Okay, this question dovetails with the question I asked a couple days ago about lawn mowers. My old lawnmower has a B&S 4.0 Horsepower engine. The new lawnmowers I'm looking at state the engine power in torque such as 700 ft-lbs. When did they (the ubiquitous "they") replace HP designation with torque? How do I relate the two? I had physics about 40 years ago and I really don't remember discussing the relationship.
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Richard -
Manufacturers were overstating hp ratings so they (someone else) changed the rules do they can no longer state hp as the main power rating.
Hp is calculated from torque
Hp = (torque x rpm) / 5252Opportunity is missed by most people because it is dressed in overalls and looks like work. - Thomas Edison -
I agree with what Tom said but will add a caution. The 5252 is a conversion factor and is appropriate as long as the torque is in ft-lbs and the desired output is horsepower. If you had torque in Newton Meters, the conversion factor would change.
I do not know what they were doing on gas engines but for universal motors, used in a lot of our power tools, they used peak torque and maximum rpm. A universal motor will not produce it's peak torque for more than an instant, if you load it at that level the rpm drops quickly. A much more realistic way to rate the motor would be to use it's maximum sustained torque and the rpm at which it is generated. HP tends to be used as an indicator of how much the tool is worth, however, so the manufacturers were anxious to come up with some way to state a higher number.
Torque is a measure of how much twisting force can be applied by the engine. Horsepower is a unit of work - how much work can the engine do.
JimComment
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Richard I have no idea as to why they have done that, but with regards to mowers,most are setup to run at a minimum rpm range and are regulated by a govenor of sorts that will automatically increase the rpm range when encountering more resistance(read as taller grass) this might very well be the reasoning for why they have decided to rate the torque readings vs actual horsepower. Many newer mowers will initially bog momentarily then increase the rpm's when encountering higher resistance to power thru the taller,wetter,etc grass. Just my 2cents
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I certainly appreciate everyone's input. l I had seen the formula that Tom quoted but since I do not know the rpms, I can not fill in the formula. In practical terms, on an average lawnmower, how much torque would I need to be equivalent to my 4 HP B&S of yesteryear. Since I'm going from a push mower to a self propelled mower, I realize that I will need more power but have no idea how much torque will be required to maintain or increase the rpms (as Tommy stated) since my old mower is rated in HP. And yes, I think Jim and C'man are right in that this is a way to distract by using large numbers. Thanks to all.Last edited by phrog; 06-12-2012, 11:10 AM.RichardComment
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If you can buy a gas engine that supplies 700 Ft. Lbs. of torque in one or two cylinders, with a bore of less than 3", I would advise you to buy it and I want to buy all of them I can.
I have a 6L71 Detroit diesel that provides 238HP with 600 Ft/ Lbs. of torque at 2150 RPM.
The 6L71 means that it is 6 cylinders and 4.5 X5" bore and stroke. for a total of 426 CID.
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According to Briggs and Stratton's website they compute hp at 3600 rpm but peak torque is generally at 3,060 rpm. Honda's might be different, I would guess higher rpm. Honda's are an overhead valve motor. B&S are built like a flathead car motor - the valves are in the block. If you compared displacement for like engine types (flathead or overhead valve) it would be a good basis for comparison.
JimComment
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Maybe it's the big numbers that sell lawnmowers. I happened to look at the latest HD flier and none of the lawnmowers had either torque or HP numbers. There should be a standardization just like automobiles as far as horsepower. Horsepower ratings on automobiles isn't at the drive wheels. I guess if they listed that figure, people might think they were lawnmowers.
Let them list the horsepower produced at the shaft of the motor.
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My vision ain't what it used to be, so cut me a little slack please.If you can buy a gas engine that supplies 700 Ft. Lbs. of torque in one or two cylinders, with a bore of less than 3", I would advise you to buy it and I want to buy all of them I can.
I have a 6L71 Detroit diesel that provides 238HP with 600 Ft/ Lbs. of torque at 2150 RPM.
The 6L71 means that it is 6 cylinders and 4.5 X5" bore and stroke. for a total of 426 CID. That was supposed to be 7.00 foot pounds. It's the 700 series. Thanks for pointing that out. 
Last edited by phrog; 06-12-2012, 05:35 PM.RichardComment
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I looked at the Toro I bought last fall. It has a sticker on it saying it has 7.0 ft. lbs of torque. It has a 190cc B&S motor. I've always found it to have adequate power. I mowed my son's lawn when it was overgrown and it did fine. I mow mine each week and it is healthy Bermuda. It's been raining a lot here in June so it is pretty tall in a week - tall enough it leaves a little on the surface in mulch mode. The Toro mows it fine. I also picked up leaves with it last fall. I have mostly big oak trees so I picked up a lot of leaves. It worked fine (except the bag filled quickly).
JimComment
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