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Discussion Starter #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?
 

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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?
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...
 

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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.
 

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Discussion Starter #4
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...
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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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.
No argument there, but you can't asses a single system component independently since overall performance depends on everything in the power train. Lawn tractor and garden tractor transmissions are built for different uses and are generally paired with different tires. Adding more power to a lawn tractor doesn't make it competitive with garden tractors when torque or heat dissipation are important. The overall design is optimized for different uses. It would help to define the intended use. If you want to use cleated tires and pull ground contact implements, you need a certain class of transmission. You can then compare various engines and horsepower ratings for that application. Otherwise, there are too many variables to know what the actual engine contribution is. (Some of the engine power goes to the mower deck as well so deck size also comes into play.) Too much horsepower with a K46 might actually be a bad thing if it increases heat buildup.
 

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Traction and torque

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.
Two other items come into play in that example. One, the two engines are very likely horsepower rated at different RPMs. Since horsepower is calculated by torque x RPM/a constant the faster the engine turns the more horsepower. It's likely the 25 hp Briggs would have been rated at 3,600 rpm while the Kubota may have been rated at 2,400 rpm. The Kubota could very well have more torque at rated rpm than the Briggs at it's rated rpm but the horsepower is different. https://www.briggsandstratton.com/na/en_us/support/faqs/browse/engine-horsepower-or-torque-value.html

Since torque is what moves things, the Kubota will outpull the lawn tractor either at the wheels or in a pto/blades. The other issue of course is traction. The subcompact tractor will generally have a lot more traction on the ground even without 4 wd.

Treefarmer
 

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Since torque is what moves things, the Kubota will outpull the lawn tractor either at the wheels or in a pto/blades. The other issue of course is traction. The subcompact tractor will generally have a lot more traction on the ground even without 4 wd.

Treefarmer
Good point. Sometimes advertised numbers are a distraction. Many pressure washer ads focus on the engine, when almost all failures occur in the pump. Sears used to brag about "max developed horsepower" -- which was only possible in the moments right before the motor burned up. On my X350, the tires spin long before I run out of power. Horsepower is an easy selling point, but beyond a certain threshold, differences may mean very little for the tasks you want to perform. These discussions set dealers apart from resellers. Good dealers know the system performance and you can make direct comparisons there.
 

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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.

26B10E2A-CB19-434B-85AD-F1D2A1F769EC.jpg
 

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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.
 

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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.
Keep in mind here that not ALL of the power from the engine is going to the hydro transmission. Some of it is for the PTO as well.

That said, the internal workings of the transmission is what decides how much of the power going into the transmission gets turned into torque applied to the axles. That tranny has the hydro pump, gears and clutches. Each one effects that path from input to output.

The K46 is only rated to a max of about 170 ft lbs of torque so not matter how much HP goes in, when it gets to 170 ft lbs of torque being applied to the axle shafts, the internal hydro systems are going to start going into bypass or the clutches are going to start slipping. Any additional HP after that point is wasted.

If you do the math and you run the K46 with input at 3600 RPM, it's internal gearing is roughly 28:1 and with a max generated torque of 170 ft lb. You end up with a figure of ~4.2 HP. So applying anything more than 4.2 HP to the K46 won't get the tractor to do anything more. It's max'd out.

If you look at the K90/K92 and do the same math, it's hydro pump, clutches and internal gears are capable of generating ~650 ft lbs of torque to the axles so you end up maxing it out with ~15.8 HP to K90's input.
 

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The K46 is only rated to a max of about 170 ft lbs of torque so not matter how much HP goes in, when it gets to 170 ft lbs of torque being applied to the axle shafts, the internal hydro systems are going to start going into bypass or the clutches are going to start slipping. Any additional HP after that point is wasted.

If you do the math and you run the K46 with input at 3600 RPM, it's internal gearing is roughly 28:1 and with a max generated torque of 170 ft lb. You end up with a figure of ~4.2 HP. So applying anything more than 4.2 HP to the K46 won't get the tractor to do anything more. It's max'd out.

If you look at the K90/K92 and do the same math, it's hydro pump, clutches and internal gears are capable of generating ~650 ft lbs of torque to the axles so you end up maxing it out with ~15.8 HP to K90's input.
That's assuming 100% efficiency as well. We all know hydrostats are not very efficient. Lots of power lost to heat and pumping losses.
 

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That's assuming 100% efficiency as well. We all know hydrostats are not very efficient. Lots of power lost to heat and pumping losses.

True dat! I have no idea what the efficiency ratings for any of these are but that would have to be figured in.
 

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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
 

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Discussion Starter #14 (Edited)
Keep in mind here that not ALL of the power from the engine is going to the hydro transmission. Some of it is for the PTO as well.

That said, the internal workings of the transmission is what decides how much of the power going into the transmission gets turned into torque applied to the axles. That tranny has the hydro pump, gears and clutches. Each one effects that path from input to output.

The K46 is only rated to a max of about 170 ft lbs of torque so not matter how much HP goes in, when it gets to 170 ft lbs of torque being applied to the axle shafts, the internal hydro systems are going to start going into bypass or the clutches are going to start slipping. Any additional HP after that point is wasted.

