HP really means nothing, it is the gearing that determines how much of the HP you can actually use. if you had a machine with 100 hp to the ground and tried to push/pull a plow it would only spin the rear tires due to lack of traction. so most are designed so that only really 5-10 hp are used for movement and the rest is partially to turn the PTO and partially just to "brag".

as far as kawa vs briggs vs kohler, the kawa is better that the briggs/kohler in box store machines because it is designed for a longer engine life. BUT keep in mind both briggs/kohler make engines designed for thousands of hours too, you just will pay more for them than their lower end lines.

yes diesels are turning slower, thus using less fuel. the pulleys/gearing systems make up for the lower RPM by bringing it up at the point of use.

 

Just give it normal care and you will be fine.

WAIT A MINUTE !! NORMAL CARE ?!?!? From what I have seen with the units I have been buying to refurb, normal care is do no maintenance at all :O

 

Haha yeah my normal care for my small engines has typically been "check oil, if it's good then pull string". The tractor cost a bit more than my previous devices though so I'll probably pay a bit more attention to it

 

... Some of the old 8 hp tractors seem to do almost as well as modern 20+ hp tractors, so what's the difference? .../QUOTE]

The difference is in how they are calculating Hp now. Using todays methods, those old 8Hp engines would most likely be rated as 14-16Hp.

The other thing that comes into play is gearing and weight. Weight helps determine how much traction you will have. Let's say that a tractor/gearing/tire combination will break traction at X Hp. You can put an engine in it with 3X Hp and it still won't pull any more that the original. The extra Hp is wasted. Now, add more weight to the tractor, and that extra Hp will come into play.

 

Just because a unit is rated at X horsepower, doesnt mean its making X amount all the time. It just means that it can make that much power. Think of it like a car, if you had a car thats rated at 300HP, its not making 300HP all the time. To maintain 55mph on the road, its probably only using 40hp. All engines are the same way. They need the perfect set of variables to make peak power.

Also, as mentioned previously, gearing plays a HUGE part.

 

How does the overall hp factor into the tractor anyway? If the transmission is a hydro, then transmission power is pretty much independant of engine power isn't it? Some of the old 8 hp tractors seem to do almost as well as modern 20+ hp tractors, so what's the difference?

And why is it that when you get to the higher end you end up with diesel engines? I thought the point of a diesel engine was to have more power at a lower rpm (1800rpm vs 3000 rpm), so do they run the same as a regular gas tractor in that they spin at the same rpm regardless of how fast you go? Or do they have some other mechanism for driving the mower blades and transmission?

There's so much I don't know...

The old 8 hp LTs and GTs weighed considerably more than the new crop of LTs and YTs and more than the entry level GTs as well. Since weight equals traction, they could pull more 'off the showroom floor' than any new tractor of equal or less weight, no matter what size engine is on the new tractor. They also had very robust transaxles, be it gear drive or hydro, unlike todays versions.

Only the heaviest GTs have hydros capable of delivering over 10 hp to the ground, and they usually break traction before hitting that horsepower limit. Most LTs and YTs have hydros that can only deliver about 3 hp to the ground. All the rest of the horsepower is for implements, accessories, and bragging rights.

Diesels are more efficient than gas engines. They will typically burn less than 2/3 the fuel doing the same task as a gas engine, are built considerably heavier, and have a longer service life. They also produce more torque than a similar hp gas engine.

Few implements can run straight off at max tractor engine rpm. The PTO must include a reduction ratio to get down to the proper rpm range. The slower turning diesels don't need quite as much reduction to get to that range. The same goes for the final drive ratio in the axle.

 

That is interesting, so now I'm going to feel like I have all this power and I need to find a use for it.

 

You actually can't measure HP. You CAN measure torque and this is converted to HP. Due to the manipulation of HP ratings you now see some motors with torque ratings instead of hp.

It's too complex to put it all down into words but the interaction of the governor means that most [ all ? ] lawnmower motors never come close to putting out their rated HP because cutting grass can't put enough load on the motor for the governor to fully open the throttle butterfly in the carb. Most motors probably never have the butterfly open even 1/2 way. The carb on my JD GT245 actually has a limiter built into the butterfly shaft that won't allow it to phyiscally open even 1/2 way.

