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Why rate engines by horsepower?

Discussion in 'Tractors' started by palmtree, May 23, 2005.

  1. palmtree

    palmtree LawnSite Member
    Posts: 13

    I am confused. Why do they even give horsepower ratings on mowers? Torque must be more important. If it isn't torque then it is something else. I have been mowing with a Craftsman garden tractor for the past 5 years. I don't like it and I am buying a Grasshopper (that is beside the point). My Craftsman has a 22HP Kohler pro engine with 46 inch deck and it has always run fine--never let me down in 5 years. However, when I need to pick up leaves I use my Father-in-laws John Deere 455 with a 21HP diesel and 60 inch deck. Let me tell you it doesn't just have a little more power than my 22HP Kohler--it has an amazing amount more. And even with a much bigger deck. I guess it is torque that makes the difference. Even still, torque values can't be that different can they? Aren't horsepower numbers derived from torque values? Thanks for shedding light.
  2. Lumberjack

    Lumberjack LawnSite Member
    Posts: 180

    Torque changes as engine speed changes.... People generally understand horsepower more then torque specs....
  3. BCSteel

    BCSteel LawnSite Senior Member
    Posts: 876

    I you look at the spec sheets of Kawi, kohler etc. on numerous engines rated at different hp they have the same torque ratings, and same displacment. How is this possable? And why would you want to spend the extra $$ on a higher hp engine that has the same torque as a lower hp engine? BTW these are all gas carb'd engines.
  4. betterlawn

    betterlawn LawnSite Senior Member
    from MI
    Posts: 426

    Arguably the HP ratings are darn near useless, but its the way its been done for ~100 years. The engines are used on all sorts of applications (mowers, augers, salt spreaders, edgers, etc.) and the HP gives a good relative indication of the power available.

    The Torque/Speed curve is what most people who know what they are talking about are looking for. The HP test is calculated using an SAE spec, I don't remember the details. It doesn't surprise me that a 6.5HP and 5.5HP engine might have the same torque at a given speed. However, the 6.5HP would have higher torque at some speeds at least. Since lawnmowers don't transmissions, all the mower manufacturer can do is optimize the running speed. In the case of a tractor where you have a transmission - there a ton of variables that could affect the performance, making two identical (HP) engines behave completely differently.

    In short, its not like they are lying about the HP, its just that the HP (as you pointed out) doesn't really give you the full story.

    It would be kind of cool if the lawnmower manfacturers would come up with standard measurement for torque at max tip speed. Barring that, the HP should be a pretty good indicator.

    In case you didn't know, HP = Torque (ft-lbs)*Speed(RPM)/5252, so torque decreases as speed increases.

  5. YardPro

    YardPro LawnSite Gold Member
    Posts: 3,563

    horse power is defined as the force required to move 500lbs one foot in one second.
    it is a measure of work performed, like watts when dealing with electricity.

    torque is a measure of rotational force, it equates to amps in electrical systems
  6. Dirty Water

    Dirty Water LawnSite Fanatic
    Posts: 6,799

    You have to remember the a diesel will make peak torque at a very low RPM, so its HP rating will be decievingly low while the actual power (remember HP is just a measure of torque) will be high.

    Comparing a gas engine and a diesel engine is pretty much apples and oranges.

    Wonder why mini excavators can have 25 HP and pick up 2 ton loads?
  7. YardPro

    YardPro LawnSite Gold Member
    Posts: 3,563

    horsepower has nothing to do with torque

    torque is rotational force.. that's it

    horsepower is a measure of work performed. one horsepower is the force required to lift 550 lbs one foot in one second..

    mini excavators can pick up 2 ton loads becuase of hydrualics, not becuase of horsepower or torque...

    torque only becomes useful after run through gearing.
    a motor with 500 ftlbs is not much good if it only spins at 100 rpm. imagine the gearing required to spin blades, or hydro pumps ar requires speeds... since horsepower is a measure of ourput per time (has a speed componet) is can be very useful.

    the two really need to be measures together to get an accurate picture of engine performance.
  8. Dirty Water

    Dirty Water LawnSite Fanatic
    Posts: 6,799

    I disagree:

    HP = Torque * RPM) / 5252

    Obviously they are directly related.

    I think thats fairly obvious, the point was that the extra torque from the diesal engine is able to power those hydrualic pumps despite having less HP than your lawnmower :D

    Again I disagree. Have you ever driven a nice diesal truck? Its really nice being able to drive 20 mph through town at 500 rpm in 4th gear (Which as we all know, is usually a 1:1 ratio), and still have tons of power.

    Apples to Oranges. You don't see low RPM engines in dragsters for a reasion, nor do you see low RPM engines running lawnmowers.

    The massive diesals that power freighter ships only turn at a 100 rpm...I'm pretty sure they put out hundreds of thousands of pounds of torque...Still want to call them not much good?
  9. xcopterdoc

    xcopterdoc LawnSite Senior Member
    Posts: 752

    rate them in KW.. that would solve everything!
  10. YardPro

    YardPro LawnSite Gold Member
    Posts: 3,563

    pick up a physics book and read it. or even a dictionary.

    the torque has absolutely nothing to do with horsepower. torque is the measure of rotational force. It has no optput (power per time) componet.

    horsepower is the measure of work performed and has a time componet....

    horsepower does have an affect on torque, but not the other way around.

    torque is one of the componets used to measure HP, but HP is not used to measure torque.

    the ship torque is not a legit comaparison ( also over a million ft lbs of torque). as i stated earlier the low rpm's need massive gearing. those ships
    gear the prop shafts. and the props are also highly pitched.

    this is why diesels are not used in most dragsters - super heavy, need massive gearboxes to get the output rpm's where they need to be.

    i am not arguing about gas/vs diesel.... i am a big diesel fan. the only way to go on work vehicles, etc.....

    i am just trying to educate on the hp/vs. torque argument...

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