# real horse power!! **URL:** \ **Category:** Factual Questions\ **Created:** [November 28, 2016, 12:09am UTC](https://boards.straightdope.com/t/real-horse-power/773246 "2016-11-28T00:09:21Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![mannmundo](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@mannmundo](https://boards.straightdope.com/u/mannmundo)\ **Post date:** [November 28, 2016, 12:09am UTC](https://boards.straightdope.com/t/real-horse-power/773246/1 "2016-11-28T00:09:21Z")
How does rated horsepower translate into real world horse pulling power? ---
**Author:** ![DorkVader](https://avatars.discourse-cdn.com/v4/letter/d/73ab20/32.png) [@DorkVader](https://boards.straightdope.com/u/DorkVader)\ **Post date:** [November 28, 2016, 12:25am UTC](https://boards.straightdope.com/t/real-horse-power/773246/2 "2016-11-28T00:25:06Z")
are you asking how a 1hp motor compares to an actual horse or are you asking how much of that 1hp gets used for actual pulling of a load? ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [November 28, 2016, 12:41am UTC](https://boards.straightdope.com/t/real-horse-power/773246/3 "2016-11-28T00:41:24Z")
From PBS’s _Nature_ ["Horses - Horsepower](http://www.pbs.org/wnet/nature/horses-horsepower/3151/): > [@](#): > > In an age of trucks and trains, it is hard to imagine that a trained team of draft horses once pulled as much as some locomotives. But the term “horsepower” — coined by the English engineer James Watt to measure how much work is done by an engine — suggests just how much respect these animal teams earned. Still, they proved no match for modern engines. Watt concluded, after careful experiments with workhorses, that a one-horsepower engine could pull about 50% more weight in a single day than a horse. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [November 28, 2016, 1:17am UTC](https://boards.straightdope.com/t/real-horse-power/773246/4 "2016-11-28T01:17:03Z")
The Master Speaks: > [@](#): > > How did we end up with the “horse” in horsepower? The term was coined by James Watt (1736-1819), the British inventor best known for his improved steam engines, who used the term to relate steam engine performance to that of horses. At the time horses were the primary energy source for applications ranging from pumping water from mines and turning grinding mill wheels to pulling carts and loads. Although sources differ on exactly how Watt arrived at the number, it’s generally thought that in 1782, he noted how quickly a brewery horse could turn a mill wheel of a certain radius, estimated the amount of force the horse needed to exert to turn the wheel, did the math, and came up with a value of 32,400 ft-lbf/min, later rounded to 33,000 ft-lbf/min. Comparing the power output of a steam engine to an equivalent number of horses was an easy way for prospective engine purchasers to compare power ratings, so the term stuck. > > What type of horse was a brewery horse? In England at the time a work horse most likely would have been one of the three British “heavy breeds” – the Suffolk punch, the shire horse, and the Clydesdale. The Clydesdale is said to have originated in the latter 1700s, perhaps too late to be a common work horse at the time Watt was doing his horsepower calculations. So it seems likely the horse in question was either a Suffolk punch or a shire horse. > **[What’s the difference between horsepower and torque?](https://www.straightdope.com/21343353/what-s-the-difference-between-horsepower-and-torque)** > > A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD Looking around the web, the consensus seems to be that Watt used the average power of the horse to determine his HP unit, but a horse is actually capable of up to 15hp. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [November 28, 2016, 1:19am UTC](https://boards.straightdope.com/t/real-horse-power/773246/5 "2016-11-28T01:19:58Z")
James Watt basically measured how much work a horse could do with tasks like turning a mill wheel or lifting heavy objects a certain distance via ramps or pulleys. He then rounded off the number he arrived at to 33,000 ft-lbs per minute, and that became one “horsepower”. Most of Watt’s measurements were made over a period of about an hour or so and extrapolating from there. People who really worked with horses all day long realized that most horses couldn’t keep up that pace of work for an entire day. I assume that the Nature quote provided by **watchwolf49** was made later after Watt had done a lot of comparisons and challenges against real horses, and found that a typical horse didn’t really make one horsepower over the long haul. ETA: Ninja’d by **Bear\_Nenno** ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [November 28, 2016, 1:40am UTC](https://boards.straightdope.com/t/real-horse-power/773246/6 "2016-11-28T01:40:14Z")
