Small engine power ratings

Whats with the advertising of the power rating on small engines? Bought a small power washer to cleaning my tractors and equipment.On the shipping box it states 5.5 of torque,no H P rating at all. We bought a riding mower several years age, same thing,both are powered with Brigs.

Small engine HP used to be wildly overstated. About ten years ago a class action lawsuit put an end to that.

I think: torquexrpm/5252=HP

In this case, if it is governed to 3,600 rpm, it would produce 3.77 hp at 3,600.

Someone said"HP is what you buy, torque is what you drive". Kind of makes sense.

Garry

As ss55 indicated there was a class-action lawsuit brought against engine manufacturers for over-stating their engine's power output. From what I understand the ratings were being taken at engine speeds that were not attainable in the real world, primarily because blade tip speed regulations limited the maximum engine speed to around 3000 rpm for walk-behind mowers. The old power ratings were generally taken at 3600 rpm. My thinking is that the companies didn't want to lose face by de-rating their engines to account for the reduced speed so Briggs and others started rating the "power" using torque instead. This is actually quite silly since torque is not a unit of power at all and it tells you nothing about the work capacity of an engine until it is combined with the rotational speed at which it is being delivered. The propaganda video that Briggs has out on YouTube trying convince us that torque is such a good "power" rating is really laughable.

Funny now a days. I bought my wife an X340 Deere lawn tractor a few years ago and WON 10 dollars in a class action suit because they over rated the 25 hp Kawasaki engine. My old Cub 1863 with an 18 hp Kohler could out-pull 8 of those Kaws.

There was a big court battle about HP. It even got into the big standby generators. I spent several months doing load bank test. To prove a generator could pull full load.

You can thank the lawyers for Hp rating going away. Briggs got in trouble for over rating hp.
Now most engines post just the CC's.

HP calculation is a messy equation but boils down to this: HP = [Torque (in ft-lbs) x rpms]/ 5252. Most 4 cycle (stroke these days) engines are rated without anything attached including muffler at 3600 rpm. So take your 5.5 x 3600 divide by 5252 and you have your net hp: 3.8.......nice funny number. So, considering the old days and the same engine design and all, very well be, was it advertised as a 4 hp? Probably.

Briggs, Kawasaki, Honda, Kohler, et. al. got involved in a class-action law-suit over HP claims. So now - it is kind of rare to see any small engine maker cite HP figures.

What exactly IS horsepower? It used to be common for makers to use different ratings depending on how hard the engine was used. "Rated" power versus "max" power. Not just claims with a momentary max rating the engine could never endure all day long.

Specs were much more complete for such engines back in the 30s-60s.

Yup. The horsepower on cars is calculated with the motor spinning at something like 10,000 RPM. Good luck with that.

Same with the small engines, hp was calculated at 3600 RPM but the motor can't run that fast without flying apart.

Outboard motor manufacturers used to do the opposite. They'd advertise a 10HP motor that had been measured at 4500 RPM but designed to run at 5000. Then they'd show it running away from the competition's 10HP motor.

We bought a new zero-turn mower last year, it seemed like everything had a hp rating.

I was actually thinking about this while I was mowing yesterday. I have a 1983 ZTR with a 60 inch deck. It uses a hydro on each wheel to power it. It has a 16 horse Kohler Magnum in it. If that mower were made today it would likely have 25 horsepower and have no more power than that old Kohler has. I've never found anything that it bogged down doing or I felt was lacking power. I'm not sure there's a new engine I trust enough. I think I'll just keep plugging away with this old thing.

Good explanation of the horse power bugaboo. One of my pet peeves is horse power talk.

Peak torque was probably measured at 2400rpm . Torque will have dropped up at the 3600rpm level.

Minor problem with the standards used to measure the horsepower- US used SAE method that meant peak power with no added equipment- like mufflers, generators, air cleaners- and the small engine got the same bare of equipment, run to peak power ratings. Germany used a different method- DIN- and the the N was 'normal' in use, meaning mufflers, etc. classic was the VW Beetle 1200cc rated at 30 hp in Germany that became a 36 hp in US market. Some OEM engines had multiple ratings- a 'peak rating' by SAE standards, then the power as the user meant it to be used, Tecumseh and Kohlers had 15 hp@4400rpm or more peak engines that were also rated at 1800 rpm at 5 hp, 8 hp at 2200 or 2400 rpm. The generator maker would advertise the 5hp rating, competitor would advertise 12 hp(peak or close to peak) and customers would get confused sometimes- and buy the one with more advertised power. Mid season the Tecumseh would be advertised at same power as competitor, same leftover stock, just a new sticker slapped on shroud over old sticker. Cars in mid 1970s or so had to advertise 'wheel horsepower' instead of SAE- the small block chevy 327 that was 225 hp previous year became a 150 hp engine. Small engines had the advertising race as engines- the manufacturers would sometimes advertise as user hp, other as peak and finally legal hassle that said same comparable method would be used or payment to lawyer expected. Torque rating at specified rpm is comparable, more so than varying hp depending on governor settings. And actual displacement is specific, easy to measure, less subject to legal hassles by bone picking bar buzzards. Some small engines were advertised by real old 'taxable' power ratings- simple measure the bore size, no concerns with governed rpm, stroke, compression ratios, added equipment- and the advantage for market was everyone met same standard for the 'taxable' hp. The Brigges 196cc is maybe a 5hp (or 3.5)- the Harbor freight 'Predator' 212cc is 6.5 hp advertised. Need to check governed speed as installed for own figure of hp, maybe it is advertised as to be used, maybe as peak- then you check warrantee papers. RN

I have a shop vac that claims to be something like 5 peak horsepower which is of course ridiculous as I think it draws less than 10 amps.

Thanks. Mfgr. misrepresentation on the subject of motors and engines is one of my peeves too.....the vacuum cleaner is the one that really chaps me.

True, question is how much. Lotta curves are pretty flat.

The vacuum cleaner draws more than 10 amps for the few seconds it makes a peak of 5HP.