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lkbum_gw

Article on HP vs torque

lkbum_gw
16 years ago

The following is an article from the Atlanta Journal Constitution. It's not that great technically, but I was surprised to see it in the Atlanta Newspaper. The link may not work (you have to join up to see on line articles, so I posted the whole thing here.

Horsepower loses its grip on mowers

In terms of engines, talk turns to torque

By Rick Barrett

McClatchy Tribune

Published on: 02/23/08

When you buy lawn and garden equipment this spring, a familiar old term - horsepower Â- will be missing from many engines.

Blame it on lawyers, or engine makers who might have fudged the numbers, but horsepower is no longer the gold standard for small gasoline engines.

Sears, for example, advertises some lawn mowers rated by horsepower, others by torque, and still others by cubic centimeters. And some mowers have no such designation at all.

"Unfortunately, we are not giving consumers the answers they want," said Bill Rotter, an owner of National Ace Hardware stores in the Milwaukee area.

There's no longer a horsepower rating for many Briggs & Stratton engines. Last year, Briggs chose torque as its rating system for push mowers, snow throwers, pressure washers and generators.

In basic terms, torque is a measure of the force needed to turn something like a wrench or a lawn mower blade.

"We think it's a better measurement of a mower's ability to cut grass," said Rick Zeckmeister, North American consumer marketing director at Briggs & Stratton, the world's largest manufacturer of small gasoline engines.

Horsepower, on the other hand, evolved from a measure of the rate at which a horse could pull coal up a mine shaft into a more technical measurement related to watts. Although most people don't know its technical meaning, many have found it useful in comparing the power of engines.

So now, consumers may face confusion over how torque equates to horsepower. There isn't a practical conversion chart because torque and horsepower are two different things.

"Torque doesn't mean much to the consumer," Rotter said. "And it's more complicated for us because it's almost impossible to try and explain what gross torque means" to someone buying a lawn mower.

Rotter said he wouldn't be surprised if, down the road, engine manufacturers return to horsepower ratings.

Lawsuit spurs change

The shift away from horsepower ratings came after a lawsuit in Illinois claimed that engine manufacturers were overstating the horsepower of lawn mower engines.

In some cases, the lawsuit alleged, identical engines were labeled with different horsepower ratings, misleading consumers into believing they were getting more power by purchasing more expensive models.

Briggs advertised one engine as having 6.75 horsepower and yet told the Environmental Protection Agency the same engine had 3.6 horsepower, an 88 percent overstatement, according to the lawsuit.

Since at least 1997, engine manufacturers Briggs, Tecumseh, Kohler, Toro and Kawasaki have reported horsepower ratings to the EPA that were significantly lower than the ratings advertised to the public, the lawsuit said.

For Briggs, it wasn't an attempt to mislead anyone, according to Tom Savage, a senior vice president at the company.

There are different testing protocols for the EPA than for the general public, Savage said. The EPA ratings are based on a "composite" of test results at different engine loads, while results for the general public are based on an engine's full power capabilities.

An Illinois judge dismissed the suit last March, but it may resurface.

"It's still not totally resolved because the judge did not tell us what portions of the suit he dismissed with prejudice or not. So in effect, it allows the lawyers to come back," said James Brenn, Briggs' chief financial officer.

The suit included plaintiffs from across the nation, including Susan Barnard, a librarian from Green Bay, Wis.

Barnard bought a Yard Machines mower for $263.70 that was supposed to have a 5-horsepower Briggs engine. Although she was happy with the mower, she was miffed when lawyers involved in the lawsuit contacted her and told her the engine was less powerful than billed.

"I said, 'Those dirty buggers. You get them to stop doing that. Put me on the lawsuit,'" she said in an interview.

'Horsepower sells'

Over the years, manufacturers in the intensely competitive small-engine business have used horsepower ratings as a marketing tool.

"Horsepower sells," said Jeff Hebbard, a vice president at Ariens Co., a Brillion, Wis.-based manufacturer of lawn tractors and other outdoor power equipment. "It doesn't always sell for the right reasons, but it does sell."

