i was doing a few searchs and i found that someone posted

"One of the reasons modified engines have a higher hp than torque "number" is because the peak torque is probably occuring in the high 4000s or low 5000s, rather than high 3000s, and peak hp is @ 6000 or higher. Remember that hp is a derived (or calculated) number from torque and rpm. HP= torque x rpm / 5252. At 5252 rpm, torque and hp numbers are the same. At any point above that the hp number will be larger than the torque number. At 15756 rpm, easily within the hp peak range of an F1 engine, the hp number is 3 times the torque number."

well then im interested to know if horsepower is directly related to torque and rpms... how does the equation get the 5252 figure?

and it seems to me that one needs to know the horsepower curve to figure out what the constant value is in the equation.

and it u need to know the curve then it must not be calculated...

can someone please clear up my thoughts?

 

It simply has to do with the definition of HP. Horsepower is not a fundemental unit, rather it is a concept that describes a combination of fundemental units. It is defined as 746 watts or 33,000 ft*lb/minute.

Looking at the units in dimensional analysis we have tq*rpm (lb*ft*rot/min), the 5252 is unitless.

HP is ft*lb/min so we have to multiply by 2pi radians per rotation. The units of radians is a funny thing, it doesn't really exist but is used as a description much like a unit is.

tq*rpm/5252 = lb*ft*rot/min*2Pi/rot

The rotations cancel out and we are left with the units of HP.

so 5252 = 33000/(2*pi)

I'd recommend reading up on dimensional analysis, the concept of the radian, and fundemental units if you don't understand that or want to know more. You could ask another question too, but i don't know your backgroud and you might want to look over that stuff first.


You could have asked this over in LT1 tech, I and others would have answered it there too.

-brent

 

so basicly horsepower is torque per minute?

and yet when i calculated my horsepower in physics class... my legs got like .6 horsepower... i could caclulate how much torque my legs have?

cool thx for the info

 

RADIAN : A unit of angular measure in which the angle of an entire circle is 2 pi radians. Thus, there are 360º/ 2 pi radians or 57.296º/ radian.

 

A radian is a pure number. It's a common misconception that it is a 'unit'.

The proper definition of a 'radian' when talking about angles is:
theta=s/r
Where s is the arclength, r the radius, and theta is a 'angle in radians'. It is a ratio of arclength over radius. S is a measured length and r is a measured length.

theta = s(meters)/r(meters) OR s(in)/r(in), where are the remaining 'units'?

In engineering angular velocity is commonly measured in rad/s OR 1/s OR s^-1, all the same thing. Some people just prefer the r to be there because it makes them feel comfortable, IMO it clouds the issue.

Look at any geometric relationship involving a circle. For example:
Circumfence, c=2*pi*r. Whatever standard unit you use to measure r that is what you get your answer of circumfence as.
Consider a full circle of 2pi, in that case the arclength equals c, the circumfrence.
thata = s / r = c / r = 2*pi*r / r = 2*pi

So yes, there are 2pi 'radians' in 360 degrees, but more correctly:
In any circle of 360 degrees, the ratio of the arclength over the radius is 2*pi. In the pesky example above we couldn't have any pesky 'radians' hanging around with nothing to cancel them out.


-brent

 

Good **** Brent, I don't remeber that far back in math classes. lol

Newbie, I'm pretty sure you quote Old SStroker on that post too.

The new Hot Rod has a HP vs TQ debate in it. Personally I like as much of both as possible ;-)

Bret

 

My first definition was taken almost verbatim from a trigonometry textbook. To say that a radian is not a unit because it is frequently defined in degrees is as incorrect as saying a kilometer is not a unit because it is based on meters. The value of a radian never changes, it is always 57.296° (using degrees as a comparison). This why we can say a circle, which alway has just 360°, has 2 pi radians.

It would be more correct to say radians and kilometers are supplementary units because they are based on standard units. The value of the supplementary unit is always a fixed value though, because the standard unit upon which it is based is also a fixed value.

 

I can't help with the hp/torque discussion. But I think I remember math to the point that a radian is a "unit". The definition I remember is "a radian is a unit of angular measure". How does that differ from other "units"?

Rich Krause

ps: Hehehe: he said "unit".

 

Busting up here.

Which one are you?



