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How much does engine horsepower really matter?

12K views 32 replies 16 participants last post by  mike01 
#1 ·
How much does engine horsepower matter in a tractor? What exactly does the engine do besides spin the transmissions hydraulic pumps? Isn't the transmission the thing that's putting out all of the actual power, driven by the engine?

Like...if you take two 20hp engines and hook one up to a powerful transmission, say a TuffTorq K90, and the other to a K46, will they put out the same power to the wheels? Or will the K90 give you more wheel hp? How does this work and why?
 
#2 ·
The only things that provide power are the battery and the engine. Different transmissions may transmit that power more efficiently, or have more durability, or provide relatively more torque at the low end or high end, but the engine HP determines power. Also, tires make a difference in utilizing the power and torque based in their gripping ability. All of these things determine overall performance, which is what I think you mean by power.
I'll let someone else present the math...
 
#4 ·
Well it's a little different with hydrostatic tractors. In a car, the engine, via the transmission and axles, is what spins the wheels. In a tractor, the engine spins the hydraulic pumps in the transmission, and the hydraulic pressure generated by the pumps (powered by the engine) is what moves the tractor.

So the engine matters, obviously. But what I want to know is how much it matters, and how its power is transferred. For example, you can get a 25hp lawn tractor at Lowe's. Its engine really makes 25hp, that's not a lie. But if you put it up against an 18hp Kubota subcompact tractor, that Kubota is going to wreck that lawn tractor in every respect, even though the lawn tractor has almost 50% more horsepower.
 
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#3 ·
Horsepower is a unit of work per time. More HP can either do more work in the same amount of time, or the same amount of work in less time.

IF,( and I say IF, because I am MAKING THESES NUMBERS UP!!!!) One horse can lift (think rope and pulley on barn beam) 2000lbs 1 foot in one minute. TWO horses will lift this same 2000lbs 1 foot in 1/2 minute. OR 4000lbs 1 foot in one minute. More horse power either can do the SAME amount of work (lift 200lbs 1 foot) in less time, Or do MORE work (lift 4000lbs 1foot) in the same amount of time.
 
#8 ·
I feel as far as homeowner type equipment goes (lawn tractors etc.) that horsepower has become more of a marketing tool than anything else.

When Joe homeowner goes to Lowe’s or Walmart to look at lawn tractors he will look at 2 different units - one with 20hp and one with 25hp. Well the 25hp one has to be better, right? He will never look at the build quality of the deck or anything else for that matter.

When a discussion like this comes up I always like to use an example like this....

Take a Deere model B. It has 12hp at the drawbar and 16hp at the belt. Look at the size/weight of that machine and all the work it can do.

Land vehicle Tractor Vehicle Agricultural machinery Plantation
 
#9 ·
HP is a mathematical figure based on torque x RPM / 5252. So if you increase either torque number or the RPM number, you make more HP.

12 HP in a riding mower engine turning 3600 RPM is quite a bit different than the 12 HP being produced by the 1200 RPM engine in the B. :good2: They both make 12 HP, same amount of work can be done, at least in theory. :mocking: But I would rather have the B.
 
#13 · (Edited)
Like...if you take two 20hp engines and hook one up to a powerful transmission, say a TuffTorq K90, and the other to a K46, will they put out the same power to the wheels? Or will the K90 give you more wheel hp? How does this work and why?
First off, as Zebrafive already pointed out, HP is a measure of work. Torque is a measure of force. Forces are static.

A tractor is a system. The transmission is part of that system whose job is to convert the rotational speed and torque (roughly speaking, torque x RPM = HP) from the engine into values usable at the axle or other power consuming device (e.g. PTO).

A 100% efficient transmission, which does not exist, would transfer all the input power to the wheels. In the real world, there are losses due to friction and thus HP at the wheels is always less then the input power. And every transmission is different in this regard.

You cannot take a 20HP petrol engine, connect it to the same transmission as a 20HP diesel engine and expect the same results. The two engines have vastly different torque and HP curves. The diesel tractor transmission is designed to accommodate the diesel engine. In theory, you could design and build a tractor transmission for the petrol engine that converted engine power into the same torque and speed to the axle as the diesel, but its usability as a system may be severely compromised.

I once built a mule-like vehicle from an old truck and a 1970 Fiat 124 coupe engine and transmission. Why? Because I had the parts. Did it work? In the sense that it moved under its' own power, yes. Did it work well? Hell no!

Al
 
#18 · (Edited)
Horsepower and torque are funny things
The other day I saw an Allis Chalmers steam engine 80 RPMs at 1100 horsepower
This is a 2 cylinder 180° opposing,
Look at the difference between a V8 and inline 6 with identical horsepower how the power stroke from each piston translates to the crank is a variable to throw into the equation
 
#19 ·
I remember reading that HP is equivalent to the amount of energy it takes to lift 500 lbs 1 foot in 1 second.
 