The horsepower race sounds like what has occurred with electric motors, where power claims have been embellished, said Kevin Brady, a Minneapolis attorney and engineer not affiliated with the horsepower lawsuit.

"You can exaggerate a bit and not get in trouble," Brady said. "It's called puffing."

In reporting to the EPA, engine manufacturers have some leeway to fudge horsepower ratings by about 15 percent.

Sometimes, the same engine is advertised as having different horsepower ratings depending on how it's sold.

"There are slight adjustments that get them there, but it's the same engine," Hebbard said.

Ariens buys engines from Briggs, Kohler, Honda and other manufacturers.

It has been challenging for the engine makers to find a rating system that works, said Dan Ariens, company president.

"Americans are very familiar with horsepower. It's a number they kind of understand," Ariens said.

It's uncertain which power standard the small-engine manufacturers will settle on, if they agree at all.

"Some guys like to have cubic centimeters as their standard, some like torque, and some like horsepower," said Savage of Briggs & Stratton. "I don't know if there will be a one-size-fits-all solution."

SHOPPER'S GUIDE

What to look for, according to Peter Sawchuk, a Consumer Reports power equipment expert.

Ignore: horsepower, torque or engine displacement

Pay attention to: The mower's cutting width and overall performance, rather than engine statistics.

Look for: An engine with an overhead valve system. It might be more expensive, but it will last longer.

GLOSSARY OF ENGINE TERMS

Torque: Briggs & Stratton says torque is the best way to rate an engine that powers a push lawn mower, snow thrower, pressure washer or other equipment where the engine is turning something. Torque, in basic terms, is a twisting force that causes rotation.

Horsepower: Engine manufacturers typically measure horsepower by operating a "bare" engine, not equipped with accessories or installed in power equipment, at a given engine speed. In technical terms, a unit of horsepower is equal to 745.7 watts, another measurement of power.

Since torque and horsepower are two different things, Briggs says it can't do a direct comparison.

Cubic centimeters: The volume of the engine's cylinder chamber. An engine with more cubic centimeters should produce more power. But that could be affected by other features of the engine, including its fuel injector or carburetor.

Comments (40)

  • turnage (8a TX)
    16 years ago
    last modified: 9 years ago

    To me, the only useful HP rating would be measured at the blade spindle. Course that would have to be measured by the mower mfg - not the engine mfg.

    Farm tractors have had two ratings for years - engine HP and PTO HP.

  • wally2q
    16 years ago
    last modified: 9 years ago

    I can't believe how stupid people can be.....

    Torque is a useless number, if not stated with an RPM figure. Why?... because without RPM, you have no idea how much work the engine can do.

    So what if your snow-blower engine can have a reasonable amount of torque... but it's spinning at some low RPM. If all you get is effectively 1HP, then take a wild guess as to how much snow you can blow out of your blower per hour? not a heck of a lot.

    Stipulating torque is like stipulating that your oven is rated at 115/230 Volts.

    the idiots.

  • xlindax
    16 years ago
    last modified: 9 years ago

    The problem with the advertised horsepower of mower engines is the fact that the manufacturers have tended to rate horsepower of a given engine at 3600 rpm. Government regulations regarding the safe tip speed of a lawn mower blade limit the rpm to an average of 3100 rpm. Inasmuch as the formula for horsepower is torque times rpm divided by 5250, it is obvious that the higher the rpm, the higher the horsepower rating.

    The simple, overlooked solution is to measure the horsepower rating of lawn mower engines at 3100 rpm instead of 3600 rpm. Most mower engines have a fairly flat torque curve between 2500 and 3600 rpm; this change in the engine speed at which the engine horsepower is rated would result in an engine rated at 6.75 horsepower at 3600 rpm being rated at approximately 5.8 horsepower at 3100. A 5.5 horsepower engine would have about 4.7 horsepower.

  • steve2ski
    16 years ago
    last modified: 9 years ago

    I think you will find the top selling small engines(under 31HP) have HP/Torque rated per SAE J1940 standard.
    Unless you specify under what conditions hp and/or torque are listed you will chase yourself in circles.
    Briggs, Kohler, Tecumseh, Kawasaki, Honda, Robin and other manufacturer's rate per this standard - also this standard is revised from time to time, so the revision is important also.

