How to compare mowers rated in HP vs ft-lb

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In trying to make a buying decision for a new lawn mower I got stumped when I finally realized (it took me 2-3 days) that the rating system had changed.

How can you compare older mowers (before mid-2007), that are rated in HP, to the newer ones (mid-2007 onward), that are rated in ft-lbs of torque, or even worse only give you size in cc's? How do you make a decision on what to buy?

I have decided to summarize the pertinent information I gathered from my research in a very concise way so any consumer can have a starting point for comparison in a SIMPLE way.

First, if you have an "older mower" you need to know its effective or "real" HP based on its operating RPM.

Example:
Want to know the actual HP of your older mower?
Older mower is 6 HP. Multiply by 0.85 = 5.1 HP ("Real" HP)

0.85 = 3060/3600 (to convert from tested RPM to actual RPM)

Too hard to calculate in your head on the spot?
Try multiplying by 0.8, then add 0.3 to your answer. This is a close approximation.

Want to know the torque of your older mower?
First, make sure you know the "real" HP of your motor (as above).
Then use the "real" HP as follows:
Older mower is 5.1 HP. Divide by 0.6 (or multiply by 10 and divide by 6) = 8.5 ft-lb.

Want to know the HP of a newer mower?
Newer mower is 7 ft-lb torque. Multiply by 0.6 = 4.2 HP

Remember, this is only to give you simple way to start to be able to do a comparison but it is based on several ASSUMPTIONS. They are not important, just to keep it SIMPLE. Even the "real" HP is NOT the real HP. But that is too much detail to cover here.

If you can AVOID IT, don't bother trying to compare size or displacement of a motor shown in cc's for the simple example that even B & S has the same 190 cc motor that is sold as a 6.75 ft-lb torque model all the way to an 8.5 ft-lb torque model. Unfortunately, in many cases that is all the info that is supplied. Then your only fallback is the following:

Want to know what the HP is for a certain size engine? Use the old rule of thumb:
Engine is 190 cc's. Divide by 28 = 6.8 HP (this is the "rated" HP. Now multiply by 0.85 = 5.8 "real" HP. This is a VERY ROUGH estimate.

Too hard to calculate in your head? Try dividing by 30. (I think this is a slightly better estimate anyway.)

I hope this helps someone. It sure did for me when I had to buy a new one yesterday. (I registered on this forum just so I could share this.)

------------------------

Want to know more about WHY the rating system changed? Go to these 3 links. Each contains slightly different info to give you a more complete picture:

http://www.washingtonpost.com/wp-dyn/content/article/2008/02/15/AR2008021501732.html

http://forums2.gardenweb.com/forums/load/tractor/msg0215391823783.html?26427

http://www.inspectionnews.net/home_inspe...-vs-ft-lbs.html

Want to know more about the CALCULATIONS? Go to this link for a good explanation:

http://www.revsearch.com/dynamometer/torque_vs_horsepower.html
 
Hi.

Useful information. However, there is no way to convert torque to horsepower. They are different things. Horsepower is a unit of power. N.m is a unit of torque. In physics, they are two totally different things.

Nevertheless, useful.
 
welcome2.gif
to BITOG

Thanks for the effort, but don't stop there. What mower did you buy and what oil are you going to use in it?
 
They are two different things, but if you know one, and the RPMs at which it was measured, you can easily calculate the other.
Torque by itself is a measure of twisting force, while horsepower is a measure of work performed, whether cutting the grass or cruising the interstate.
OTOH, SteveS is probably right.
Unless you place really unusual demands on a mower, any engine offered will work.
Also, deck and blade design play a big role in how well a mower will cut.
Finally, without regard to output, an old two stroke Lawnboy remains the ultimate mower for getting a clean cut in heavy grass, in my personal experience.
 
Seems simpler to just get the highest rated (torque) engine you can find.

Without a parts list, it's difficult to know what (or if) specific parts might be different between two different rated engines on the same block.
 
Originally Posted By: dtt004
Hi.

Useful information. However, there is no way to convert torque to horsepower. They are different things. Horsepower is a unit of power. N.m is a unit of torque. In physics, they are two totally different things.

Nevertheless, useful.


Of course there is. A dyno measures torque which is converted to horsepower with a simple formula.
 
well, if that torque number by itself is at the running rpm, it will be a good comparison for a given blade size. of course, a smaller mower can spin at a slightly higher rpm.

i suspect OHV vs flathead throws a big chink in the displace-to-HP conversion, as the flatheads are not as efficient for a given displacement.

Good info, welcome to bitog.

M
 
Originally Posted By: meep
i suspect OHV vs flathead throws a big chink in the displace-to-HP conversion, as the flatheads are not as efficient for a given displacement.


It does. I did these charts YEARS ago, but there's still at least some validity to them.

http://www.jnjhome.net/ope/engines.htm

The flathead engines had the worst displacement-to-power ratio, at the worst displacement-to-torque ratio. Essentially, to get the same power as many OHV designs, they had to have a larger displacement (and likely used more fuel).

On the flip side, they were typically (and predictably) lighter for a given power or torque produced.

You can download the Excel file to see the raw data and the graphs.
 
