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.