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#20 ·
So there is still one part of my question that remains unanswered. In an automotive transmission, gear reduction is mechanical. In a hydrostatic transmission, that reduction is not. It's not a matter of gears, right? It's a matter of the engine spinning up hydraulic pumps and the hydraulic pressure being what accomplishes that gear reduction. So that's where the inefficiency/loss is happening, I'm assuming. Is that right? If so, is there a rating available for how much power a given transmissions saps in this conversion process?
 
#21 ·
So there is still one part of my question that remains unanswered. In an automotive transmission, gear reduction is mechanical. In a hydrostatic transmission, that reduction is not. It's not a matter of gears, right? It's a matter of the engine spinning up hydraulic pumps and the hydraulic pressure being what accomplishes that gear reduction. So that's where the inefficiency/loss is happening, I'm assuming. Is that right?
Essentially, yes.

If so, is there a rating available for how much power a given transmissions saps in this conversion process?
There is.... somewhere! :laugh:

The problem with things like the CUTs is that Deere doesn't tell us what the max torque is that the transmissions on these things are capable of generating nor any sort of gear ratios. I just checked the service manual for mine again and nothing. No spec for highest gear ratio or torque output. There are probably engineers at Deere that know this sort of stuff but they don't seem to make it public. I've never seen numbers listed anywhere.

But there are a lot of guys that love tractors that have a clutch, a shifter and gears. They usually admit that the clutch is a pain sometimes but those machines get more of their power to the wheels.
 
#22 ·
There are inherent losses in a car's automatic transmission torque converter and I assume it's more or less a corralory to a hydro tractor. Those humongous ship diesel engines run I think at 100 rpm.
 
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#24 ·
Thanks everyone! This has been extremely informative!
 
#26 ·
On a hydrostatic transmission no gear ratios change as the pedal is depressed. As someone else said, the hyd pump for the hydro is a variable displacement pump. The more the pedal is pushed the more flow is created and sent to the hyd motor. The motor turns the trans input shaft. In the range box section of the trans is where the gear ratio is changed. Thats why you have a 2 or 3 or more speeds. Hi, low ect..
There is also a gear reduction at the final drive gears where the axles connect. It is a fixed ratio and can't be changed with a lever.
Also on the small compacts without a power reversor trans, there are no clutches except for the pto clutch. So.. No slippage. You will only have a pressure relief. Example would be.. You hook up to a large log on your draw bar and try to move it in high range. That noise you hear is not clutch slippage. It is the hydrostatic system going into relief. Much like dead heading a function on your loader.

Sent from my LGL52VL using Tapatalk
 
#27 ·
Ag Equipment has traditionally been rated with 3 separate HP ratings: Engine, PTO, and drawbar.

I'd love to see the drawbar HP ratings for these hydrostatic (s)CUTs. I think this would be useful information, especially when comparing to tractors of yesteryear (like the B Coaltrain mentioned).

Folks will argue that 'drawbar HP' is not relevant for a CUT. I disagree. Most of us have tried to pull (or push) something which caused the hydrostat to go into relief. This rating would be enlightening. For example, are all SCUTs similar in pulling/pushing power? How does a 3-series compare to the 2-series, etc?

Tim
 
#29 ·
The engine only makes enough HP to do the job. If it made more, it would speed up. The throttle is maintained by the governor to keep the engine at the set RPM by your throttle lever. So the trans will only use so much before the engine can’t give any more and it starts to slow down and bog.
 
#30 ·
Okay, so please help me understand this.

Let's say I have an engine rated at 25hp at 3400 RPM and a transmission whose maximum HP output is 15. If I run this engine in neutral/standstill at 3400 RPM, it is producing 25hp, or at least, potential hp. If you hooked it up to some mechanical device, it would put 25hp into that device, because that is what it is rated for at that RPM. Yes? No? If no, then what does that rating even mean?

If you then set your transmission to L or H, or just step on the pedal of a lawn/garden tractor, the engine doesn't then bog down. The RPMs stay the same.

So what's happening to the extra hp in that scenario? The engine is running at its rated RPM and supposedly putting out its rated HP at that RPM.

Also, if that transmission sapped 4hp due to inefficiency losses, does that mean a 19hp engine would perform exactly the same paired to it as a 30hp engine?
 
#32 ·
It may also help to think of a hydrostatic drive system more like an electric motor, or light.

If you have a rheostat switch, the more you turn the faster the motor spins.
Same with a light on a dimmer. All these do is limit the voltage to the motor or bulb.
Its the same in a hydro. The more you open the valve (by pressing the peddle), the faster the input pump spins. The flow is already available, just like the full 120v or 240v is available before the switch, its just not being used yet.
Or an air hose. Ive youve ever used a blow gun, the more you press the button and open the valve, the more air comes out.
 
#33 ·
Thanks! I greatly appreciate the time you took to answer.

I'll be in the corner processing this info for a while.
 
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