How a dyno measures horsepower

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When it comes to superbikes, everyone appears to be very concerned with horsepower. Michael Czysz and Chip Yates go out of their way to make fantastic claims of over 200hp for their motorcycles. BMW claims the S1000RR makes 193 and the ZX10R is supposed to be 197. Even my GSX-R 600 has a claimed 126. But what is horsepower? How do we measure it? And most importantly, why does it matter?

What is horsepower?
Back in high school physics, you should have learned that horsepower is work over time. To get technical, horsepower is 33,000 ft·lbf/min (supposedly, the amount of work one actual horse can do in one minute). Back in the day when James Watt was making steam engines, he estimated how much work he believed a horse could do in one minute. He came up with a number slightly over 32,000 and rounded that up to 33,000. From the looks of his math, he also rounded off decimal points, and relied heavily on estimation. Horsepower has always been mostly about marketing, and less about actual work being done. He was a savvy guy. By exaggerating the amount of work a horse can perform (a thing people could understand then), Watt’s steam engines all of a sudden became that much more impressive to prospective buyers.

How do we measure it?
These days we measure horsepower with an extremely complex device known as a dynamometer, most commonly known as a dyno. The first step is to strap a bike down to the dyno; in this case we’re testing an Aprilia RSV4 on an industry-standard Dynojet dyno.

During the test, the bike stays stationary while the back wheel turns a large roller braked by a computer. The technician hooks up the necessary cables to the bike. These electronic sensors measure variables including ambient air temperature, humidity and oil temperature (if you want to get fancy), while also recording the bike’s RPM.

At the start of the test, the bike is warmed up, and then the technician initiates the dyno run. In the case of the Aprilia, he ran the bike at wide open throttle in fourth and fifth gears until it hit redline in each. It’s a rather quick process. The force the bike outputs on the roller is torque. To get horsepower, the dyno uses that recorded number, RPM as a measure of time, and a number of other variables in a complex calculation (read: estimation) that spits out the magic horsepower number.

The dyno takes the information it has gathered and uses sophisticated software to print out a report. The y-axis represents horsepower. The two power curves show performance in fourth and fifth gears. We can see that the Aprilia builds horsepower somewhat steadily, has a dip in the mid-range, and builds more horsepower until it hits redline. The best run on the RSV4 showed a horsepower of 163.91 at the rear wheel.

In a perfectly mathematical world horsepower should be calculated as torque multiplied by RPM divided by 5,252 (a constant derived from (33,000 ft·lbf/min)/(2π rad/rev)). This isn’t quite the way a dyno works; a Dynojet dynamometer also applies a proprietary correction factor with an explanation that would make your head spin. It’s supposed to account for all those above operating variables, like ambient air temperature and humidity. In theory, it helps produce numbers that are repeatable and comparable with reports from other dynos from other parts of the world in different environmental conditions. In reality, there are still a number of dyno manufacturers and variables in play with proprietary “correction factors” and different hardware, so claimed horsepower numbers should usually be taken with a grain of salt. Or several.

Why does it matter?
The need for the correction factor explains EXACTLY why horsepower numbers matter. They allow us to compare engine performance between different bikes.

How did the Aprila RSV4 stack up to manufacturer claims?
Aprilia advertises 180 horsepower at the crank. The crank is the first point where rotational force is created. We historically give value to horsepower numbers measured at the crank because when it comes to cars, that’s a measurable number. The transmission sits outside of the engine in an automobile, so the power sent from the engine is coming straight from the crank. However, there’s no way to test horsepower output at the crank on a motorcycle.

Horsepower is output through the primary drive to the transmission and finally leaves the engine casings at the countershaft where the sprocket sits. Your first opportunity to measure horsepower of a motorcycle would be at the countershaft, well after the crank. There are dynos that simply read power from that point, instead of sending the power down the chain through the rear sprocket, through the cush drive, and then finally through the wheel and tire.

