What Does Horsepower Get You?

[8ntruck 6,365]

In drag race terms, torque makes acceleration, horsepower makes top speed.

 

A Buick 455 will come off the line harder than a Buick 3.8L turbo V6, but the turbo car will likely have a higher speed through the traps.

 

In tractor terms, torque is more important for ground engaging chores, and horse power is more important for PTO work.

 

In engineering terms, torque is force times the radius of rotation.  Horsepower is a unit of work, which is force times distance divided by time.

[Pullstart 59,687]

6 hours ago, 8ntruck said:

In engineering terms, torque is force times the radius of rotation.  Horsepower is a unit of work, which is force times distance divided by time.

That completely backs up Dan’s explanation of stroke and a wrench length!  That increases the radius of rotation!

[8ntruck 6,365]

7 hours ago, pullstart said:

That completely backs up Dan’s explanation of stroke and a wrench length!  That increases the radius of rotation!

The light comes on!

[pfrederi 17,147]

More HP gets you a bigger gas bill....

[Howie 882]

And bragging rights!

[Greentored 3,201]

On 10/14/2020 at 9:54 PM, tom2p said:

 

 

 

can only imagine what the 520 is like 

 

They are an animal! Nearly impossible to load em hard enough to make em bark. 
I miss mine.....

[oilwell1415 563]

There is a lot of misinformation in this thread.  The simplest way to put it is that torque determines how much work you can do and power determines how fast you can get it done.

[pfrederi 17,147]

Horsepower brings you the beer...Torque gets the cap off....

 

 

 

 

[WVHillbilly520H 10,369]

10 hours ago, Greentored said:

They are an animal! Nearly impossible to load em hard enough to make em bark. 
I miss mine.....

Depends...

[wh500special 2,053]

On 10/14/2020 at 10:35 AM, 71_Bronco said:

... I have a Commando 8 that mows very nicely and pushes snow without issue. The 8HP seems to be plenty strong for what I need.

 

What would more HP get?...

 

These questions about horsepower and torque come up regularly.  Conceptually, the attributes of each are pretty simple to understand but the anecdotes and on-the-road automotive analogies sometimes cloud things.

 

Horsepower is a rate measurement.  As such, it is a way of expressing how fast work can be performed.  It is not a contrived unit like is sometimes implied when we say “you can’t measure horsepower, but you can measure torque.”

 

Higher horsepower translates to being able to do work faster.  So if you were able to max out the power of your engine by mowing grass really fast, you’d be able to run through the same patch of grass with the same deck twice as fast with a 16 hp engine than you can with an 8 hp engine.  If you’re mowing with a 36” deck and 8 hp acceptably on your lawn, you’d probably be able to use a 7.5’ deck under the same conditions if you upsized to a 20 hp engine.

 

A lot of times we like to say that torque is what gets the job done, but that’s not really the case if you care at all about how fast you can get work done.

Torque measures rotational force.  It is completely independent of any speed concerns.  In cases where max torque is generated at zero RPM it’s really not that beneficial of a statistic.  When you’re trying to break loose a big rusted nut with a monster breaker bar and it won’t budge you might be generating a heck of a lot of torque, but you’re not doing any measurable mechanical work.

 

Older engines seldom had torque specs presented, so they don't usually enter the conversation on these enthusiast sites.  Newer tractors have torque displayed since there was a controversy and lawsuit a decade or two ago that was a consequence of some manufacturers reporting horsepower values at speeds other than the rated speeds, so the values were sometimes inflated.  Publishing peak torque on the other hand doesn't require it be presented at rated speed.  Torque values became the default marketing statistic as a result.  it is a much less useful number than the power number.

 

Through speed manipulations (gearing!) we can multiply the available torque by decreasing the speed of the output shaft on the downstream side of a transmission.  Add enough gear reduction, and even a little k91 can tow a monster camper trailer up a steep mountain.  It will be slow, but it can eventually do it.  Hook the same camper to a 350 hp engine in a big pickup and it will also get it up the mountain, but has the option of doing it a lot faster because it can generate more horsepower.

 

In either case, the camper needs the same torque applied to the drive wheels to physically pull it up the hill but the big engine can turn them a heck of a lot faster. 

 

How does this translate to our little tractors? 

