Horse Power question

Discussion in 'Lawn Mowing' started by gonna.cut.grass, Mar 24, 2005.

  1. gonna.cut.grass

    gonna.cut.grass LawnSite Member
    Messages: 14

    My wondering has led me to this post- Seems like everyone wants more HP on their machine.
    Does that mean the lower HP machines performs less than the higher HP machines? in what noticable ways ?

    for instance, one Scag Cub comes with a selection of four engines: 19 Kawa, 20 Koh, 21 Kawa, 23 Kawa, 24 Hond.
    Would riding the 19 feel like a 4 cyl car in comparison to the 24 HP?
    I must admit - I dont have any ZTR riding experience.
    so I have to assume there is a lot I dont know yet. But at this point I am trying to make sense out of this using engineering logic.
  2. Lawn Wolf

    Lawn Wolf LawnSite Member
    from 77090
    Messages: 116

    More hp less RPM's to operate the engine . The less hp more RPM to cut the same amount of grass.In other words the mower with the bigger engine spins less in revolution vs. a smaller engine that spins faster for the same work load.
  3. Precision

    Precision LawnSite Silver Member
    Messages: 2,995

    Generally speaking, more horsepower is better. But more horsepower really only helps to power through thick tough grass or situations where you are cutting overgrowth.

    This or course assumes the mower is properly powered to start with.
    Example my Gravely PM 260 has a 27 HP engine. It was offered with a 25 hp or a 28HP liquid cooled. The mower is about 1150#. The 25 HP would do just fine in 80% of my applications. Weekly mowing situations would be fine. For an extra $300 I went with the 27HP because in spring growth the 2 HP will make a difference and I have one property where I mow a 5 acre ditch once a month and that 2 hp is a godsend. Why not the 28HP liquid. That was yet again $900 more. Not worth it.

    Down side. The 25HP and 27 HP have the same block, piston and everything else, so it means they are running slightly higher compression, higher spark and the like. So long story short, the 27 hp is working the same components harder so it will potentially lower the lifespan. Also it will drink more gas.

    The other thing to realize is that horsepower really isn't what we need in a mower. Torque is much more important. Torque is power generated at low RPM and since our mowers don't run over 4000 rpm or move faster than 10-12 MPH that is the much more important number. Also torque is a cyclical description of energy where horsepower is a linear description of energy. And what we are more concerned about is spinning the blade which requires torque, obviously low end and a flat torque curve are more important than a high peak horsepower. So you might be better off with a 19 HP Kawa over a 20 HP Kohler depending on the torque curves.

    Hope that helps.
  4. Mark McC

    Mark McC LawnSite Bronze Member
    Messages: 1,565

    I was under the impression that the manufacturers set most of their engines for this kind of application to run at 3,600 revs per minute. If so, I'm not sure your explanation washes.

    I believe that a lower horsepower/torque engine will not last as long as one with more horsepower/torque, but my impression is that the determining factor is reserve capacity. An engine that is always running near capacity will burn out faster than one that rarely does so...or that's what I always thought, anyway.
  5. barnard

    barnard LawnSite Senior Member
    Messages: 618

    You are exactly right.
  6. MMLawn

    MMLawn LawnSite Gold Member
    Messages: 3,569

    That's right. Commercial mowers in all HP ranges the RPM's are set at the factory between 3600 and 3750 depending on the maker.
  7. Evergreenpros

    Evergreenpros LawnSite Bronze Member
    Messages: 1,154

    Isn't the 3600rpm limit just on the 21" mowers? I think it's an EPA regulation. And as far as lasting, like any engine 75% is how you take care of it.
  8. LB1234

    LB1234 LawnSite Gold Member
    Messages: 3,208

    Precision, that was an excellent post...very informative...thanks.
  9. rodfather

    rodfather LawnSite Fanatic
    Messages: 9,501

    Frankly, I wish the manufacturers would invest more $$$ in R&D on other aspects of the machines instead of just making the engines bigger and bigger in hp rating. The more the hp, the more fuel being consumed. Right?

    I rather see some new designs in baffling systems, discharging, operator comfort, db levels, etc.
  10. CuttingCrew

    CuttingCrew LawnSite Member
    Messages: 193

    I agree RodFather. Show me lower center of gravity for the hills in Cincinnati, better seats for comfortable long days and better deck designs for a cleaner cut! Re-engineer the thing from front to back using alloys to reduce the weight and you have a lighter, more efficient machine that leaves a better cut. That I will pay more money for. At that point with the reduced weight they can use a lower horse power engine.

    Back to the question. I never go with the lowest horse power offered. More power is always good in my area due to the hills and strong growth in the spring.


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