As the horsepower travels to the point of measurement, a fraction of the power is lost. At the wheel, our dyno test indicated a horsepower of 163.91 for the RSV4. From that number, it’s safe to assume that Aprilia’s 180 horsepower at the crank claim is on par with the bike’s capable performance since measuring at the wheel gives you a smaller number than measuring at the countershaft, and measuring at the countershaft gives you a smaller number than the untestable number we could get from the crank. But the reality is that they have not measured crank horsepower on a production motorcycle engine (pre-unit Triumphs and Harleys excepted, though as far as I know there’s nothing there to measure). And neither has any other manufacturer because there is simply no way to do so.

So motorcycle manufacturers are lying?
Where the manufacturers’ numbers come from is a mystery to me. They might just take the engineers out to a bar, feed them booze for a few hours and then ask them to estimate (read: fabricate) some numbers. They certainly don’t strap the bike to a dyno, do some runs and then print out a number, so I consider the math to be fuzzy and the claims to be estimates. They are estimates that help us compare bikes and really the actual numbers aren’t the lie we should be concerned about.

The important lie is one of marketing: More horsepower equals a better bike (car, etc.). The automotive and motorcycle industry has been lying to people and making up ridiculous power numbers since the dawn of the combustion engine, thanks to the savvy Mr. Watt. The actual peak horsepower number itself doesn’t really mean much in terms of superior performance. The market demands high ones, but doesn’t necessarily understand what they mean. A smooth power curve with lots of power everywhere is much more useful than a very high peak number. If you have a motor that makes 200hp and redlines at 8000 rpm, but only 100 at 7000 rpm and less than 80 anywhere below that, you have a steaming pile of dog shit that’s not very useful for accelerating a vehicle. But what if it makes 200hp from 10,000 rpm to it’s 19000 rpm redline? That would be quite the trick. A smooth 9000 rpm wide band of power would be very useful indeed. The Aprilia RSV4 demonstrates a similar smooth power curve, and that’s why it is an impressive achievement in motorcycle technology. Not because of an arbitrary and unmeasurable number equaling 180.

  • Denzel

    Love these types of pieces Sean. Very well put together. Makes me want to believe everything I read on the internet…

    • Ben Incarnate

      Agreed. This is a great article, Sean. I hope there’s a followup that goes into the importance of and how to read full dyno curves – both HP and torque.


    • octopodes


      These types of articles make me so proud to support this site. Thanks!

  • Eric

    Good write up.

    Check your units early in the page.

    • Sean Smith

      What are you referring to?

      • Justin

        In your “What is horsepower?” section, you refer to lbs as a unit of work. To be pedantic, a pound force or lbf (less ambiguous than lbs or lb) is a unit of force, and a foot pound or ft-lbf is a unit of work. If you just would’ve gone with SI units you could’ve called a hp 745.7W and avoided all this unit conversion mess. ;)

        • Sean Smith

          Ha. Yes; I may have oversimplified in my incredibly basic explanation. Thankfully most of my target audience isn’t engineers. ;) Thanks for the explanation.

          • Eric

            There’s always a few of us to mess it up for everyone else. ;-)

        • Joe

          For those of us that are not european, SI is French for Metric System.

          Good write up though, I love the technical stuff.

  • Myles

    But the 180hp bikes are faster than the Ape? So it’s not just a marketing scheme, but a measure of performance?

    Raperilla ftw?

    On a happier note, running a bike on the dyno is fucking fun – everyone should do it. Even if you’re bike makes literally half the power of the Roman’s like mine.

    • Sean Smith

      This isn’t about what’s faster.

      It’s an inside look at how horsepower is measured and what that means to consumers. Is a ZX10R faster than an RSV4 R APRC? Or an S1000RR? Or an RC212V? Who knows.

      If you put all those bikes on the same dyno, one after the other, you would be able to compare their horsepower numbers. If you put them on the track, it would depend mostly on the rider.

      Even using the same rider, lets say Dani Pedrosa because he’s fast and is used to the referenced RC212V, he wouldn’t be used to the other bikes in question, would have his own riding style that would work well with some bikes, but not with others and we haven’t even taken into account gearing, tires and suspension.

      A dyno number has very, very little with how fast a bike actually is.

