posted on February 09, 2011 23:00
by Dave Coleman
In my continuing quest to right the wrongs of the modern automotive world, I hereby propose the retirement of the horsepower. Why? Simple. Horsepower, as a measurement of engine output, is a bizarrely arbitrary unit of measure. If the history books are right, a horsepower is supposedly 1.5 times the amount of work an average mine pony could do, as measured sometime in the late 1700s. This is stupid.
Supposedly a late 1700s horse could do 22,000 foot-pounds of work every minute. Basically, that means if you tied a 220-pound bucket of coal to the backside of a horse, it could drag the bucket 100 feet up a mine shaft in a minute. Or it could drag 2200 pounds 10 feet up the shaft. Or, with the right gearing (block and tackle in those days), even 22,000 pounds one foot up the shaft. You get the idea.
So along comes James Watt, who must have been pretty smart, since he managed to get his name on every lightbulb ever made, and decides one horsepower should be defined as 33,000 foot-pounds per minute.
A horse can do 22,000, but a horsepower is 33,000? Perhaps James Watt was visionary enough to build this in as a hedge against exaggerated horsepower claims.
Automotive terms and definitions have a long history of being stupid and arbitrary, and I have no intention of changing that, but if we’re going to be stupid, let’s at least use monkeys. So, monkeypower it is, then. We’ll start with an average monkey: Josh Jacquot.
Despite the fact that Jacquot claims to be a highly advanced form of monkey, he throws an absolute fit if you try to tie a rope to his backside and drop said rope down a mineshaft with 220 pounds of coal on the other end of it. Party pooper.
So instead of coal, let’s define monkeypower with mountain bikes. In racing trim, a Jacquot monkey is 124 pounds of unmitigated fury. His monkey bike weighs 21.25 pounds. A bottle of monkey water, some monkey tools, and a spare monkey innertube add up to another pound. Total: 146.25 pounds. According to actual data logs, Jacquot can climb 3000 vertical feet in one hour. That’s 50 feet per minute, or 7312.5 foot-pounds per minute. Of course, we still have that arbitrary factor of 1.5 to throw in to compensate for monkeypower escalation. Throw in the factor and the official definition of a monkeypower is 10,968.75 pound-feet per minute. Or 0.332 horsepower.
This is the monkeypower definition I put forth in the February, 2004 issue of Sport Copmact Car, but as with much of what we published back then, a few years of reflection have called that original calculation into question. Simple observation of a monkey suggests it can't do one third the work of a horse, and, indeed, if you try a couple thought experiments, you'll quickly see this conversion factor is highly improbable. A good rider, for example, can travel 50 miles on a horse in one day, but there's no way I could ride Josh for more than about half a mile. Even accounting for my shortage of riding skills, we clearly need to adjust the math.
The problem, it turns out, is that Josh's refusal to haul buckets of coal led us to an erroneous substitution. We calculated the weight of Josh and his monkey bike, but when when Watt's horse dragged that 220-pound bucket, he ignored the 1200 pounds of horse that was also moving. So, to be fair, we should cancel out the monkey meat. Doing that leaves us only with 22.25 pounds of bike, water and tools, and when you re-do the math with that figure, one monkeypower is only 0.05 hp.
Now you see why they don’t use monkeys in coal mines.
So, did you catch the point of this story? It was up in the beginning. 22,000 foot pounds per minute can be 22,000 pounds going one foot in one minute, or 2.2 pounds going 10,000 feet in one minute, or anything in between. Know what the difference is? You guessed it. Torque.
That 22,000 pound job took one torquey horse. And the reason the torquey horse and the high-revving horse (that’s the one that only lifted 2.2 pounds) can do the same work is the magic of gearing. A Clydesdale could do all 22,000 foot pounds per minute just by grunting forward a few feet. To get the most out of that kind of horse, you'd want tall gearing that moved the bucket a bunch with only a little horse stepping.
A Thoroughbred, on the other hand, is only at its best in a sprint, so you'd want really short gearing that let the horse cover some distance while lifting the same 22,000 foot pounds per minute. Both horses are making one horsepower, each is just doing it with its own style.
So the only difference between the 3,460-monkeypower / 99 lb-ft Hayabusa engine going into the Miatabusa and the 3,460-monkeypower / 295 lb-ft 2.2-liter turbodiesel in a European market Mazda CX-7 is the style. The gearing required to make each of those engines deliver that monkeypower in a reasonable speed range will have them delivering their monkeypower in a very similar way, but the spinny bits under the Miatabusa's hood will be spinning three times as fast, and sounding three times as good.
