce. This accounts for the feeling you often get with a diesel that it's kind of digging in and pulling harder, whereas with a gas engine you have to keep the revs up. FWIW yesterday I helped my neighbor move a '71 corvette stingray out of his barn, where it had been sitting for some years. Flat tires, mostly locked brakes, you name it. Pulled it out with my little B7200D in first gear at idle. Wasn't till we had it out of the gravel and up over the hump onto the asphalt that we realized the corvette was still in gear. Further I just ran a compression test on the kub the other day; factory specs are 450psi new, 325psi lower service limit. Mine were 405, 425, and 425, with about 1100 hours and 22 years on the engine. Not many gas engines can match that kind of durability.

 

[morningwood]

No argument here! I especially like the direct inject diesels. 10 or so seconds with the air heater (does not require a glow plug for fuel heat up) in 20 degree weather and it fires right up. BobG in VA

BobG,

I thought that most diesels since the beginning of there production have been direct injection ??? I have heard of a few companies ( MF comes to mind ) that made indirect injection but they usuallly stopped making them after a few years due to problems.

Off Topic

I have a 06 Duramax, it has glow plugs and a grid heater. It got down to 0 degree F one day last year and she fired up, no questions asked. During normal OH winter days the glow plus usually stay on for about 5 - 10 seconds and she is ready to go.

Scott

 

[BobG_in_VA]

BobG,

I thought that most diesels since the beginning of there production have been direct injection ??? I have heard of a few companies ( MF comes to mind ) that made indirect injection but they usuallly stopped making them after a few years due to problems.

Off Topic

I have a 06 Duramax, it has glow plugs and a grid heater. It got down to 0 degree F one day last year and she fired up, no questions asked. During normal OH winter days the glow plus usually stay on for about 5 - 10 seconds and she is ready to go.

Scott

Any diesel that has real "glow plugs" (sort of spark plug lookin' things screwed into the intake manifold is an indirect injection engine. There are still a bunch out there and a lot of current production diesels are still indirect. If I'm not mistaken, the Ford (Navistar) PowerStroke is indirect. No biggie...BobG in VA

 

[morningwood]

You are referring to indirect injection as the fuel gets sprayed onto or near the glow plug ??? From what I was told about the duramax ( not sure about the powerstroke ) that the glow plugs are only there for starting.

In my eyes indirect injection can be compared to fuel injection or carbeuration that most ( audi just came out with direct injection for gassers ) gas motors use. The gas and air are pre - mixed and then burned. I thought most modern diesel motors inject fuel directly into the cylinder and then it is mixed with air and then compressed.

Scott

 

[schmism]

To make it more muddy... i understood it like this

standard injection was spayed directly into the cylinder but on the intake stroke.

direct injection was spayed directly into the cylinder but durring the compression stroke. thus much higher pressures are involved and therefore much more complicated injectors which is why they have been a "new thing"