    • Wes Siler

      Yeah, it’s much more about feel, confidence and components like suspension and tires and brakes than it is about peak horsepower. I’m faster on the RSV4 APRC pictured here than I am on an S1000RR, R1 or 1198 S. The engine feels better when you’re using it than any other liter bike I’ve ridden too, that nice full curve is way more user friendly than a peaky one. That plateau in the top 1/3 of the rev range is just beautiful to use.

      • Myles

        I totally hear you guys when it comes to usable power on the track, but at the end of the day a higher peak horsepower number (when vehicles are of similar weight) almost directly correlates with how “fast” a bike is. 1/4 mile, standing mile, top speed, etc.

        The closing paragraph seems to argue that high dyno numbers are bullshit, but in the real world the bikes with high dyno numbers (s1k and zx10) have hecka motor.

        Also, Pedrosa is fast on everything as long as Simoncelli isn’t around. The federation needs to put that guy on a Suzuki as punishment.

        • Sean Smith

          1/4 mile, standing mile and top speed aren’t really relevant when you’re talking superbikes though. Sure, a guy like Skank will buy the bike with the most power because he’s heavy and rides wide-open in sixth all the time, but how many other guys are around doing that?

          The flip-side to that is: Peak power is extremely easy to make. Just bolt on a turbo, or if you’re feeling spendy, swap out the stock bottom-end with unobtainium, port the head and have some monster cams cut. Anything will make big power with that treatment.

          For proof, look no further than any bike setup for racing 1/4 mile, standing mile or land-speed.

          I firmly believe that most dyno numbers are bullshit. The variance in equipment, operating procedures and machine prep couldn’t lead to anything else.

          Throw a well-lubed, non-o-ring chain on anything and watch it pick up at least 2.5 hp. Run an old tire and the same thing happens. Get the bike on strapped on just a hair sideways and you’ll lose power. Is there an exhaust sucker? How powerful are the cooling fans? Is there a giant blower pointed in the intakes? What correction factor is being used? Does that particular dyno happen to read high?

          There are very few places in the world where all of these things have been addressed. Yoshimura is one. Hasselgren Engineering is another. None of them are small businesses where you can take your bike and have some pulls made.

          The last point I have to make is so important that it should be restated: You cannot measure hp at the crank on a motorcycle engine. It’s just not possible. Sure, I suppose you could tear the stator or primary drive apart, plug up some oil passages and bolt the thing to an engine dyno, but that wouldn’t be a configuration of the motor that would ever be used. Pull power from the end of the crank instead of the middle (which is how modern 4 cylinders are setup) and the dynamics of the motor will change.

          The altered rotating mass of missing and changed accessories would also throw that number off.

          Yet, manufacturers continue to put out crank hp figures.

          • Case

            This comment from Sean explains why I get frustrated when somebody starts quibbling about dyno numbers. (I don’t mean frustrated with Sean, I mean frustrated with the morons.) DUDE I GOT A 3 HP IMPROVEMENT FROM MY TWO BROS SLIP ON BRO!!!11!! That kind of number isn’t statistically relevant, even with the adjustment algos used by the dyno software. Also, peak HP is only a small element of what makes an engine useful.

            HP is a starting point for a discussion but not as relevant as how the power is delivered. Thank you for explaining this.

          • Janak

            Great Article… Can you please do suspension… & Setup

            • Sean Smith

              Sorry, I’m not that great with voodoo.

      • Chris

        I agree with your comment Wes. I’ve spent a fair bit of time on various generation R1′s, for example, and the power delivery makes them comparatively difficult to ride fast on a twisty road – too little response down low and too frantic up top was my feeling.

        The 1198S is amazing, but its tendency to want to wheely in side-to-side transitions under power, even when running a gear high makes it a beast to ride.

        I have a 2010 R w/ race mapping and a Leo slip-on, and the power is even more linear. I have not yet ridden a APRC (2011) mapped RSV4, but I understand they’re smoother and even more usable.

        Thanks for the article and to Wes for volunteering his bike/data for it.

    • Steven

      I didn’t see any claim about a motorcycle being faster than another motorcycle in the article.

    • Chris

      Faster where/how? Around what circuit?

      Top speed, the BMW is probably going to be faster at the end of a 3 mile airstrip than any other widely available production bike. That said, MCN’s 2010 top-speed test had the ape as #2 at 189mph.

