Comments from others on Torque vs Horsepower


I came across this page while looking to confirm my definition of one HP for a letter to Super Ford magazine who claimed there was "no magic 5,252" rpm relationship between torque and HP when responding to a letter to the editor. The letter was in response to some dyno charts in a previous issue that did not accurately reflect the proper "crossover" point for test results on several intake manifolds. I remember that article and chuckled at the graphs and figured someone else would point it out. But Super Ford still did not "get it" when their error was exposed. Anyway, radians are not unitless, they are their own unit. A radian is the arc of a circle that is equal to its radius. The circumference of a circle is equal to 2 * pi * radius, which leads to the relationship that 360 degrees is equal to "2 pi radians." Thus a radian is approx. 57.3 degrees. Your derivation of the HP = Torque * RPM/5,252 is much better for the lay person than getting into radians.


-Steve Schmidt, MSEE

I had some further questions for Steve and here is his reply.


No problem, this is interesting as you have made me think about this some more. And it does make more sense to use the actual data (150 lb., one minute, and 220 feet), as you have done, instead of the common definition of 550 lb. by one foot in one second. It helps tie it together much better.

Yes, exactly, the units cancel. That is our constant of 5,252 has the units of, hold on to your hat,...

(ft * lb * RPM) / HP

...or converting one Revolution (as in RPM) to its equivalent in radians and putting "the per minute part" in the denominator we get...

(ft * lb * 2 * pi * radians) / (minute * hp)

Remember that one revolution (the diameter) is 2 * pi * radians? Now you can see why you don't want to go to radians. ;-) So how did that constant get such "ugly" units? The "problem" is Watt was simply observing that the horse lifted the 150 lb. object by 220 feet in one minute. We can't change that. With the definition of lb., pi (3.14159 etc.) , and feet already existing and defining one horsepower by what he observed (which comes to lifting 550 lb. by one foot in one second as the commonly listed definition of HP) he simply put the two actions (torque and HP) into an equation and derived whatever constant related the two. Since the two actions are "equivalent", the units of the constant are whatever makes both sides have the same units. This brings up the topic of "unit analysis." In the above Watt was deriving a basic relationship, but when we are trying to calculate something complicated we often use "unit analysis" as a check. That is we just look at the units for all the data (observed or given to us) we will plug into an equation and set up an "equation" with just the units on both sides and make sure they are equivalent. It is interesting to note that had Watt simply observed a bigger (say, a Clydesdale) or smaller horse (like Ford used to measure the '99 Cobra!) the definition of HP would be different. I get to make cracks like that since I am a Ford fan and own a '98 Cobra and my wife has a '95 Lightning.



Here is a comment from Keith Menges

I came across the 'Horsepower and Torque' page while looking for the magic conversion number for horsepower (5252). Great page. It is a really good description of the relationship of horsepower and torque. The comment about the crossover point of horsepower and torque was really interesting. I never really thought about it but it is definitely true. Pretty cool.

I have to disagree with Steve's Schmidt's point that radians have a unit. A radian is a ratio. It just so happens that there is an angle that goes with it, but that is simply coincidental. When talking about radians you are referring to an arc LENGTH, not the angle related to that arc. So the radian measurement actually refers to the arc length relative to the radius of the arc. Because both of these have length units and the arc length is divided by the radius, all the units cancel and you end up with a unitless ratio.

Now, as Steve said, there really is no need to get into radians when breaking down horsepower. Since it's just a ratio there is no unit corruption. It just becomes a part of the bigger conversion factor.


Keith Menges

Here is a comment from Aivars Lelis,


A simpler way to understand the relationship between torque and horsepower, and how to calculate it, may be as follows;

Torque is tangential force * distance from the fulcrum. Power can be defined as work (force over distance) per unit time.

Applying 1 lb of force 1 ft from the fulcrum for a complete revolution will lead to;

W = F*2*pi*r = 1 lb * 2*pi * 1 ft = 2*pi lb-ft = 6.283 lb-ft

If it takes one minute to complete this revolution, then the power is;

P = W / time = 6.283 lb-ft / min

1 hp is defined as 550 lb-ft / s = 33,000 lb-ft / min

Therefore, applying 1 lb-ft of torque in one minute (1 rpm) = [6.283 lb-ft / min] / [33,000 lb-ft / min] = 1 / 5252 of 1 hp.

From this you can then calculate the number of hp from any given torque and rpm:

hp = torque (lb-ft) * rpm / 5252

Aivars Lelis

Here is a comment from Marty Rosalik,

For those hung up on radians per second we can stay in SI units and convert directly.

Torque(Nm) * Omega(rad/sec) = Watts.

Or kilowatts.

Now divide by about 746 to get horsepower.

Good luck finding a tachometer in radians per second.

Here is a comment from RJ67ChevySS,

I have to say that I belive that radians are in fact a unit. I belive that when you are refering to radians you are not refering to the length of an arc of a circle, but actually to the angle corresponding to that arc. A specific arc in a circle will change as the radius changes, yet the angle corresponding to that arc never changes. So only on a unit circle will 2 pi radians actually equal the length of the arc all the way around a circle, but on any circle 2 pi radians will be 360 degrees. Keith has a point in saying that radians don't have a unit because they are there own unit, just like inches and degrees. The best comparison I can make is that an inch is to a centimeter as a radian is to a degree.

Here is a comment from Jim Rennison,

The very easiest way to derive the HP formula is to remember that the distance around any circle measured in any units is 6.28 when we use a circle with a radius of one unit. In Watts classic definition, we have a HP as 33,000 lb*ft/minute. This is understood to be work done in a straight line. Since we already have a linear measurement contained in this definition, namely one foot, logically we can set the radius of our spinning shaft at one foot. This means we can then import torque figures in lb*ft right into the formula without any further calculations. Also, we need only the constant 2(pi) as a multiplier to converts watt's 'linear' HP into HP on a spinning shaft since we have smartly set the radius of that shaft at one foot. No need to worry about radians. So then HP=R(T)(2pi)/33,000 which can be written R(T)(2pi)/1 * 1/33,000 Dividing 33,000 by 2pi, 33,000/6.283185 =5252.1131. So then Hp = R(T)/5252. This is so simple and logical, you can derive the formula yourself if you forget.


Here is a comment from Tab Rasmussen,

I don't know if you have enough room on your site to post the attached article. Please feel free. I hope it helps makes sense of the ridiculous debate about horsepower and torque. This is really not as complicated and arcane as people think. The difference between the two is clear and simple, but folks cannot relate to the debate if it involves math and physics. Post my article and I believe more people will understand. Feel free, just cite me.

Tab Rasmussen Edwardsville, IL

Torque and Horsepower by Tab Rasmussen. This is a MS word document.

Thanks for your comments.

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