If you do the math and you run the K46 with input at 3600 RPM, it's internal gearing is roughly 28:1 and with a max generated torque of 170 ft lb. You end up with a figure of ~4.2 HP. So applying anything more than 4.2 HP to the K46 won't get the tractor to do anything more. It's max'd out.

If you look at the K90/K92 and do the same math, it's hydro pump, clutches and internal gears are capable of generating ~650 ft lbs of torque to the axles so you end up maxing it out with ~15.8 HP to K90's input.
This is exactly what I was looking for! Thank you!

Can you elaborate on this? Also, the axle torque sounds high. 170 ft lbs of torque is quite a lot. How is axle torque different from engine torque (which is rated much lower for lawn, garden and utility tractors). So like a 1025R has something like 45ft lbs of torque (don't remember the exact rating), while a K46 can handle 170 ft lbs? Is the axle torque usually much higher than engine torque?

Also, how do you get the max hp number for a given transmission?
 

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This is exactly what I was looking for! Thank you!

Can you elaborate on this? Also, the axle torque sounds high. 170 ft lbs of torque is quite a lot. How is axle torque different from engine torque (which is rated much lower for lawn, garden and utility tractors). So like a 1025R has something like 45ft lbs of torque (don't remember the exact rating), while a K46 can handle 170 ft lbs? Is the axle torque usually much higher than engine torque?

Also, how do you get the max hp number for a given transmission?
Engine hp is a function of torque and rpm...that’s it.
You can have 2 engines of the same hp, and one has lots of torque and runs at a low rpm, the other low torque and rev’s like a Japanese motorcycle.

You have to look at a transmission, hydrostatic or otherwise, as a variable gearbox. Hp in equals hp out (minus efficiencies of course). They can convert rpm’s to torque and vice versa.
An important aspect of an engine is the torque curve... as in how the torque changes with rpm...the flatter the better usually is what you want.
 

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This is exactly what I was looking for! Thank you!

Can you elaborate on this? Also, the axle torque sounds high. 170 ft lbs of torque is quite a lot. How is axle torque different from engine torque (which is rated much lower for lawn, garden and utility tractors). So like a 1025R has something like 45ft lbs of torque (don't remember the exact rating), while a K46 can handle 170 ft lbs? Is the axle torque usually much higher than engine torque?


It's a simple mathematical conversion based on the transmissions gear ratios.

As others have said HP = torque and RPM. If you have a transmission with a 4:1 gear ratio, it's going to reduce the RPMs by a factor 4 and increase the torque by a factor of 4.

If you have a 20 HP engine running at 3600 RPM, it puts out ~29 ft lbs of torque.

If I run that through a 28:1 gear ratio transmission (in perfect conditions!) then I divide the 3600 engine RPM by 28 and get 124 RPM on the output (i.e. the axles).

In a perfect transmission you don't lose anything so 20 HP going in has to equal 20 HP going out. If I divide RPM by 28 basic algebra says you multiply torque to balance things out. So I multiply engine torque of 29 ft lbs by 28 and get 812 ft lbs of torque at the output. 124 RPM and 812 ft lbs still gives you ~20 HP.

Also, how do you get the max hp number for a given transmission?
Ok, so we know if you have 3600 RPM going in and a 28:1 gear ratio you get 124 RPM coming out.

Torq Tuff's spec sheet lists the K46 with a max output torque of 171 ft lbs. So 124 RPM and 171 ft. lbs = ~4.2 HP. (I cheated on the math and used an on-line calculator!) So anything more than 4.2 HP going in is wasted.
 

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Discussion Starter #17
It's a simple mathematical conversion based on the transmissions gear ratios.

As others have said HP = torque and RPM. If you have a transmission with a 4:1 gear ratio, it's going to reduce the RPMs by a factor 4 and increase the torque by a factor of 4.

If you have a 20 HP engine running at 3600 RPM, it puts out ~29 ft lbs of torque.

If I run that through a 28:1 gear ratio transmission (in perfect conditions!) then I divide the 3600 engine RPM by 28 and get 124 RPM on the output (i.e. the axles).

In a perfect transmission you don't lose anything so 20 HP going in has to equal 20 HP going out. If I divide RPM by 28 basic algebra says you multiply torque to balance things out. So I multiply engine torque of 29 ft lbs by 28 and get 812 ft lbs of torque at the output. 124 RPM and 812 ft lbs still gives you ~20 HP.



Ok, so we know if you have 3600 RPM going in and a 28:1 gear ratio you get 124 RPM coming out.

Torq Tuff's spec sheet lists the K46 with a max output torque of 171 ft lbs. So 124 RPM and 171 ft. lbs = ~4.2 HP. (I cheated on the math and used an on-line calculator!) So anything more than 4.2 HP going in is wasted.
Words cannot describe how much I appreciate this information! This is exactly what I was looking for!! Thank you!
 

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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
 

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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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Discussion Starter #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?
 
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