      What the Aprilia loses in peak HP, it gains in reduced drag due to the width of the motor, which is far narrower than the I4′s.

      Read this month’s issue of Motorcyclist for one set of opinions – all of their riders were fastest on the RSV4 Factory SE over the BMW, 11′ ZX10 and 1198SP (TC equipped liter-bike comparison) around Chuckwalla Valley Raceway. The difference between the BMW and the Ape was about 1 second a lap.

      I would expect that a UK version ZX10 (or one with shorter than stock gearing) or on a bumpier track, or perhaps with the 2011 V4R the order would get mixed up. They’re all great bikes – buy the one which suits you best.

  • Kevin

    In case anybody else was wondering, that character that looks like a lower case N is actually pi.

  • Gregory

    Mine goes to eleven.

    Portland, OR
    2008 Kawasaki KLR 650

    • tomwito


    • Liquidogged

      Why don’t you just make ten louder?

      • Sean Smith

        “But this one goes to eleven.”

  • Chris

    I never even stopped to consider how an oem would test power at the crank. This is a great article, thanks for being so informative. You earned my dollar this month for sure!

  • Your_Mom

    The manufacturers do use dynamometers. Are you seriously implying that they don’t? I worked for one and their posted numbers were always *lower* than tested numbers. Why? Lawyers.

    • Your_Mom

      Oh – this is high-school physics. you don’t need to be an engineer to know the difference between units of force and units of torque.

    • Sean Smith

      So the numbers they put out are fabricated by the lawyers and not engineers?

      • Andrew

        *clap* *clap* *clap*

  • BuellDoc

    A Dyno is just another tool to bring in the money. I have seen many a sad face after a bike is dynoed…”I thought It had alot more HP!”
    The numbers game will drive you insane..and Broke!

  • mugget

    Nice one. I vaguely recall reading something about the manufacturers claims… but good to be reminded and keep that in mind.

    Keep up the good work!

  • Sam

    By ::UNDER:: exaggerating the amount of work a horse can perform (a thing people could understand then), Watt’s steam engines all of a sudden became that much more impressive to prospective buyers.

    Horses have substantially more than one horsepower… In fact, most adult males have about one horsepower in their legs alone, another easy high school physics experiment.

    • Sean Smith

      Watts numbers certainly had some guesstimation in them.

      While it’s true that a horse can inexplicably churn out 15 or so hp for a short burst, Watt claimed that they would do 32,400 ft-lbs per minute while they were working. He then took that number and rounded up to 33,000.

      This meant that you could take a horse that was a ringer, put it up against one of his steam engines and the steam engine would over-deliver on power claims.

      He was under-promising and over-delivering. And there we have the story of the time when Horsepower was invented as a marketing tool.

  • Skank NYCF

    Just learned something new. Never knew where HP originated from.

  • jon

    Good article… but it felt like you ony scratched the surface. What about torque vs HP?

  • Ola

    Torque vs hp is already well explained. Torque is the force, the push of the piston. How often the piston can push with that force makes hp. Let’s say you’re moving house. If you can lift 50 lbs box and your friend only 25, you can do twice the work if your trips are equal. But if he can make 4 times the trips, he’s developing twice the hp.

    Also, crank vs wheel hp is no black magic. It’s easy (for engineers) to measure the force needed to spin the drivetrain and wheel hp + drivetrain losses = crank hp.

    • Eric

      Good analogy.

  • Гена

    “But what if it makes 200hp from 10,000 rpm to it’s 19000 rpm redline?”
    This is wrong. A good engine would make constant torque over a range of rpm, not constant horsepower. As you mentioned in the article, horsepower is a product of torque and rpm. So the horsepower should steadily rise over the range of RPM, exactly as it does in the graphs above.

    • HammSammich

      Maybe I’m reading it wrong, but the second HP curve seems to be pretty flat over what appears to be about 5,000 rpm…(the revs aren’t indicated on the x-axis, so I’m guessing).