That's a good grasp of the concept. Make the same torque at higher rpm and you have bigger horsepower. Your .6 hp, if you made it on a bicycle or Schwinn Airdyne might be at 70-75 pedal rpm. At 75 rpm, that would be 42 lb-ft. but probably not for long. If you measured it running up stairs, torque would be hard to figure.

 

Look at any geometric relationship involving a circle. For example:
Circumfence, c=2*pi*r. Whatever standard unit you use to measure r that is what you get your answer of circumfence as.
Consider a full circle of 2pi, in that case the arclength equals c, the circumfrence.
thata = s / r = c / r = 2*pi*r / r = 2*pi

I'm sorry, but I have to disagree with you on this one because you appear to be using radius and radians interchangeably. Radius is abbreviated as r, while radians are abbreviated as rad. r and rad do not cancel each other out.

You cannot calculate the circumference of the circle with the formula C = 2*pi*r, if you do not know the length of r. And we can't calculate the length of r unless we know the subtended arc length s of the angle (in the case of a circle, we already know that the angular displacement -represented by the greek letter theta or Q when that is unavailable- is equal to 2*pi*rad).

So for your example, the radius r = s/Q

Unfortunately, we still cannot calculate r because we do not know the value s. So it would be meaningless to substitute s/Q for r into the formula C = 2*pi*r, as you are just replacing one unknown variable with another while trying to calculate an unknown result for C

 

In engineering angular velocity is commonly measured in rad/s OR 1/s OR s^-1, all the same thing. Some people just prefer the r to be there because it makes them feel comfortable, IMO it clouds the issue.

From a physics standpoint, I totally agree that angular velocity can be expressed in rad/sec, but you should not replace rad with r, nor sec with s unless you wish to cloud the issue. rad is the angular displacement, r is the displacement of the radius, sec is the elapsed time and s is the subtended arc length.

Even if you allow yourself to replace sec with s, the expressions "1/s" or "s^-1", don't mean much unless you are looking for a fancy way to say 0.1 seconds. No angular velocity is calculated because no angular displacement (i.e., rads) is given.

V = d*t

 

I apologize, your right, when talking about the units of angular velocity i should not have used rad/s because i already used s to represent a variable. For every engineer who abbreviates 'radians' with 'rad' there is another who abbreviates it 'r' and yet another wiser/lazier engineer who doesn't bother to write the silly thing all together.

Need another variable? abcdefghifklmnopqursuvwxyz
-my calc teacher had a poster like that haha.

If you look at my above correction in quotes, it should make my whole post clear, everywhere else r is a variable radius and s a variable arclength.

I stand by my original post, the radian as a unit is a mere figment of your imagination good friend. I'm sure in that same textbook you could find evidence to support my claims, it is just spreading common misinformation at the same time.

rich, like i said, it is a commonly held misconception. I'm not argueing that there are not 2pi 'radians' in 360 degrees. I am saying that it is an unneccesary label and not a true engineering unit. 'Radian' angles are a ratio or arclength over radius where the units clearly cancel each other out.

I'll be glad to post more later today but for now i have to get to work.

-brent

 

Brent: I see what you mean. As a History major, I am a little out of my depth here anyway!

Jon: I think I am more of a "Beavis" than a "Butthead". So on that "score" I will stop alluding to cartoons.

Rich Krause

 

Although I like to picture myself as "Pops Racer", I think "Mr. Magoo" is closer.

NewbieWar, don't get lost in the "units" discussion. It's fun but slightly OT...but then that's what makes Advanced Tech so interesting.

 

http://www.off-road.com/hummer/tech/power.html

I cut and pasted from that site:

horsepower: 1hp = 550 lb-ft/s

So what about power? If we remember from above, power is torque*distance/time or is torque*velocity. For this discussion, we want to take torque in lbs*ft and speed in revs/minute and calculate horsepower:

Horsepower = torque * revs/minute * minute/60 s * 2*pi * 1/550
Horsepower = torque * revs/minute * 1/5252

In the first line, torque is in lbs*ft, speed in revs/minute. The third term just converts RPM to revs/s. The fourth term converts revs to radians. The last term is the conversion factor from lbs*ft/s to horsepower. The second equation just multiplies out the constants to make life easy.

.