Thursday, February 10, 2011 1:18 AM
What the hell!!!??? That makes perfect sense!!!
Thursday, February 10, 2011 3:25 AM
So in the end torque is the most significant measure. Hp = torque x RPM so it is sort of easy to understand that HP is just a result of the produced torque.
Typical american V8 = bunch of torque @ low RPM => X hp
Hayabusa = insignificant torque @ zillion RPM => same X hp
No I wonder if one calculated the AREA bellow the torque line (think of a dyno chart) would be significant and what would it mean. Just thinking out loud...
Thursday, February 10, 2011 3:26 AM
X hp and same X hp although the same number but at very different RPM, of course!
Thursday, February 10, 2011 7:39 AM
Area under the curve would be the integral of power over time, which would give you energy (joules). So more area under the curve equates to more total energy output.
Thursday, February 10, 2011 8:01 AM
So, more total energy output for the same engine size (or even better, for the same fuel consumption in predetermine conditions) = better overall efficiency...
Still doesn't say anything about how the engine will deliver (for that, there is the torque x RPM graph) but should be helpful for something! :D
Thursday, February 10, 2011 10:23 AM
Since HP is a measure of work over time, and torque is measured (in America) as ft/lbs, this is why HP will always cross the torque curve at 5280rpm. There are 5280ft in a mile.
Or something like that.
I love that Coleman is putting up his old Technobable stuff. I'm still re-reading old SCC mags. It's amazing how many small things you miss the first time or how much you have learned since the first read that you can apply to your current knowledge. Case in point, Coleman corrected his first article with current knowledge.
Thursday, February 10, 2011 11:19 AM
It would also be cool to tell readers how Dynojet horsepower numbers are again completely arbitrary. From what I remember a Dynojet dyno reads your horsepower based off some motorcycle as the constant.
Thursday, February 10, 2011 11:38 AM
@Dusty: Nope. You forgot that the X-axis is RPM, not time.
The result of the integral under the Torque Curve would essentially be average horsepower over the RPM range you integrated over (depending on what constants you multiply or divide by).
Check the units: torque X angular velocity = power
There's obviously no shortage of debate (or would that be babble?) about the meaning or importance of HP and Torque. I'm of the opinion that there is no "best", just "most well suited for your application". You can get to power through higher angular velocity or torque, each has a place.
Motors that make high torque across band from low-to-mid RPM are great for street cars that need most of their power accelerating from idle to some mellow shift point.
On the other hand, in competition you're willing to run a motor where it'll give you maximum power output, regardless of consumption, wear, or the gearing it takes to get there. Most race motors are all about higher revs and lower gearing, as it's easier to get an extra few monkeypower by spinning an extra 500 rpm.
Thursday, February 10, 2011 11:53 AM
@Cobymoby: The 1985 Yamaha Vmax
Thursday, February 10, 2011 1:10 PM
When revving an engine, Rpms increase over a given time period; they do not change instantaneously. Therefore, the power vs rpm graph could be thought of as being power vs time and integrated accordingly. You would simply need to know the amount of time the engine required to rev during the creation of the dyno graph and adjust the x-axis accordingly.
Thursday, February 10, 2011 1:24 PM
When you say "You would simply need to know the amount of time the engine required to rev during the creation of the dyno graph and adjust the x-axis accordingly."
What you're really saying is "create a graph that's Output Vs Time" and integrate under that.
If the graph is Power Vs RPM or Torue Vs RPM, integrating under the curve gives you a value that has units of Power X RPM or Torque X RPM.
If the graph is Power Vs Time, then integrating gives you a value that has units of Power X Time (=work done, in units of energy), which would be the average power output over the given time duration.
The first quantity is a representation of a motor's output over a rev range, a property of the motor (or monkey) itself.
The second quantity depends on what you're doing with the motor/monkey at any given time. It's a function not just of the motor's output, but the load on it and throttle applied at a given time.
Thursday, February 10, 2011 5:20 PM
If throttle output and load are held constant at unity (100%), the curve would have the same shape as the dyno graph.
Thursday, February 10, 2011 6:37 PM
Josh won't haul coal bags out of a mine? What a party pooper.
Thursday, February 10, 2011 6:45 PM
This is slightly tangential, but humor me. One of the benefits to a high revving engine, I deduce, is that it's easier to use any amount of horsepower without spinning the tires.