 

It only takes a fraction of a horsepower to move a tractor around your yard at the speeds we care about.  We can approximate it if we want…let’s say it takes a 25 lb push to push a Commando 8 tractor through the yard if you were just shoving on the back of the seat.  Let’s move it at 3 mph.  That calculates to 0.2 hp of actual power required to move it at constant speed.  Seriously, 1/5-hp.

 

The engine, if it was doing the job instead of your leg muscles, would see about the same power requirement.  That leaves a lot of power in reserve to do other things.  There are some efficiency losses through the transmission (that’s why it gets hot when it is being used), so the load on the engine is a little higher but still it’s pretty darn low.  Double the speed, double the power needed.  Deflate the tires and make it harder to propel, the power requirement increases.  Incline the ground so that the constant push won’t keep it moving at the same speed and if the engine can’t respond by generating more power - or your legs give out if you’re still pushing - the tractor comes to a stop and the horsepower goes to zero.

Horsepower and rate intertwined.

 

Clearly, the implements are where the power goes with snowblowers and mowers being the biggest power suckers.  Offhand I don’t know how to calculate their power requirements but it probably isn’t very complicated.  Very little power is used to propel the tractor.  Hydro tractors are constantly squishing thick fluid through small passageways so tend to use a little bit more power just keeping themselves primed for action, but even then it’s pretty small.   If you need to mow more and faster, you need a more powerful engine.  If not concerned with speed, then lower power will do it and you’ll chug around for more time.  Either way, you’ll have done the same amount of work when finished.

 

Steve

Edited October 16, 2020 by wh500special

[lynnmor 6,775]

1 hour ago, wh500special said:

 

A lot of times we like to say that torque is what gets the job done, but that’s not really the case if you care at all about how fast you can get work done.

 

Steve

Thanks for your explanation, hopefully your clear examples will be understood.

[AHS 1,411]

I grew up on a C101; that was hooked to a snow blade, a 36” mower, and garden work. That list of chores for the 10hp Kohler...not a problem!! Skip ahead a couple years... now I have a 00’ WH 312 with a 12.5  command horizontal drive. The flywheel of that 312 is puny compared to that of the 101!! I’ll bet that the 101 has more torque or the same as my 312. 
 

Just today I idled uphill with my Kohler 10hp carrying a 8x5 steel trailer loaded with kids...try doing that with an mtd/craftsman. The torque is where it’s at!

[ebinmaine 63,110]

5 minutes ago, AHS said:

The flywheel of that 312 is puny compared to that of the 101!! I’ll bet that the 101 has more torque or the same as my 312

Well now boys and girls I'm going to go ahead and add another question right into the mix here. I certainly understand what AHS is indicating but does he have more torque or does he actually have more INERTIA???

I'm not arguing the point, I'm asking the question to be defined.

[AHS 1,411]

18 minutes ago, ebinmaine said:

Well now boys and girls I'm going to go ahead and add another question right into the mix here. I certainly understand what AHS is indicating but does he have more torque or does he actually have more INERTIA???

I'm not arguing the point, I'm asking the question to be defined.

Well.. that’s a good point... ask the scientists!!😀

[wh500special 2,053]

The engine was generating pulling force (torque) and was moving it (horsepower) at some speed.  They go hand in hand.  Had you wanted to go faster it would have taken more horsepower but probably similar torque.  Go fast enough, and at some point I'm sure other factors start to compound the increased torque and hp requirements.

 

Heavy flywheels have more angular momentum to temporarily help carry the engine through an intermittent tough spot.  So they help smooth out the dips in power on the three piston cycles that aren't producing power.  However, their increased inertia also keeps the engine from revving up or responding as quickly in other circumstances.  Give and take.

 

If you think of the energy carried by the flywheel as a product of its rotational speed and its mass and diameter, you'll see why those super old hit and miss engines had such large, heavy flywheels.  They ran soooooo slow they needed a lot of help from the flywheel.  Smaller engines that run at much higher speeds - and those with more than one cylinder - don't need so much iron along for the ride.

 

You gotta pump the energy into the flywheel to be able to get it back out.  No free lunch.

 

Steve

Edited October 16, 2020 by wh500special

[ebinmaine 63,110]

15 minutes ago, wh500special said:

You gotta pump the energy into the flywheel to be able to get it back out.  No free lunch.