  • johndeere2210
    16 years ago
    last modified: 9 years ago

    It wasn't until I owned a diesel truck and tractor that I realized the torque advantage in these engines.

  • danman1
    16 years ago
    last modified: 9 years ago

    I'd like to state again...
    The confusion for consumers is because of the fact that the advertised power rating is done at a high speed ( usually at a high speed the manufacture would not gaurantee the engine surviving long term ) the for the engine which is different than the governed speed of the complete product. It's good to see a paper at least take a stab at the problem, but they also are clueless and i think they only confused more people. Stating "Ignore: horsepower, torque or engine displacement" is just plain wrong.
    The word to get out is "We the consumers, request an H.P. rating of the complete product tested at the speed that the product is intended/waranteed to run." PLAIN AND SIMPLE!

  • deeredoctor
    16 years ago
    last modified: 9 years ago

    What xlindax is saying is that at max hp/rpm you have less torque in these small engines. If you look at the hp/torque graphs you will see this.

    Example, engine X is set max at 3600 rpm 20 hp and say 29 pound feet of torque.

    Same engine X is factory set to run at 3200 rpm with engine torque at 33 pound feet torque.

    HP varies between 16.5 to 19.5

    The link below has the torque/hp graphs.

    MODEL Courage SV600
    MAX. POWER @ 3600 RPM hp (kW) 20 (14.9)
    DISPLACEMENT cu. in. (cc) 36.4 (597)
    BORE in. (mm) 3.7 (94)
    STROKE in. (mm) 3.4 (86)
    PEAK TORQUE @ MAXIMUM lbs. ft. (Nm) 36.9 (50.0) @ 2200
    COMPRESSION RATIO 8.5:1
    DRY WEIGHT lbs. (kg) 79 (35.8)
    OIL CAPACITY U.S. quarts (liters) 1.6 (1.5)
    LUBRICATION Full pressure w/full-flow filter
    DIMENSIONS L x W x H in. 19.4 x 16.4 x 12.9

  • bill_kapaun
    16 years ago
    last modified: 9 years ago

    The new "Torque Ratings" are taken at 3060 RPM, which is more meaningful for typical, homeowner, walk behind mowers, since they operate in that range. At least for that application, you get about as valid of a comparison as one could expect.
    It still boils down to - How well does it cut the grass! Deck design etc. is a major factor too.

  • deerslayer
    16 years ago
    last modified: 9 years ago

    "Torque is a useless number, if not stated with an RPM figure."

    I agree.

    At each RPM within the power curve, torque and HP are directly proportional.

    Power = torque x 2pi x RPM

    Choose either torque or HP. You can convert one to the other.

    -Deerslayer

  • booster
    16 years ago
    last modified: 9 years ago

    I agree that torque is a pretty useless rating for a mower without the rpm, and as soon as you factor in the rpm, you are right back to hp.

    It takes work to mow grass (hp) not force (torque). 500 ft-lb of torque at 50 rpm (4.75 hp) wouldn't cut much grass, but would sure look good on the rating plate to someone who doesn't know any better.

    I do also agree that the hp should be given for the actual speed the engine is being run at. To do otherwise is pretty cheesy IMHO.

    I also think they need to differentiate between the "max" hp rating that is on the curve, and the "continuous" hp on the curve. Both are useful, but you need to know which one you are looking at, and how that relates to the job the engine is being used for. A tiller, or pump, might be running on max output for long periods, but most mowers would see full load sporadically, for example. In general, the closer the two ratings are to each other, the more I would consider the engine to be of higher quality and design.

  • wally2q
    16 years ago
    last modified: 9 years ago

    torque and HP curves can be full of peaks too. And if you rate it all at 3000RPM, and then design the engine to have a torque (or HP) peak at that RPM, then as soon as you load the engine down enough to slow it down, you are off that peak and into the sh!tty-performance part of the curve.

