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How to compare mowers rated in HP vs ft-lb #2274273
05/30/11 12:36 AM
05/30/11 12:36 AM
Joined: May 2011
Posts: 5
Toronto ON, Canada
smayer97 Offline OP
smayer97  Offline OP
Joined: May 2011
Posts: 5
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

Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2274298
05/30/11 02:34 AM
05/30/11 02:34 AM
Joined: Aug 2010
Posts: 314
Orange County, CA
dtt004 Offline
dtt004  Offline
Joined: Aug 2010
Posts: 314
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.


Technical Design Engineer
--
1991 Toyota MR2 Turbo
1993 Mazda RX-7 Turbo
1996 Toyota Tacoma V6 4x4
2005 Acura NSX
2008 Lotus Elise
2004 Kawasaki Ninja ZX-10R
Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2274319
05/30/11 06:19 AM
05/30/11 06:19 AM
Joined: Oct 2005
Posts: 7,094
Colorado
Kuato Offline
Kuato  Offline
Joined: Oct 2005
Posts: 7,094
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.


Thick vs Thin test: 15k / 43k miles complete
Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2274332
05/30/11 07:05 AM
05/30/11 07:05 AM
Joined: Jan 2003
Posts: 18,449
East of IGO
Steve S Offline
Steve S  Offline
Joined: Jan 2003
Posts: 18,449
East of IGO
It is just a number. There are only a few engines available on push type mowers.


Why do people post I want the best for my car,,, When there isn't anything that is the best on the car to begin with.
Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2274340
05/30/11 07:21 AM
05/30/11 07:21 AM
Joined: May 2002
Posts: 14,013
Retired | Wausau, WI
Johnny Offline
Johnny  Offline
Joined: May 2002
Posts: 14,013
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?

Re: How to compare mowers rated in HP vs ft-lb [Re: dtt004] #2274353
05/30/11 07:34 AM
05/30/11 07:34 AM
Joined: Sep 2009
Posts: 16,026
OH
fdcg27 Offline
fdcg27  Offline
Joined: Sep 2009
Posts: 16,026
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.


17 Forester 18K VME 0W-20
12 Accord LX 92K PP 5W-20
09 Forester 95K M1HM 10W-30
01 Focus ZX3 118K PP 5W-20
96 Accord LX 104K T5 10W-30
95 318i
Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2274487
05/30/11 11:05 AM
05/30/11 11:05 AM
Joined: May 2006
Posts: 4,015
Central Wisconsin
dwendt44 Offline
dwendt44  Offline
Joined: May 2006
Posts: 4,015
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.


There's no such thing as:
Too big of a battery,
Too large of a gas tank,
or too loud of a horn,
or too bright headlights.
Re: How to compare mowers rated in HP vs ft-lb [Re: dtt004] #2274512
05/30/11 11:29 AM
05/30/11 11:29 AM
Joined: Dec 2004
Posts: 1,530
Pennsylvania
Slick17601 Offline
Slick17601  Offline
Joined: Dec 2004
Posts: 1,530
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.

Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2275307
05/31/11 10:53 AM
05/31/11 10:53 AM
Joined: Feb 2007
Posts: 3,647
Southeast
meep Offline
meep  Offline
Joined: Feb 2007
Posts: 3,647
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


2018 F150
2015 crv (wifey!)
Re: How to compare mowers rated in HP vs ft-lb [Re: meep] #2275571
05/31/11 03:25 PM
05/31/11 03:25 PM
Joined: Jun 2004
Posts: 14,505
Top of Virginia
Hokiefyd Offline
Hokiefyd  Offline
Joined: Jun 2004
Posts: 14,505
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.

Re: How to compare mowers rated in HP vs ft-lb [Re: dtt004] #2275622
05/31/11 04:24 PM
05/31/11 04:24 PM
Joined: Dec 2006
Posts: 13
United States
maineiac Offline
maineiac  Offline
Joined: Dec 2006
Posts: 13
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

Last edited by maineiac; 05/31/11 04:26 PM.
Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2276262
06/01/11 11:34 AM
06/01/11 11:34 AM
Joined: Jul 2005
Posts: 11,315
NorthEast
Vikas Offline
Vikas  Offline
Joined: Jul 2005
Posts: 11,315
NorthEast
What is the origin of 33000 in the above explanation? I am sure I used to know that in the past :-(

- Vikas

Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2276289
06/01/11 12:26 PM
06/01/11 12:26 PM
Joined: Jan 2010
Posts: 1,736
Rochester, NY
NateDN10 Offline
NateDN10  Offline
Joined: Jan 2010
Posts: 1,736
Rochester, NY
33000 foot-pounds of work per minute = 1 horsepower

Last edited by NateDN10; 06/01/11 12:27 PM.

2016 Mazda3 - 20,000 mi.
2011 Toyota Sienna - 100,000 mi.
Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2276404
06/01/11 03:05 PM
06/01/11 03:05 PM
Joined: Jul 2005
Posts: 11,315
NorthEast
Vikas Offline
Vikas  Offline
Joined: Jul 2005
Posts: 11,315
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 :-)

Re: How to compare mowers rated in HP vs ft-lb [Re: smayer97] #2285582
06/11/11 03:14 PM
06/11/11 03:14 PM
Joined: Oct 2005
Posts: 7,094
Colorado
Kuato Offline
Kuato  Offline
Joined: Oct 2005
Posts: 7,094
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.


Thick vs Thin test: 15k / 43k miles complete
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