I asked some similar questions a long time ago. Here is one of the threads but some of the links don’t work anymore. The point of the question was that smaller cars and SUV’s often have much higher horsepower than really large agricultural tractors and even some 18 wheelers. The quick summary is that horsepower is only one measure of true power and isn’t the most important one for many uses. Torque is more useful measure for lower speed, high load applications. I know that “horsepower” is an arbitrary measure but I still think it is misleading to do endurance comparisons between biological creatures that tire out over time and mechanical ones that don’t. I have seen tractor pulls with vehicles rated at many thousand horsepower than couldn’t make it to the end of a short run. I am pretty sure that a team of ten thousand+ real horses hitched up properly could have made it just fine if there was a way to coordinate them. > **[Why Do Farm Tractors Have Such Low Horsepower Compared to Cars?](https://boards.straightdope.com/sdmb/showthread.php?t=389226)** > > I am the proud owner of a new John Deere lawn tractor. That has always been a dream of mine. The thing has 19hp which can do a lot of work but seems puny compared to even a crappy car. Ok, it is a lawn tractor and will be used mainly for mowing.... ---
**Author:** ![Dewey\_Finn](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dewey_finn/32/4222_2.png) [@Dewey\_Finn](https://boards.straightdope.com/u/Dewey_Finn)\ **Post date:** [November 28, 2016, 1:50am UTC](https://boards.straightdope.com/t/real-horse-power/773246/7 "2016-11-28T01:50:41Z")
Top Gear formerly used the term “brake horsepower”. What does that refer to? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [November 28, 2016, 1:54am UTC](https://boards.straightdope.com/t/real-horse-power/773246/8 "2016-11-28T01:54:33Z")
> [@Dewey\_Finn](#): > > Top Gear formerly used the term “brake horsepower”. What does that refer to? It refers to the measurement device - the [Prony Brake](https://en.wikipedia.org/wiki/Prony_brake). ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [November 28, 2016, 2:10am UTC](https://boards.straightdope.com/t/real-horse-power/773246/9 "2016-11-28T02:10:46Z")
> [@Shagnasty](#): > > The quick summary is that horsepower is only one measure of true power and isn’t the most important one for many uses. Torque is more useful measure for lower speed, high load applications. I know that “horsepower” is an arbitrary measure this is over-simplified to the point of being incorrect. Horse **power** is a measure of the engine’s actual output power (i.e. the rate at which it can do work.) The “horsepower” _unit_ may seem arbitrary, but the actual stated power isn’t. hp maps pretty cleanly to Watts, the SI unit for power. 1 SAE horsepower = 1.014 PS = 746 Watts. Torque is a static force, and torque alone cannot do any work therefore it is not a measure of power. what people misunderstand about an engine’s torque output is that it doesn’t have anything to do with the engine’s actual power output. In a vehicle application- say, a truck- the engine’s horsepower tells you how much of a load it can pull. The engine’s peak torque and torque curve _tell you how you’re going to need to gear the vehicle so it can pull that load effectively._ Theoretically, a 400 hp 5.0 liter gas V8 in a truck could haul the same load as a 400 hp, 12 liter turbodiesel. you would just need a transmission with an insane ratio spread (read “many forward gear ratios”) and it would be screaming at peak rpm constantly. or, to put it into an example of stationary equipment, lets take a generator set. both the 400 hp 5.0 V8 and the 400 hp 12.0 turbodiesel could drive an equally powerful generator. The V8 would be screaming at about 6,500 rpm while the diesel would be loafing along at about 1700. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 28, 2016, 2:41am UTC](https://boards.straightdope.com/t/real-horse-power/773246/10 "2016-11-28T02:41:28Z")