    What really makes sense is a "minimum" rating of HP or torque that can be delivered between 2000 and 3000 RPM for that given engine. Then and only then do you know what your limits for performance really are, and what will separate the men from the boys in engine-speak.

    But as is the case with any standard defined by a governing body - what I suggest is just too-much-to-ask.

  • danman1
    16 years ago
    last modified: 9 years ago

    "torque and HP curves can be full of peaks too. And if you rate it all at 3000RPM, and then design the engine to have a torque (or HP) peak at that RPM, then as soon as you load the engine down enough to slow it down, you are off that peak and into the sh!tty-performance part of the curve"

    yes and no. If you tax an engine enough that it can't maintain it's power level @ that rpm then you have forced that engine to an rpm where it has a torque output that equals that load. Normally this is a higher torque but a lower HP. Torque always equals load. BUT you can have torque with 0 rpm. this equals 0 power=no good.
    In the 'real' world, as soon as you load your governed engine to the point where it can not maintqain it's speed you have asked the engine to drop to an rpm that develops more torque, but the loss of rpm is greater than the increase in torque. This results in less power( work ) being produced by the machine.

  • wally2q
    16 years ago
    last modified: 9 years ago

    Danman.... Although what you say is theoretically possible, it is not what typically happens...
    In the "real world", when you load the engine to a point where it can not sustain RPM, you drag it down "not" to the point of higher torque, but to the point where the hp demand has dropped to a level that the engine can sustain. That typically means a point of "lower torque" too. Why?... Well pretend you're running a mower, and you slow down the blades... The HP required to cut grass is reduced, but the same with the force that the blade exerts on the grass is also reduced. Torque and HP demand drop, until en equilibrium is reached with what the engine can provide. So it's not that the engine has more torque at the lower rpm, but it's that the load on the engine dropped to a sustainable level. That is why when your mower deck starts choking on heavy wet grass, and the engine rpm starts to drop - it quite often results in a stall, rather than a recovery.

  • danman1
    16 years ago
    last modified: 9 years ago

    wally2q, Assuming you run your small governed engine at the recommended full speed, which I assume is what we are discussing, everything I just wrote is 100% true. If you feel the need to come up with your own theories and doubt me, then that's your choice, I'm not going to argue.

  • wally2q
    16 years ago
    last modified: 9 years ago

    Danman,

    It's a shame that you took my note so personally and so negativelly... jeesh - take it easy... it's just a debate...

    Although nearly everything you said is right on the money (we actually said the same thing), the one part that you stated that I disagreed with is:
    "you have asked the engine to drop to an rpm that develops more torque"

    So let me ask the question differently: following your logic, what would happen if for the given engine in question, the entire RPM range below the "original higher RPM" had a specific torque that was lower than the original torque?... in other words, if the highest torque peak was at the original higher RPM, and you loaded the engine down to drop the RPM... please explain what would happen then?

    thanks...

  • deerslayer
    16 years ago
    last modified: 9 years ago

    At least the discussion is now addressing the key issue. That is, "at what RPM should rated HP or torque be measured?". I suggest that operating RPM is the most useful. Does anyone disagree?

    -Deerslayer

  • steve2ski
    16 years ago
    last modified: 9 years ago

    Deerslayer,
    The govenor on both my LT and GT is set at about 3000 rpm. This starts opening the throttle to maintain that rpm, when operating rpm falls below that point with the throttle set at wide open.
    Is this the operating RPM you are refering to?

  • wally2q
    16 years ago
    last modified: 9 years ago

    "I suggest that operating RPM is the most useful."

    Agreed. But as i mentioned earlier - I'd take it further... and provide the worst-case (lowest) power or torque rating in the "useful range" of RPM.... not just 1 RPM number. This is because even with a governor, the RPM fluctuates quite a bit under load... and what is important to distinguish is whether the engine is designed to produce a hhigh peak HP or Torque at just a very narrow RPM band (which is useless in the real world, and would serve only the engine-ratings-number-game), or is it a bascially flat torque curve across the broad range or RPM, which we all know is an indication of true capability of the engine...