Let me explain. 300 hp engine delivering its power at 10,000 rpm is not delivering less force to the tires than a 300 hp engine delivering its power at 1,000 rpm is, assuming the gearing on the first engine is 10 times higher than the second engine (which would result in the same speeds and accelerations for identical cars/boats/whatevers).
However, in order to produce 20 percent wheelspin coming out of a corner, the 1,000 rpm engine only has to gain 200 rpm - barely a twitch on the throttle. The 10,000 rpm engine would have to spike upwards by 2,000 rpm, which should be much easier to modulate.
At least, this is part of the reasoning my friends and I came up with when trying to explain how a 1,000 hp Viper spins its tires at 140 mph and an old school mid 80's, 1500 hp F1 car can get out of a hairpin without toasting the tires.
Friday, February 11, 2011 7:36 AM
i knew i recognized this, i'm gonna pull that issue out today!
Friday, February 11, 2011 8:19 AM
I think I just got dummer.
Friday, February 11, 2011 10:07 AM
"If throttle output and load are held constant at unity (100%), the curve would have the same shape as the dyno graph."
Not true, as RPMs wouldn't increase linearly with time.
With a constant load and WOT, the rate
of RPM increase will increase as the motor makes more power at higher RPM. e.g., it'll go from 4000-5000 a lot faster than 1000-2000, something you can verify on the street with a complete run of the revs in 1st gear.
The end result of a HP (or torque) Vs Time graph of a dyno pull would be steeper than HP Vs RPM.
I promise, I'm not trying to pick on you or be "that guy", but I make my living out of getting little details like these correct.
Saturday, February 12, 2011 11:22 PM
@Daewoo of Death
Remember, you're also forgetting all the other factors of a Formula 1 car vs a Viper/car. Tires: Even though tire technology has advanced with leaps and bounds throughout history, the tires that the late 80s turbo F1 cars were using are still incredible pieces of technology that still are leaps and bounds better than a street tire that Viper is roasting through(I imagine this Viper has street tires?). Also remember that aerodynamics (even at low speeds) can play a significant advantage. And finally remember that the turbo era Formula 1 cars were piloted by some of the most amazing men of our time. Similar to the guys who were driving the Group B rally cars of the time, their reaction times as well as pure unadulterated driving talent can make up a lot of disadvantages.
Sunday, February 13, 2011 2:15 PM
Daewoo of Death is correct. Trent_FSAE, the tires on a Viper are huge, they're like 355 width in the rear, far bigger than anything F1 ever used. And Dodge used very good tires on the Viper because of the huge torque. A 1000 hp Viper spins its tires because of the 1000 lb-ft wall of torque that's pushing that car forward. Remember Viper's have very flat torque curves, a product of the 8.3L displacement. The F1 car is peaky due to the low displacement motor and high boost turbos.
Ina short drag race the Viper has the F1 car covered. In longer races (maybe half a mile or so), the F1 car would win because of the extra top end power. Over a half mile, the F1 car hits an aerodynamic brick wall and the Viper wins again.
Sunday, February 13, 2011 2:17 PM
This was actually tested in Motor Trend a few years ago on a standing mile, except it was a Lola Champ Car with a Cosworth XFE engine in road course trim. The Champ Car was the quickest accelerating because of the light weight, short gearing, and massive power. The 1500 hp Hennessey Viper was the fastest overall.
Sunday, February 13, 2011 9:53 PM
Generally speaking, increasing tire width does not equal more straight line traction or a larger contact patch.
As for why those 80s F1 cars don't destroy the tires leaving a corner: the rubber on those 80s cars is still amazing technology for street cars, the car weighs considerably less (less resistance to acceleration means less wheelspin), and more of the weight is on the rear tires (weight and inflation pressure are primary determinates of contact patch). Also, the Viper is probably on cold tires, on a greasy, dusty concrete freeway at night in a YouTube video, and the 80s F1 car is spit hot tires on a groomed track.
And the 80s F1 car would absolutely crush the Viper. The Viper would need over 3000hp to match the 80s F1 car's qualifying power:weight ratio. In a drag race, it'd be the F1 car, by about an aircraft carrier length.
Thursday, February 24, 2011 7:48 PM
@Daewoo of Death
You're forgetting that (in general) engines that make high torque/power at high RPM respond very quickly to RPM change. Enough so that I would estimate they respond at the same relative quickness at peak power as the high torque/power at low RPM engine does at it's peak power range.
Rev a Viper and it revs fairly slowly. Rev an F1 engine (not off idle, mind you) and it hits the rev limiter before you finish blinking.
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