Huh, and all this time I’d assumed that “brake horsepower” was the horsepower capability of the brakes, not of the engine (that is to say, 1 brake horsepower could decrease the energy of the vehicle by 746 joules each second). And an engine’s torque divided by the transmission’s gear ratio will give you the torque at the wheels, and the torque at the wheels divided by their radius will give you the force that the vehicle can exert. So that’s what determines if you can budge a load at all. You can take a very low power engine, and gear it down enough to pull an arbitrarily large force. But if you do that, you’re going to be pulling it only very slowly. Your force times your speed will give you your power. Or, equivalently, your torque times your rotational speed, which is convenient because you can make that measurement at any point in the drivetrain (before or after the transmission gears) and get the same answer. ---
**Author:** ![lazybratsche](https://avatars.discourse-cdn.com/v4/letter/l/ba8739/32.png) [@lazybratsche](https://boards.straightdope.com/u/lazybratsche)\ **Post date:** [November 28, 2016, 1:23pm UTC](https://boards.straightdope.com/t/real-horse-power/773246/11 "2016-11-28T13:23:37Z")
> [@jz78817](#): > > Theoretically, a 400 hp 5.0 liter gas V8 in a truck could haul the same load as a 400 hp, 12 liter turbodiesel. you would just need a transmission with an insane ratio spread (read “many forward gear ratios”) and it would be screaming at peak rpm constantly. > > or, to put it into an example of stationary equipment, lets take a generator set. both the 400 hp 5.0 V8 and the 400 hp 12.0 turbodiesel could drive an equally powerful generator. The V8 would be screaming at about 6,500 rpm while the diesel would be loafing along at about 1700. Could that V8 maintain that kind of load for long? A typical truck’s engine has to produce peak power output long enough to climb a mountain range (an hour or more?) Even on a track, a sports car’s engine isn’t going to be producing uninterrupted peak power for more than a few minutes at a time, or for more than tens of minutes in total. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 28, 2016, 2:02pm UTC](https://boards.straightdope.com/t/real-horse-power/773246/12 "2016-11-28T14:02:58Z")
> [@Chronos](#): > > Huh, and all this time I’d assumed that “brake horsepower” was the horsepower capability of the brakes, not of the engine (that is to say, 1 brake horsepower could decrease the energy of the vehicle by 746 joules each second). Brakes can be far more powerful than the engine. At 30 MPH, 1 G of acceleration on a 3500-pound sports car requires (30_5280/3600)_(3500)/550 = 280 horsepower. Now imagine you’ve gotten that same sports car up to 150 MPH, and you slam on the brakes, decelerating you at 1 G; same force (3500 pounds), but five times the speed, so 280\*5 = 1400 horsepower. Now imagine a 1500-pound Formula 1 car: 180 MPH, 5G of decel = 3600 horsepower. You can start to see why the brakes on those cars are so damn big and use exotic materials and lots of venting to manage the heat. As for the term “brake” in reference to measuring engine power: as described upthread, it refers to the power as measured at the flywheel by an external mechanical brake. The “brake” force may have been named for the Prony brake, but these days it refers to any externally applied torque used to limit engine speed while the engine is making power. This can be an electric motor/generator, an eddy-current absorber, a water brake, or the classic Prony brake. The “brake” distinction is important because there are a few different ways to measure engine power: -brake power: this is the mechanical power output at the flywheel, the useful thing you want. -indicated power: this is the power transferred from the combustion gases to the piston face. You calculate this based on instantaneous cylinder pressure (measured with a high-speed pressure transducer) and piston speed. -pumping power: this is the power being spent to move fresh air/mixture into the engine and move exhaust gases out. At high speeds and light loads (especially on a throttled gasoline engine, as opposed to an unthrottled diesel engine), pumping power can be significant (relative to the brake power). -friction power: this is the power lost to friction in all of the engine’s mechanical interfaces: bearings, seals, crankshaft windage, and so on. Shadetree mechanics and hotrodders generally only need to care about brake power, but if you’re in a test cell doing engine development, then all of those things start to matter. Generally speaking, total crankshaft power output isn’t referenced in engine development as often as [mean effective pressure (MEP),](https://en.wikipedia.org/wiki/Mean_effective_pressure) which normalizes power relative to speed and engine displacement; ultimately, MEP is proportional to torque divided by displacement and provides a convenient way to compare engines with different displacements operating at different speeds. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [November 28, 2016, 2:58pm UTC](https://boards.straightdope.com/t/real-horse-power/773246/13 "2016-11-28T14:58:05Z")