 

You actually can't measure HP. You CAN measure torque and this is converted to HP. Due to the manipulation of HP ratings you now see some motors with torque ratings instead of hp.

It's too complex to put it all down into words but the interaction of the governor means that most [ all ? ] lawnmower motors never come close to putting out their rated HP because cutting grass can't put enough load on the motor for the governor to fully open the throttle butterfly in the carb. Most motors probably never have the butterfly open even 1/2 way. The carb on my JD GT245 actually has a limiter built into the butterfly shaft that won't allow it to phyiscally open even 1/2 way.

You actually can't measure HP. ????

Dont tell my dyno that...

 

engines are also rated by their manufacturer, not the tractor manufacturer. so Briggs/Kohler/Kawa/etc.. may say it makes X HP at 4000 RPM, but the tractor manufacturer governs it at 3000 RPM which is way below the peak power.

 

That depends on what grass you're cutting I've choked a 70HP diesel dead cutting grass of course it was about 30" tall,extremely thick and I was cutting with a 10ft wide bushhog.I push my Simplicity Sunstar that I cut between my garden plots to the limit all the time.What engine is personal perference I'd just as soon have a 16HP single cylinder Kohler as any garden tractor engine I've erver run.

 

Just to add to the confusion horse power is a combination of torque (rotational force) at speed (typically revolutions per minute) sort of like how amps+volts=watts. Pretty much all small engines spin at a max of 3600 RPM's and have a varying degree of max torque based on the size of the engine.
What you should really look at is displacement rather then an arbitrary hp rating published by the engines own manufacturer. Other things like flywheels, compression rates and ignition timing will affect hp too. But the rule of thumb I follow is the older and bigger an engine is the more hp it will have.

The reason diesels have an advantage in slow moving tractors and trucks is because they make the same horsepower with more torque at a slower speed then a comparable gasoline engine so you don't have to have as much gearing so you don't lose as much power spinning extra gears and whatnot so you can get more of the engines power to the ground.

I hope that made sense.

 

So coming from the car world I know the value of good torque, pretty much anytime you're trying to get something moving having more torque is better. I also see that when it comes to hydro trannies on the lawn/garden tractors, that's where your torque comes from to get moving and most of the ratings are fairly simple to find. Are those torque ratings they give for the hydro trannies fairly accurate when compared to car ratings?

Would a better way to think about it be to just think of any ground work in terms of transmission torque while running the mower deck and accessories would depend more on the engine power?

I've mowed through some fairly thick stuff with mine (3-4 foot tall buffalo grass fields) and didn't have any issues. The only time I had any issues with powering the blades was after I let my friend drive it at night and he ran over a mesquite tree stump.

 

My Dyno is for HP.
It does the toque and speed at the same time to give HP.

 

Not to hijack the thread, but I am intriqued by your dyno.:howdy: Can you make a new thread on it?????:thThumbsU

 

Saying HP can't be measured to me is like saying square footage can't be measured as you'd need to multiply the widthXthe length to get sq.ft..That IS how one measures it.

 

Not quite a square foot is a 12' x 12' area. A Horsepower is an arbitrary unit loosly based on the work some generic horse can do. Earlier someone mentioned it was an advertising gimmick.
From wikipedia:
The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses.

 

The manufacturers have been playing all kinds of games with HP ratings in recent years. Resulting in nonrepeatable numbers in the real world. There have been lawsuits over it.

Most engines now have overhead valves. Which make power at different rpm ranges than the older flatheads. They are also smaller in displacement than the older flatheads. That's important. If I remember correctly the rebuilder told me my old M18 Kohler flathead is around 45 ci. It is a torque monster for 18 hp. It is also a bit of a gas hog. Just like old school big block car engines.

In the same way the newer engines use less fuel and make more hp for a given displacement and they run cleaner at the exhaust. But the cost is low rpm torque. So they use a bigger hp engine to get the same work done.