So does that indicate that a 10 horse older iron Kohler engine FEELS LIKE it has more torque than a modern 12.5 because it is using the torque multiplication of the heavy flywheel in a similar manner to dividing gears?

 

I do certainly realize I'm splitting hairs here but I'm just curious...

[SylvanLakeWH 24,129]

6 minutes ago, ebinmaine said:

So does that indicate that a 10 horse older iron Kohler engine FEELS LIKE ...

 

 I know, but...

My  C 105 10 hp is adequate... she does everything I want..

now what she “feels like” is a loaded question... .

 

I’ll never know cuz that’s kinda like you asking the Paint Department to pick up the pace on Colossus... 

 

 

[ebinmaine 63,110]

2 minutes ago, SylvanLakeWH said:

that’s kinda like you asking the Paint Department to pick up the pace on Colossus

I think we all know it's not The Paint Department that's holding up any progress

[AHS 1,411]

2 hours ago, ebinmaine said:

So does that indicate that a 10 horse older iron Kohler engine FEELS LIKE it has more torque than a modern 12.5 because it is using the torque multiplication of the heavy flywheel in a similar manner to dividing gears?

 

I do certainly realize I'm splitting hairs here but I'm just curious...

Yup. The heavy flywheel. Just seat to seat...they feel the same. Hey @ebinmaine, are you calling me fat?😀 with that inertia comment??😀

[ebinmaine 63,110]

2 minutes ago, AHS said:

Yup. The heavy flywheel. Just seat to seat...they feel the same. Hey @ebinmaine, are you calling me fat?😀 with that inertia comment??😀

I'll bet I out weigh you.....

 

[WheelhorseBob 1,548]

Wow some great explanations. For my pea brain, think of it like this, if your running a Snow thrower in light fluffy snow all is well. When it gets heavy and wet things tend to slow down. That’s when torque takes over. When resistance overtakes speed you need torque. Hope that makes a little sense.

[tom2p 2,393]

On 10/15/2020 at 3:52 PM, pfrederi said:

More HP gets you a bigger gas bill....

while I agree with this - more horsepower with a bigger deck and increased ground speed basically negates the fuel consumption increase for me 

 

I basically use the same amount of fuel to cut our grass with a 416-H and 48" deck as with a 312-8 with 42" deck 

 

bigger cut - and cut faster 

 

[wh500special 2,053]

7 hours ago, ebinmaine said:

So does that indicate that a 10 horse older iron Kohler engine FEELS LIKE it has more torque than a modern 12.5 because it is using the torque multiplication of the heavy flywheel in a similar manner to dividing gears?

 

I do certainly realize I'm splitting hairs here but I'm just curious...

I don’t know.   
 

i really wouldn’t expect the flywheel to compensate for more than a very short amount of time.   I’d think maybe a second or two at the most.  
 

While it’s possible one engine’s power drops off faster than the other as speed drops down the power curve I’d sure not expect the 10 horse to feel stronger than the 12.  I guess it is possible though.  
 

Are the older k241’s bigger in displacement than the Command 12’s?   Could be since technological progression has let us do more with less.  
 

steve 

[wh500special 2,053]

4 hours ago, WheelhorseBob said:

Wow some great explanations. For my pea brain, think of it like this, if your running a Snow thrower in light fluffy snow all is well. When it gets heavy and wet things tend to slow down. That’s when torque takes over. When resistance overtakes speed you need torque. Hope that makes a little sense.

I think you’re on to something, but I offer a slightly different interpretation.  
 

in the light snow you’re not using all the power the engine has to offer at the set rpm.   As the load increases from the wet snow at some point the engine can no longer process the snow at the same rate because the loading rate exceeds the available power, so the engine begins to bog down. 

Concomitantly the thrower impeller and forward speeds also slow down.  The rate of snow ingestion slows and eventually matches up to the available power of the engine at the reduced speed and everything reaches a new equilibrium of sorts.   When the load lessens, the engine speeds back up and so does the rate of work.  
 

so we’re really still relying on the power generation of the engine.  But with insufficient torque, the engine wouldn’t have been able to generate enough force to keep rotating.  So thinking of it as torque taking over seems reasonable too. 
 

steve

[ebinmaine 63,110]

Lots of Awesomeness in this thread....