  • ervie
    16 years ago
    last modified: 9 years ago

    The diameter, lift and duration (degrees of opening) of the intake valve in relation to the volume of the cylinder determines at what RPM peak torque occurs. At that RPM the
    cylinder is being filled with the maximum amount of air mixture. As you run an engine faster than the peak torque RPM, not enough time is available and the cylinder does not fill as well. More power is produced because more firing strokes per second, but each stroke has less punch (torque). Eventually an RPM is reached where the cylinder breathes so poorly, peak horsepower goes down too.
    These small air cooled engines are designed to not overheat, and to produce modest levels of dependable power.
    Look at engine displacement as a gauge of which one will produce the most power. Forget the advertised horsepower.

  • steve2ski
    16 years ago
    last modified: 9 years ago

    If you are on a hill and tie a wagon to a dead horse that is heavy enough to prevent the wagon from rolling down that hill, how much work or "horsepower" is that dead horse producing.
    If you ain't moving the load you are at 0 power or "Horepower". High School Physics, 1950 something.
    I guess a dead horse can produce torque, but can it work?

  • mownie
    16 years ago
    last modified: 9 years ago

    steve2ski, I like that analogy.....it is a humorous way to get the word "horsepower" into the paragraph. But let me toss a little twist into the scenario. In a situation such as you described, the heavily laden wagon (reacting to gravity) might become the potential motive power and the dead horse (or horse shaped sculptured rock, or horse shaped bronze statue) might assume the role of "load". The power or motive force effected by the wagon depends on variables such as the gross weight of the wagon, the pitch of the hill, direction and strength of the winds and so forth. So long as no objects are moving, they only express potential energy and inertia, which is a reflection of the mass of the object. Many may argue that inertia is only present in moving objects, but inertia in moving objects can be called "momentum" (inertia, from root word "inert"). Objects at rest (motionless) are said to be in a state of "equilibrium". "Neigh"?

  • steve2ski
    16 years ago
    last modified: 9 years ago

    mowie,
    18c in your link is the example - it is 2 weights, neither being a dead horse, but then again this is a dead horse with potentional also. ;-)

  • deerslayer
    16 years ago
    last modified: 9 years ago

    "The govenor on both my LT and GT is set at about 3000 rpm. This starts opening the throttle to maintain that rpm, when operating rpm falls below that point with the throttle set at wide open.
    Is this the operating RPM you are refering to?"

    I was referring to full throttle RPM. Maybe my tractor is unusual, but I bet it operates at full throttle RPM (plus or minus 2%) 95%+ of the time. IMO, that's the most relevant RPM for torque and HP ratings.

    Yes, under heavy load, RPM will decrease until the governor takes over. In my case, this happens very infrequently...probably because I mow my lawn using the 1/3 rule.

    Whether the rating is expressed as torque or HP doesn't matter...one can be converted to the other.

    Regarding a dead horse doing work, of course it can!

    Work = Force x Distance

    A falling dead horse can do plenty of work! In fact, many inanimate objects can do work. The weights in a grandfather clock are an example.

    -Deerslayer

  • den69rs96
    16 years ago
    last modified: 9 years ago

    Evrie,

    I wouldn't say its not enough time to fill the cylinders once you pass peak rpms, but that your heads, intake, and valves are too restrictive to flow the volume of air/fuel needed to sustain that rpm. Duration and lift are ground into the camshaft so thats not going to change unless you have some kind of variable valve timng system working. Reducing those restrictions will increase hp and your rpm range. However its not just the volume of air thats important, but increasing velocity into the cylinder as well.

  • hortsense
    16 years ago
    last modified: 9 years ago

    I was catching up on my late Winter reading when I found this string...and I found it fascinating for a while. But I think ya'll are starting to beat a dead horse. (S)nickers...

