View Full Version : Lawnmower Torque?

07-13-2007, 08:15 PM
Did Briggs & Stratton and the others get nailed by the watch dogs like the electric motor folks? I've got two tractors and three lawn tractors but I need a mower to use around the house so I went shopping. Seems all they say on the engine now is x.x Torque or Ft/Lbs. There's no more horsepower rating. Just when you think you're getting a great deal, I'm scratching my head trying to do the conversion for a "relative" comparison. When did they start this crap?

07-13-2007, 08:51 PM
I don't know what happened, but have started noticing the same thing. It is a PITA to figure out what you are looking at with the "new" system. :rolleyes:

The meddling bastards can't leave well enough alone!

07-13-2007, 09:11 PM
Yea, and did you notice Air compressors are rated at "X "hp, but the motor says nothing but "Special" or some such in the HP rating box.

07-13-2007, 09:19 PM
The meddling bastards can't leave well enough alone!
Well, a rating of 7 instantaneous horsepower doesn't tell you much either. The torque rating is also meaningless unless it is tied to rpm. Virtually any amount of torque can be produced by any motor if it is sufficiently geared and rpm doesn't count. If they give the torque at a specified or "standard" rpm then you can calculate the horsepower easily.

07-13-2007, 09:26 PM
Think metric. They sell these things international. HP is out Watts is in so is Torque.

07-13-2007, 09:35 PM
Both watts and torque are meaningless without reference to something else, such as watt hours. If all you give is a torque value then it is no different that it was before. I'm guessing that there is a standard rpm they must use to determine the torque. They probably have the option of reporting the power in various ways as long as it is accurate. Since it wouldn't look so good that what used to be a "7 HORSEPOWER" engine is now a "NEW IMPROVED 2 HORSEPOWER ENGINE" they have chosen to report the power rating in a manner that most people aren't familiar with and don't know how to calculate.

07-13-2007, 10:07 PM
Watts = 0.142 * ft-lbs * RPM
HP = ft-lbs * RPM / 5249

Plug in your Briggs and Stratton's rated RPM in the above to get its HP or Watts.

Watts-seconds don't apply. Watts and HP are units of power. Watt-seconds are units of energy or work (1 Joule = 1 Watt-second). 1 Joule is 1 HP for 0.00134 seconds. Can't mow much grass in 1/1,000 of a second with only 1 Joule of energy.


07-13-2007, 10:15 PM
To mow a lawn requires a specific amount of work be done. A measurement of the ability of the machine to perform work is appropriate. We should assume that the product will last longer than one millisecond.

1 Joule is 1 HP for 0.00134 seconds

Right. So given a watthour rating we can determine actual horsepower.

Plug in your Briggs and Stratton's rated RPM in the above to get its HP or Watts.

That is the question, what rpm rating is used with the advertised torque specification?

07-13-2007, 10:22 PM
Evan, the last I checked Power = Watt.Motors are rated in Watts. Diesel, Electric,Gas or Wind or what ever not Watt/Hr.

07-13-2007, 10:56 PM
The torque is just like any other gas engine. The peak torque rpm is far below the horsepower rpm and NONE of the lawnmower engines run at peak horsepower rpm. They're normally set to around 2800-3200 rpm max and the hp rating was at 3600 rpm. This was done to keep the blade tip speed and engine longevity at optimum. So while the engine was capable of producing X horsepower, you weren't cutting grass as that figure.

It's not clear if they're giving peak torque or the torque at which the engine is set to cut grass. It's just as worthless as the horsepower figures but it would be nice to know for "traditional" comparisons. I guess I'll just have to take my calculator with me next time.

07-14-2007, 02:32 AM
Ken, for the official explanation from Briggs & Stratton go to the link below and click on the "* Information On Power Ratings" tab near the bottom of the page.


Don't know what was wrong with the old horsepower rating...at least it gave you an indication of how much work an engine could do in a given amount of time.

Forrest Addy
07-14-2007, 03:02 AM
Stop complaining. Enough of BS specs grounded in marketing hype. Be glad that there's a move towards honestly rated stuff. I recall a couple years ago a shop vac that all had 12 Amp motors. The only difference between them was the "gallon" size of the cannister.

"Horsepower," "torque," "RPM," and other units have legal existance as well as technical and scientific existance. If mnarketers can play fast and loose with units to exploit the ignorance and gullibility of their customers what to keep thme from using this same scam to sell us drugs, foodstuff, and allegedly safe transportation.

Eternal vigilence is not only the price of freedom it's the only way to get safe reliable honesly rated products to consumers. A momentary confusion when transitioning from a bogus system of lies and deceit to honestly presented and rated produce seems like a small price to pay.

07-14-2007, 04:50 AM
Don't know what was wrong with the old horsepower rating...at least it gave you an indication of how much work an engine could do in a given amount of time.
It was a lie, that's what was wrong. Here's that fine print they don't want you to copy.

Based on that statement for the purposes of the test they get to fit the most advantageous combination of performance accessories they want, not what they are selling. It's the same as telling you the engine in the car you are buying has been known to produce 1000 horsepower without telling you that it was only done when tricked out for drag racing and has nothing to do with what you are buying.

07-14-2007, 05:06 AM
About this nonsense on the BS web site:

Torque: Defined as the twisting force that tends to cause rotation.
In the outdoor power equipment, torque is the most direct way to determine an engine's ability to get the job done.
More torque means improved performance, and the more torque you have, the better![my emphasis]
The easy way to provide more torque in a lawnmower is to gear down the blade to 1 rpm. Gobs and gobs of torque.

Without knowing the power over time rating you know nothing. Of course that pretty well describes the average consumer.

Your Old Dog
07-14-2007, 08:54 AM
My friend told me years ago that many if not most HP ratings were given at the point the product failed. That was OK with me as who in their right mind would put a 7 HP motor on a job requiring 7 HP and not up it for a little safety margin.

I agree with Forest that any movement in the direction of truth is a good day for the consumer. But I shouldn't have to be engineer to make a simple purchase and the HP concept worked great for me for 60+ years.

07-14-2007, 09:22 AM
As stated on the B&S site, horsepower and torque are subject to many factors and won't always be the same. It gives a general idea of expected performance. I perfer the HP rating rather than watts or torque but they sell in a world market.