      I agree that for most street riding, it would be preferable to have a very wide/flat torque curve, but for racing, that doesn’t generally seem to be true…

      • Гена

        The curve is at an angle. If the bike delivered a constant power over the range of rpm, the curve would be straight and parallel to the X-axis

        • HammSammich

          There are two HP curves. As you note, the first is fairly steep and drops off at a much lower RPM. The Second is at an angle up to (I’m guessing) maybe 10-11k RPM, where it levels out and remains between 150-160hp all the way to the redline. You’re never going to have a curve that is perfectly parallel to the X-Axis, as no internal combustion engine can produce 100% of it’s peak power through it’s entire RPM Range…but this engine is still indicating relatively constant horsepower for a significant portion of its range.

          Again, I agree that torque is a more telling number, and it’s what you’re going to feel with your “butt-dyno” but it’s not represented in these charts, as far as I can tell.

          • Sean Smith

            The first curve is from a fourth gear run and the second is from a fifth gear run. It drops off at 13,700 rpm.

  • Thom

    I can’t speak for the other M/C OEM’s ( over stating their bikes horsepower ) but as for Aprilla .


    Overstating horsepower by Italian M/C and Auto Makers ( especially Ferrari ) has been a decades long tradition .

    Along with overstating Top Speed as well as Acceleration figures . You kind of get used to it after awhile .

    Hyperbole being Italy’s 2nd Official Religion :o)

    • Wes Siler

      Well, making 163+hp at the wheel puts them on par for 180+ at the crank, so there’s no suggestion of a lie there.

      I think you’re missing the message here that HP is a calculation, not a measurement, with numerous variables, some of which are more or less guesstimates.

  • Xenophya

    I wish power-to-weight ratio was published more. As you say, a hp figure alone tells little of the story. What good is 200bhp if you have to lug around half a ton?

    That way we might see a shift to more efficient bikes as a whole. Less weight requires less power for equivalent performance, which in turn improves handling, fuel economy etc. etc.

    • Ben Incarnate

      OEMs usually provide peak power and wet weights, so it’s easy to determine. A full dyno chart, with disclosure of conditions, would be awesome to have from the onset.

      • Xenophya

        You’re right it is easy to get to, but how many ‘Joe Bloggs’ punters do the maths? If reviewers started using bhp per tonne a bit more prominently rather than just a bhp headline it could open the market up a little I think.

        On the surface 50-60hp sounds a bit lame when you look at something like an RGV250 or ZXR400 vs an SV650 with 70. But people get more interested when you tell them the power-to-weight is comparable if not better than the 650. I know which I’d rather have.

        • Ben Incarnate

          @Xeno – That’s a fair point. I concede this. With our powers combined, OEMs could provide both a power-to-weight stat and full dynos. The motorcycle world would turn upside down.

    • rohorn

      A tuned to the death GoPed (6 hp?) has great power to weight numbers – until you stick a rider on it. That’s why power/weight is meaningless ’til the (M)Ass Of Reference determined and included in the calculations.

    • rohorn

      And speaking of meaningless numbers, since when are the OEM weight numbers honest? An article on how that is derived would be amusing, especially with some OEM responses.

      • Ben Incarnate

        So true on both counts.

  • Maxwell

    strange how similar it is to my article:

    • Wes Siler

      Yeah, that was a nice article Max and one of the reasons I asked Sean to put this together.

  • Brammofan

    Hmmm. In what way is it similar? Because it talks about horsepower, Watt, and horses? So does Wikipedia . . . and every article written about horsepower.

    • Ben Incarnate

      Thank you for saying this first.

      Oh, and Sean’s a dirty thief. (This is a title far more nefarious than a plain thief.)

  • Liquidogged

    On the flipside, sometimes manufacturers have lied the other way and claimed low numbers to avoid regulation/tariff/taxation and such. A good example is late 60′s mustangs, which were actually significantly more powerful than Ford was letting on. I wonder if that kind of thing might become a factor again as safety regulations get more stringent with bikes.

    Nice article, btw.

    • Sean Smith

      Like with the new ZX-10R that wakes up and makes a LOT of power when you do minor things like flash the ECU and throw a pipe on it? The manufacturers use devices to limit sound and emissions but it’s always easy to remove them and get full power.

  • alang

    Nice article, good job. P.E. in structural engineering…