  • lkbum_gw
    Original Author
    16 years ago
    last modified: 9 years ago

    Funny, when I originally posted this article. I didn't expect to start a debate on Newtonian Mechanics. What Caught my eye in the article, was that the Lecherous Layers were seeking to start a class action suit on the matter. It stated that "Susan Banard" felt she had been duped (after an attorney contacted her) and therefore wanted the Class Action suit to go forward. Here is a lady who was content until some attorney told her she was duped and had EPA data to prove it. I have noticed in Lowes and Home Depot flyers, they now state disclaimers regarding horsepower. It's clear most on this board understand the issue and I think this article points out the problems with goverment regulations and ATTORNEYS seeking to line their pockets based on technicalities and ignorant consumers.

  • oneway
    16 years ago
    last modified: 9 years ago

    excellent article thank you.

  • bus_driver
    16 years ago
    last modified: 9 years ago

    For larger tractors, the makers frequently brag about "torque rise". My Diesel tractor has maximum torque at 1500 RPM and maximum horsepower at 2400 RPM. If a load is applied that pulls down the engine, it pulls wonderfully as the RPM drops nearer to 1500. Since the travel speed drops directly proportionally to the engine RPM, the horsepower requirement for the load drops also while the torque is rising. Often it pulls right on. But by the time my Diesel drops to 1200 RPM, it is time to quickly declutch and find a lower gear- or the engine will die. We had a couple of identical buses with relatively small engines. Factory RPM max at 2800 RPM. Somebody adjusted one of those to 3200 RPM. On level and downhill, the 3200 RPM gave higher top speed. Going up long hills, they each had the same maximum top speed. Torque does the work, horsepower is a measure of how much work.

  • sdeg3stannic
    15 years ago
    last modified: 9 years ago

    I feel the issue is best addressed by setting standards for Net HP ratings as rated at the mower deck under normal operating rpm's...much as the auto industry changed to in the early 70's, instead of the gross HP ratings. That would reflect "reality". Toque is a system forced upon us by the lawyers...and that alone is enough to make me say...BULL...T It is not a mearsurment that consumers will relate to. HP is. Torque may be relavant to capability, but not for most consumers.

  • danman1
    15 years ago
    last modified: 9 years ago

    Bus driver,
    Welcome to the exclusive small club of people who understand torque on this site!

  • bill_kapaun
    15 years ago
    last modified: 9 years ago

    {{gwi:315110}}

  • mownie
    15 years ago
    last modified: 9 years ago

    bill kapaun! I didn't know cartoonist was in your resume too! :^)

  • oldlawnmowerman81
    13 years ago
    last modified: 9 years ago

    According to the Tecumseh Service Replacement Engines catalog dated October 1, 1985, the recommended operating speeds for the TVS/TVXL, and possibly the ECV and TNT series as well, is between 2000 rpm low and 3200 rpm high.

    The second page of the Horsepower & Torque Rating Curves section list three suggested no-load governed speeds depending on how much available torque is gained at any one of those three speeds: A is 3200 rpm, B is 2900 rpm and C is 2600 rpm. The TVS105, TVXL105 and the TVS120, and also the ECV100, ECV120, TNT100 and TNT120, have their highest torque ratings gained at 2600 rpm no-load. This may explain why the Toro-exclusive TNT series as well as the Sears-exclusive ECV series have notably lower governed speeds.

    I have a 1984 Toro 20672 equipped with the TNT100.

    ~Ben Edge (oldlawnmowerman81)

  • strongteamaker_aol_com
    13 years ago
    last modified: 9 years ago

    SWITCHING ENGINE RATINGS FROM HORSE POWER TO TORQUE IS A GOVERNMENT EPA SCHEME TO DECEIVE THE PUBLIC.

    The US EPA demanded under penalty of Federal Law Suits that all US engine manufacturers comply with new arbitrary small engine clean air regulations. The engine manufacturers screamed bloody murder that these draconian emissions standards would decrease small engine performance and power causing consumers to blame the manufactures and purchase Chinese made small engines. The EPA solved the issue by ordering the American Standards Association to devise a new way to rate engines that would hide the fact that the new emissions regulations would result in weak engine performance, premature engine failure, and increased fuel consumption. Foreign manufactures could still rate engines in horsepower but US manufactures would be compelled to switch to the new deceptive method of rating small engine performance. The torque numbers are larger than the old horsepower and the new emissions conforming engines have less horsepower. There isn�t anyway to equate torque to horsepower they are different; it�s like trying to compare luminosity to velocity.