Originally Posted By: dtt004
Hi.

Useful information. However, there is no way to convert torque to horsepower. They are different things. Horsepower is a unit of power. N.m is a unit of torque. In physics, they are two totally different things.

Nevertheless, useful.


This is a common misconception. Torque and horsepower ARE directly related and one is easily calculated from the other if you know the RPM at which the torque or horsepower value is measured:


Calculating Braking Torque:

Full-load motor torque is calculated to determine the required braking torque of a motor.
To Determine braking torque of a motor, apply this formula:

T = 5252 x HP
rpm

T = full-load motor torque (in lb-ft)
5252 = constant (33,000 divided by 3.14 x 2 = 5252)
HP = motor horsepower
rpm = speed of motor shaft

Example: What is the braking torque of a 60 HP, 240V motor rotating at 1725 rpm?

T = 5252 x HP
rpm
T = 5252 x 60
1725
T = 315,120
1725
T = 182.7 lb-ft
 
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What is the origin of 33000 in the above explanation? I am sure I used to know that in the past :-(

- Vikas
 
For some reason, I thought horsepower would be defined to be some "non-metric" number i.e. 28288 ft-lb of work per minute :)
 
It's based on one horse pulling a 1000 pound weight up a well, which is 33 feet deep, in one minute. 33000ft-lb = 1 HP

I read the story years ago in a history book; tried to locate the story online but couldn't locate it.
 
This is a simple matter!

Forget HP, Forget torque. Although HP is the ability to do work, and therefore the more relevant number.

Simply compare displacement. For example, an 11HP, 3600RPM engine is typically around 340cc.

So, using my example, let's purchase a generator. There are 2 available that are rated at 5000 watts. One has a 270cc engine, the other, a 340cc.

Which one will actually make 5000 watts continuous? Obviously the 340cc one.

There really is very little difference between the torque curves of various engines of the same displacement. So, look at the size!

Remember, the Acura 2.0L can make 200HP, and so can the older ford 302 (5L). Honda/Acura makes the HP through excellent cylinder filling at very high RPM. But, at 3600 RPM, the 5L is the clear winner.
 
@cujet, your analogy is partially true when it applies to motors with variable RPM, such as car.

But in the context of mowers, as per this thread, their operating RPM is fairly constant, except for some variation under load, e.g, long or thick grass.

As I point out, 2 motors with the exact same displacement, by the same manufacturer, can have totally different torque, and therefore, HP.

But as "meep" and "Hokiefyd" point out, OHV (overhead valve) engines are more efficient, and the claim is that they should last longer, whatever that means, as your "mileage" may vary.

So all this said, if you do not have the actual charts showing the torque curve or the HP curve, and if price is not a major concern, go with the highest torque with the largest displacement and OHV.
 
Now for those of you wondering what I decided on, first let me tell you what I found.

After looking at MANY sites, retailers, motor brands, and models, all the mowers I found with the best value (price for performance) all seem to be Craftsman mowers with Briggs & Stratton motors from Sears (Canada).

1. the highest torque with OHV has 8.5 ft-lb Briggs and Stratton motor (approx. 5 HP). It is a Craftsman mower from Sears (Canada) for only $450 (list price).

http://www.sears.ca/product/craftsman-md-professional-22-rear-drive-mower/671-000202721-944-36973

2. the highest torque with flathead valves has 8.75 ft-lb. I found 2 models.

The first model is also for only $450 until June 16 (list price: $550)
http://www.sears.ca/product/craftsman-professional8482-22-rgd-eco-gas-mower/671-000202721-944-36127

The second model is also for only $450 until July 10 (list price: $580)

All the above models have lots of extras, e.g., key start, self-propelled, etc., all with slightly different features (it seems the only way to get the higher torque/HP motors is to pay for the extras too).

3. If you want one without all these features the next best value is one with flathead valves and 8.5 ft-lb for $400.

http://www.sears.ca/product/craftsman-md-21-low-emission-3-in-1-gas-mower/671-000202721-944-36933


WHAT I CHOSE

My previous mower was a Murray with a "6 HP" Briggs & Stratton motor (which really is approx 5 HP at 3100 RPM), from Home Depot. I had it 8 years. The motor was still fine (even though I never changed the oil--I did top up) and use stale gas from the previous season. It was the deck that gave way because the blade got damaged and eventually the motor broke off from the vibration (no one told me about it until it was too late).

Then I also found a 6.75 ft-lb flathead (approx 4 HP) for only $230 until June 16 (list $260). So, finally, I decided that I could not justify paying an extra $200 just to get a 5 HP motor (zilch between 7 ft-lb and 8.5 ft-lb). See what I bought here.

http://www.sears.ca/product/craftsman-md...275-11A-A25K599

It is a small trade off between power and cost. It has an easy start motor without priming. Tried it the other day. I could feel the lower power but I think it will meet my needs for now.

And hey, at this price I can almost buy 2 for the price of the pricier ones.
 
Last edited:
Commercial mowers generally have higher rated engines, but you pay big time.
They will last a lot longer though. Ball bearing wheels, iron liner in the engine, etc...
 
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