Just like every other product on the market it's about bigger and more features is better for most folks. Bragging rights count. Tim Allen lives and my tractor is 32hp and yours is only 27.

Bottom line is if it gets the work done enjoy it! Personally I like old iron. The sound of a big flat head Kohler twin putting it's shoulder to the work is music to my ears!

 

I thought that you can measure horsepower but its not something tangible that you can feel when operating a machine?

 

One of the games engine manufacturers play is rating engine HP at 5500 RPM.
24 HP at 5,500 RPM's works out to 23.8 lbs/ft torque which translates to about 16 HP at governed speed of 3,600 RPM's

 

If you've ever tractor pulled,run a large bush hog,chisel plowed,drag raced or other power demanding job then you'd know that Horsepower isn't some abstract but a real enity
that 'lives' in an engine.My explaination of HP and Torgue when tractor pulling is Torque will get you thru the hard spot but HP will take you out the end of the track.Nebraska Tractor tests did a great job of measuring HP and I've found their comparative figures between tractors to be accurate over the years as I owned some of the tractors that were tested.Where testing can have issues is alot of things affect engine performance
and how much HP they put out.A few are air flow,air temperature,relativity humidity,Altitude(the higher up the less dense the air) fuel especially gasoline varies and so does the resulting HP it'll put out.HP an engine puts out can change due to all these and other forces so a test one day won't necessarily be exactly what it is another day,its not the test is wrong its that the engine is putting out different HP each day due to conditions.When tractor pulling I'd do all sorts of things to compensate for external conditions to increase my HP on a given day that I'd never bother with when mowing the grass or cutting hay because they weren't needed or worth the time and effort.

 

good discussion! Didn’t realize there were so many gear heads here, not sure why im surprised though. i throw my hat into the HP is a derivative of TQ when talking about motors. dynos measure TQ and do the equations to come up with a HP number.

 

I think the thing that throws me off the most is that the driving power (power to the wheels) is based of the transmission, which is pretty much independant of engine power. It just seems weird that a tractor can have only 25 hp but more torque than my car that has a v8.

 

A HP rating is important on an engine that runs at varied RPMs, such as an automobile. For tractor operation where much work is done at a specific RPM, torque at that RPM is far more important. However, for a lawnmower which needs to run at WOT to maintain blade speed it doesn't matter since on most modern engines 3600 RPM is the peak speed, the mowing speed, and at the top of the torque curve.
Not all engines are like that and tractors often run at much less than full throttle. HP is always listed as maximum developed - usually at WOT - which may not be in the range you're actually going to be using.

If I'm doing work at a set RPM, the engine which develops more torque at that RPM is the more powerful engine, regardless of the peak HP rating.

That is one of the reasons we are seeing less HP ratings on engines because of the lawsuit. For example, Briggs now lists "Gross HP*" with an asterisk, poke around in the fine print long enough and you'll discover that if the engine has an exhaust sytem and a muffle (DOH!) then you have to look for net HP, which isn't listed. Got a 23HP Vanguard engine? If there's a muffler on it it's not 23HP anymore, even before it does any work - they don't tell you that (I'm not picking on Briggs, just using them as an example, other mfgrs do it too). Anyway, we are already seeing less and less HP ratings so get used to it.

Torque measurements are made at the point at which rotation stops from a given RPM, so there is motion required since you can't stop something that isn't moving.

 

Torque measurements are made at the point at which rotation stops from a given RPM, so there is motion required since you can't stop something that isn't moving.

If motion was required, there would be a time factor added to the value. Torque calculations for a hydraulic motor include displacement and pressure, there is no time involved. Horsepower calculations include pressure and flow in gallons per minute.

Torque gets it moving, horsepower keeps it moving. Lose the horsepower and it stops, but the torque does not disappear unless the pressure drops to zero.

If you stop the drive wheels of a gear drive tractor without declutching the engine, the engine dies because it isn't capable of producing enough horsepower to keep the wheels turning. Stop the rear wheels of a hydro and the relief valve pops, but the torque is still there and the engine keeps running because it is making enough horsepower to push the fluid over the relief valve.