  • kompressor
    13 years ago
    last modified: 9 years ago

    Tell me... strongteamaker. What's your point?

    You drag out a long-dead thread from the archives; then add your post to it and yet.... I see nothing in those words that point to a source of information to back up what you say.

    If you are quoting something you read, then provide a URL to the source.

    If these are all your own words, then say so.

  • mownie
    13 years ago
    last modified: 9 years ago

    1 tork = 3 hosspower

  • exmar zone 7, SE Ohio
    13 years ago
    last modified: 9 years ago

    {{gwi:315112}}

    Just for "grins and giggles," this photo is of the nameplate or decal on the replacement engine provided by Briggs after the air cleaner fiasco a while back.

    What's interesting is that the original engine was a 22HP, Briggs shipped in a 26. The "authorized" Briggs tech and I examined both these engines side by side and they were identical. everything was a perfect fit up.

    I posted this on the Craftsman Club and I think it was Mr. Bill (NOTE: Heavy on the Mr. :-) ) from Orygun commented words to the effect that changing a decal from 22 to 26 almost made sense considering how these engines are rated.

    I wonder how they increased the HP on identical engines, different jets in the carb? From my standpoint, this engine seems more powerful than the two previous (changed by Sears during the evolution of the air cleaner issue) but that could simply be I was used to engines with poor compression caused by sucking dust down the carb throat.

    I have no real opinion, just glad I've finally got a good engine.

    Just sharing,

    Ev

  • smayer97
    12 years ago
    last modified: 9 years ago

    This is all great and dandy but how can we apply this info? How can you compare older mowers (before 2008) to the newer ones (2008 onward) to make a decision on what to buy?

    I have decided to summarize the pertinent information 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 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 do a comparison but it is based on a few assumptions. (They are not important, just to keep it simple.)

    Don't bother trying to compare size or displacement of a motor shown in cc's for the simple reason 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.

    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.)

  • oldlawnmowerman81
    12 years ago
    last modified: 9 years ago

    So, by many accounts, the "working horsepower" is rated horsepower x 85 percent (0.85), right?

    Say you have an old Briggs & Stratton model 92908 engine. Rated 3.5 hp @ 3600 rpm, but to obtain working HP, do this math: 5.26 (rated torque) * 3060 (85% of the rated HP's RPM) / 5252 = 3.06 working HP. Sounds correct now, doesn't it?

    ~Ben

  • Enginemaster
    12 years ago
    last modified: 9 years ago

    Gross torque is a red herring. As mentioned in a previous posting, torque is a function of engine speed, and as such is limited and reduced by any implement that does not operate the engine at its maximum allowable RPM. Thus the same engine operating a 3600 RPM generator would have a higher gross torque output at full load/wide open throttle than the same engine governed to a maximum of 2500 RPM under a wide open throttle situation, so, yes, the equipment manufacturer should be the one to assign "Gross Torque", not the engine builder. Ever noticed auto/truck engines are rated in both horsepower, -AND- Lb.Ft. torque AT A SPECIFIC RPM? Look at critiques by Motorweek for examples of this.
    CC displacement is a "serving suggestion" and has no DIRECT correlation to horsepower, torque, watts or any other actual measure of work effort or output and as such are a useless way to select an engine at purchase.
    I say, bring back HP as a rating. It worked fine for the last 100 years! We don't rate/purchase lightbulbs as X amps at Y volts, we use a convenient composite measure called WATTS. Do we really need to abandon Horsepower (a similar composit rating) for X torque at Y RPM?
    If manufacturers rated their engines honestly, by a fixed standard (SAE Horsepower, Brake Horsepower), their butts wouldn't be in a legal sling now, and we wouldn't have this marketing/rating morasse!
    BTW, Horsepower CAN be calculated using torque at a specific RPM. To claim otherwise, and refuse to provide conversion data to that effect sounds a bit like B&S to me.

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