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#2274273 - 05/30/11 12:36 AM How to compare mowers rated in HP vs ft-lb
smayer97 Offline


Registered: 05/29/11
Posts: 5
Loc: Toronto ON, Canada
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

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#2274298 - 05/30/11 02:34 AM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
dtt004 Offline


Registered: 08/13/10
Posts: 314
Loc: Orange County, CA
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.
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#2274319 - 05/30/11 06:19 AM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
Kuato Offline


Registered: 10/23/05
Posts: 7094
Loc: Colorado
Here's a link that will help:

http://www.calculatoredge.com/new/horsepower.htm

As you can see, you can convert from hp to torque and torque to hp.
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#2274332 - 05/30/11 07:05 AM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
Steve S Offline


Registered: 01/20/03
Posts: 18449
Loc: East of IGO
It is just a number. There are only a few engines available on push type mowers.
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#2274340 - 05/30/11 07:21 AM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
Johnny Offline


Registered: 05/27/02
Posts: 14013
Loc: Retired | Wausau, WI
welcome 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?

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#2274353 - 05/30/11 07:34 AM Re: How to compare mowers rated in HP vs ft-lb [Re: dtt004]
fdcg27 Online   content


Registered: 09/25/09
Posts: 15702
Loc: OH
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.
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#2274487 - 05/30/11 11:05 AM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
dwendt44 Online   content


Registered: 05/17/06
Posts: 3970
Loc: Central Wisconsin
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.
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#2274512 - 05/30/11 11:29 AM Re: How to compare mowers rated in HP vs ft-lb [Re: dtt004]
Slick17601 Offline


Registered: 12/04/04
Posts: 1515
Loc: Pennsylvania
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.

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#2275307 - 05/31/11 10:53 AM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
meep Offline


Registered: 02/20/07
Posts: 3557
Loc: Southeast
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
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#2275571 - 05/31/11 03:25 PM Re: How to compare mowers rated in HP vs ft-lb [Re: meep]
Hokiefyd Offline


Registered: 06/24/04
Posts: 14505
Loc: Top of Virginia
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.

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#2275622 - 05/31/11 04:24 PM Re: How to compare mowers rated in HP vs ft-lb [Re: dtt004]
maineiac Offline


Registered: 12/09/06
Posts: 12
Loc: United States
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


Edited by maineiac (05/31/11 04:26 PM)

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#2276262 - 06/01/11 11:34 AM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
Vikas Offline


Registered: 07/22/05
Posts: 11251
Loc: NorthEast
What is the origin of 33000 in the above explanation? I am sure I used to know that in the past :-(

- Vikas

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#2276289 - 06/01/11 12:26 PM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
NateDN10 Offline


Registered: 01/10/10
Posts: 1735
Loc: Rochester, NY
33000 foot-pounds of work per minute = 1 horsepower


Edited by NateDN10 (06/01/11 12:27 PM)
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#2276404 - 06/01/11 03:05 PM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
Vikas Offline


Registered: 07/22/05
Posts: 11251
Loc: NorthEast
For some reason, I thought horsepower would be defined to be some "non-metric" number i.e. 28288 ft-lb of work per minute :-)

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#2285582 - 06/11/11 03:14 PM Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97]
Kuato Offline


Registered: 10/23/05
Posts: 7094
Loc: Colorado
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.
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