> [@lazybratsche](#): > > Could that V8 maintain that kind of load for long? A typical truck’s engine has to produce peak power output long enough to climb a mountain range (an hour or more?) sure, so long as the cooling system is up to the task\*. that’s why a 400 hp 18-wheeler has an enormous radiator compared to the little one in a 400 hp car. The car’s cooling system can’t actually keep the engine cool making peak power for very long; it’s capable of doing so in short bursts so long as the car is moving at speed. The truck is assumed to spend much more of its time making close to peak power, and at slower vehicle speeds. > [@](#): > > Even on a track, a sports car’s engine isn’t going to be producing uninterrupted peak power for more than a few minutes at a time, or for more than tens of minutes in total. correct. water also acts as something of a “thermal buffer,” its high specific heat capacity means it can absorb a lot of energy before its temperature increases. - now, the engine’s overall longevity will not likely match that of the diesel. one of the key reasons diesel engines have a reputation for lasting a long time is that they generally operate at lower RPM; your average heavy truck engine redlines at about 1900-2100 RPM. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 28, 2016, 3:19pm UTC](https://boards.straightdope.com/t/real-horse-power/773246/14 "2016-11-28T15:19:48Z")
> [@](#): > > Quoth **Machine Elf** : > > Brakes can be far more powerful than the engine. Can be and usually are. As I understand it, there’s no car on the market that will move if you floor both pedals, and while it takes an extraordinary car to accelerate at the tires’ friction limit, most of them can decelerate at that limit. Failure to grasp this distinction has also been behind multiple perpetual-motion devices. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [November 28, 2016, 3:54pm UTC](https://boards.straightdope.com/t/real-horse-power/773246/15 "2016-11-28T15:54:25Z")
> [@Shagnasty](#): > > I asked some similar questions a long time ago. Here is one of the threads but some of the links don’t work anymore. The point of the question was that smaller cars and SUV’s often have much higher horsepower than really large agricultural tractors and even some 18 wheelers. The quick summary is that horsepower is only one measure of true power and isn’t the most important one for many uses. Torque is more useful measure for lower speed, high load applications. … Torque value by itself is meaningless, it does not tell you anything about the performance of the vehicle. But the torque _curve_ matters. As I understand, car engines only develop their rated horsepower at very high RPM. So a 200-horsepower car engine may only develop 200 horsepower at 7500 rpm, and cannot maintain it for very long. A 200-horsepower tractor engine would be much bigger and heavier, but it may output 200 horsepower at 2000 rpm, and keep doing it all day long. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [November 28, 2016, 6:49pm UTC](https://boards.straightdope.com/t/real-horse-power/773246/16 "2016-11-28T18:49:04Z")
> [@Shagnasty](#): > > I asked some similar questions a long time ago. Here is one of the threads but some of the links don’t work anymore. The point of the question was that smaller cars and SUV’s often have much higher horsepower than really large agricultural tractors and even some 18 wheelers. The quick summary is that horsepower is only one measure of true power and isn’t the most important one for many uses. From an engineering perspective, power is power; there’s no “true” power and “other” power. Power output (measured in horsepower, kilowatts, or BTUs per hour, or whatever unit you like) is just one aspect of powerplant performance; there’s also torque (more importantly, the entire torque-versus RPM curve), RPM range, turndown ratio (the ratio between