Tractor Sizing What effect does HP have on residential lawn tractors?

[chim]

In the race car world we had a saying.. There is no replacement, for displacement !

Or, "You can beat cubic inches, but you can't beat cubic dollars"

 

[CobyRupert]

At the heart of OP’s question is when does a riding mower run out of power?
Mowing? People’s grass usually doesn’t get that high. If I’m mowing tall grass and weeds, I don’t usually have to keep speed down because of (lack of) power, IMHO, it’s because stalks need time to get chopped up and discharged (i.e horsepower doesn’t change max. blade speed. However, if you hear engine bog down, you can say horsepower is a factor.
Going up a hill? IMHO, I doubt many people mow that fast or have that steep of a hill that they run out of power.
Towing a cart? Yes, going up a hill with a loaded cart, horsepower will get up quicker, same with accelerating. On flat ground a smaller horsepower will accelerate to the same top speed as the big Hp, just at a slower rate.
I can’t think of anytime mowing with a “18hp” mower that I’ve run out of power. There’s been times snow blowing when rpms dip because of load, but again, like mowing, more often one has to slow because the volume of snow overwhelms the rate (at operating rpms) the blower (or mower) can process; not because you run out of power.
...the belts and pulleys are such that mowers and snowblowers can only process so much material at their operating rpms. When you overload, people will slow down because of this, not because of lack of power. Sort of self correcting.
And when you slow down, you don’t need additional power.

Quick physics lesson: say you want to climb a 100’ hill on a ten speed bike.
It takes the same amount of Energy (or work), no matter if you do it in 1 sec, 10 sec, 1 min, 1 hour, etc... , or use a 1 hp, 2 hp, or 10 hp motor.
Energy (or work) is a total. Takes the same to get up 100’ no matter what.
Horsepower is a rate (of energy) that you can do work.
(Say) at 1 horsepower it takes 2 minutes (120 sec) to climb; at 2 horsepower it takes 1 minute, at 10HP it takes 12 sec.
That’s horsepower.
Now looking closer at the 2Hp in 2 minutes climb. If motor can do it in high gear at 100 rpms, that’s 10x the torque of a 2Hp motor that runs at 1000 rpms for the same 2 min climb. That’s torque and gearing.

Realistically, it gets more complicated because motors put out different torque and horsepower at different rpms; gear ratios have limitations, etc.. Also a machine’s weight (and traction) usually mirror the HP (so a 20hp usually can’t pull the same load as a 200hp tractor if it goes 1/10th speed...but sometimes it can).

 

[4570Man]

All of the above would assume the the ratings are actually accurate which they probably aren’t. I think you’ll notice zero difference between the 2. There’s other way more important factors to compare.

 

[rScotty]

Why wouldn't you want the larger HP engine? Turn the throttle down and it becomes the smaller HP engine....but you can't go the other way.

 

[wheelbuilder]

I don't have data, but I would imagine that the lower hp engine would use slightly less fuel, up to a point. In a mower, how much height of grass, and terrain aren't probably too extreme. It would bee deck width as the variable, which effects how quickly you get done. As far as pulling a load up a hill, I think you'd run out of traction before power.

 

[RandyT]

The truth is horsepower sells mowers. People look at HP as a selling point and in most cases doesn't add anything to the performance. In the 70's you would find 42" decks with 10-12 hp engines on them, and a 16 hp was used with the 48" deck. Now people have 22 hp for 42" decks and 25-28 for 48" decks. Also when this emissions crap started the HP of pushmower and rider engines started going up. I remember when I was seeing 7.5 HP engines on 22 inch mowers, that just a few years earlier would of only had a 3.75-4 HP engine. And I still can't believe that an old non emissions "dirty" running 3.5 HP engine put out any more emissions than the newer 7.5 HP engine they are now using.

 

[tomplum]

The truth is horsepower sells mowers. People look at HP as a selling point and in most cases doesn't add anything to the performance. In the 70's you would find 42" decks with 10-12 hp engines on them, and a 16 hp was used with the 48" deck. Now people have 22 hp for 42" decks and 25-28 for 48" decks. Also when this emissions crap started the HP of pushmower and rider engines started going up. I remember when I was seeing 7.5 HP engines on 22 inch mowers, that just a few years earlier would of only had a 3.75-4 HP engine. And I still can't believe that an old non emissions "dirty" running 3.5 HP engine put out any more emissions than the newer 7.5 HP engine they are now using.

Very true indeed.
Old, old, old riders @ 5HP

 

[tomplum]

Yet my 1978 Wheel Horse with a single cylinder Kohler (10 HP K241as) still runs like a top to this day.
Been a very reliable, trouble free engine.

You bet. That's what Kohler built a reputation on, the K series. Then the M series. Then honed their skills when they built the Command. Then came the bucket engine.....

 

[DK35vince]

Very true indeed.
Old, old, old riders @ 5HP

True, and look how slow they mowed.
I still have a 10 HP Wheel Horse garden tractor with 36" deck. I mowed with it for years, but fast it certainly wasn't. (would have taken an entire day or more to mow what I can mow now with my 35HP, 72" zero turn in 2 hours)
My first zero turn was a 21HP, 54" deck. It would mow at speeds up to 8MPH.
But in the spring/early summer when the grass was growing fast I would often need to slow down due to running out of power.
My next zero turn I didn't want to be slowing down all the time due to running low on power.
Went with the 35HP with 72" deck. (top speed 14MPH).
I wouldn't want any less power. Spring/early summer mowing once per week, the grass grows so fast here I use all the power this thing has mowing at full speed (14 MPH) with a 72" deck.
But its a mowing machine. I've never owned or used any mower that mows as fast. Mows my 8 acre lawn in 2 hours.

 

[RalphVa]

I've had about that kind of hp difference between the 4010 (18.5) and 2025R (24.5). Main differences is 1) run the 2025R at 2500 rpm vs. the "PTO" 3,100 because its 24.5 hp isn't needed; 2) can start the bush hog at 1300 rpm on the 2025R vs. about 2,000 on the 4010 without stalling.

Otherwise, cut grass is the same.

Ralph