If there was no relief valve, the pressure, and therefore the torque, would increase until either the load moved or the rear end failed. It is unlikely that the engine would die first if the operator compensated for the higher torque by reducing the flow and therefore the horsepower demand.

 

Interesting, but I'm not surprised at the way they do ratings. It's been the same way for cars for years, which is why a lot of people preferred to see numbers from a chassis dyno versus an engine dyno.

That brings up another point about riding mower engines though in that I wish I could find a way to make it quieter without breathing exhaust or sacrificing power.

 

If you go through the multiplications with gear ratios in a car and tractor, it's not quite as impressive. The GTs have a lot more reduction. Theoretical speed of my van at 3600 rpm is 108 mph, for my MF1655 it's 9 mph with a slightly smaller diameter tire (for even more reduction). That is a factor of 12 times the gear reduction of the van. The hydro max torque is 13.3 ft-lb times 12 equals 160 ft-lb compared to the approximately 300 ft-lb of torque for the V-8 at 3600 rpm.

Before anyone jumps at these numbers, I'm comparing engine speed to ground speed which takes all gear reduction into account and am working only with the difference in speed for torque comparisons, and I do not know the specific torque curve numbers for my 5.7 l. Chevy. I'm making an educated guess that may be off by 5% or so. I have the charts in hand for the hydro and am rounding down to the nearest 10. The actual max torque at the rear wheels is considerably higher for both. The van would have approximately 855 ft-lb of torque and the tractor 396 ft-lb.

The big difference is that the hydro can deliver that level of torque at a bare creep at idle. The truck can't. To top it off, that tractor has a Hi/Lo shifter that gives an extra 2:1 reduction for a top speed of 4.5 mph and of course multiplies the torque by the same factor to almost 800 ft-lb.

The other thing with a hydro, engine output is irrelevant once horsepower requirements are met. For a Sundstrand Series 15 hydro, that's 14 hp. Anything more is unuseable except for accelleration. It won't pull any harder, or go any faster, but it will get to its top speed a bit quicker. By comparison, a K-46 in a LT needs about 4 hp for max performance.

Go figure, they put 20+ hp engines in front of K-46s and only put 14 - 20 hp engines in front of the Sundstrands.

 

Engines don't necessarily reach their top HP at maximum RPM,this is where the camshaft comes into play as valve duration is a big part of the equasion.When building a tractor puller a good cam guy will always want to know at what RPM will the tractor be pulled as speed and RPM are limited by pulling rules sometime.If cam is ground to achieve maximum pull at 2400 RPM it'll run 3000 but once under load it'll drop back to around 2400 and stay there even when more load is added.
And with farm tractors it varies greatly on how much engines will pull at a given RPM as 2 different engines might be equal at maximum throttle but at 1/2 throttle one might way out pull the other.Timing is another factor thats critical and could explain why some single cylinder engines vary in HP as they are massed produced and just a very slight variance in where the keyway for the flywheel is cut would change engine output.

 

The cam would be ground for peak torque. Peak torque happens at peak volumetric efficiency. Peak horsepower will happen when the engine has lost enough of its volumetric efficiency that increased rpms no longer increase power output. Ultimately hp is arbitrary unless being used in a constant load environment, vehicles do not provide this.

 

Allright, horsepower is calculated. Any concept which includes 2 or more units of measurement is calculated.

Weight is measured in pounds.
Distance is measured in inches or feet.
Time is measured in seconds or minutes.

When these units of measurement are combined into one concept, calculations are required.

Horsepower uses 3 units of measurement, weight, distance, and time.

Torque uses 2 units of measurement, weight and distance.

They are both calculated. One shows force in motion and the other shows force potential.

The only 2 formulae that I noticed in the wikipedia articles that included both torque and horsepower included a constant. In both cases, one of the figures used to generate the constant was "33,000 ft-lbf/min", the definition of a horsepower.

Torque is necessary to accellerate a load, horsepower maintains the speed. This concept was pointed out to me several decades ago.