the maximum and minimum operating RPM of the engine), diesel vs. gasoline, number of cylinders, and on and on. As you’ve noted, a [6.2-liter Chevy LT1 engine](https://en.wikipedia.org/wiki/LS_based_GM_small-block_engine#4.06_in_bore_blocks_.282014_.E2.80.93_current.29) can make about 450 horsepower, and so can a [15-linter Cummins diesel engine -](https://en.wikipedia.org/wiki/Cummins_ISX) but you wouldn’t want to put the Corvette engine in an 80,000-pound big-rig, because you’d fry the clutch just trying to get the whole thing moving. > [@Shagnasty](#): > > I know that “horsepower” is an arbitrary measure but I still think it is misleading to do endurance comparisons between biological creatures that tire out over time and mechanical ones that don’t. > > I have seen tractor pulls with vehicles rated at many thousand horsepower than couldn’t make it to the end of a short run. I am pretty sure that a team of ten thousand+ real horses hitched up properly could have made it just fine if there was a way to coordinate them. “Horsepower” isn’t about endurance (other than back in the day, that’s what Watt figured a good draft horse could put out at a steady pace without running out of breath). It’s also very rarely the limiting factor at tractor pulls; instead, traction is the problem. A team of a thousand Clydesdales would probably have pretty good grip (by virtue of their 1.8 million pounds of weight), and would probably have no trouble dragging the sled - but the same would be true of a 1000-horsepower tractor, if you could just get a good enough grip on the soil; the tractors that don’t make it never stall their engine, they just sit there throwing dirt. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [November 28, 2016, 7:51pm UTC](https://boards.straightdope.com/t/real-horse-power/773246/17 "2016-11-28T19:51:07Z")
> [@scr4](#): > > Torque value by itself is meaningless, it does not tell you anything about the performance of the vehicle. But the torque _curve_ matters. > > As I understand, car engines only develop their rated horsepower at very high RPM. So a 200-horsepower car engine may only develop 200 horsepower at 7500 rpm, and cannot maintain it for very long. only because of cooling limitations. we run engines on dynamometers for hours on end at peak load, but they have basically infinite cooling capacity (facility cooling towers.) With a big enough radiator and enough coolant on hand, a 200 horsepower car engine could crank that out all day long. an example that comes to mind is my local gas utility’s trucks. They buy Ford F-750s, which are Class 6 medium-heavy trucks. Instead of diesels, they buy them with the 6.8 liter Ford V10 with CNG prep. Makes sense, since as a gas company they can fuel the trucks on site. I was driving alongside one, and could hear the engine screaming at 5,000+ RPM. but, they still have big, heavy truck radiators and coolant capacity, and apparently they last just fine. > [@](#): > > A 200-horsepower tractor engine would be much bigger and heavier, but it may output 200 horsepower at 2000 rpm, and keep doing it all day long. because it has a cooling system designed for a 200 hp engine being operated in a low-speed vehicle. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [November 29, 2016, 1:40am UTC](https://boards.straightdope.com/t/real-horse-power/773246/18 "2016-11-29T01:40:32Z")
Oh, and to get back to living organisms, even a human (at least, an extremely athletic one) can put out half a horsepower for periods of an hour or more, and over a horsepower in short bursts. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [November 29, 2016, 1:55am UTC](https://boards.straightdope.com/t/real-horse-power/773246/19 "2016-11-29T01:55:30Z")
> [@Chronos](#): > > and over a horsepower in short bursts. [Just enough to make a slice of toast](http://www.dailymail.co.uk/news/article-3112063/Can-Olympic-cyclist-power-toaster-Science-experiment-shows-energy-REALLY-takes-make-simple-slice-toast.html) (well, 0.94 horsepower at least). ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [November 29, 2016, 4:16am UTC](https://boards.straightdope.com/t/real-horse-power/773246/20 "2016-11-29T04:16:13Z")
> [@Machine\_Elf](#): > > … the tractors that don’t make it never stall their engine, they just sit there throwing dirt. And, occasionally, [various engine parts](https://www.youtube.com/watch?v=qBF6iZfguF0). [Next page](https://boards.straightdope.com/t/real-horse-power/773246.md?page=2)