torque and horsepower at 5250
because the units are different.
1hp = 550 ftlb/s
1 tq = 1 ftlb
If we used non-USCS, non-retarded units, the dyno graphs would cross at a different point.
EDIT- you have to understand that hp is a function of torque. Go do some reading on howstuffworks.com and see whatsup.
1hp = 550 ftlb/s
1 tq = 1 ftlb
If we used non-USCS, non-retarded units, the dyno graphs would cross at a different point.
EDIT- you have to understand that hp is a function of torque. Go do some reading on howstuffworks.com and see whatsup.
TECH Fanatic
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From: Upstate NY
Hint, the "5252" constant results from the unit conversion.
There are 2(PI) radians per revolution, and 33000 lb-ft/minute/hp. I'll leave it up to the math majors to get the 5252.1131... constant.
Yes, @ 5252.1131... rpm the values of torque and horsepower are equal in the English system of units. Below that, the torque value is higher, above that the hp value is higher. That's obvious from the equation for HP.
You could figure things in metric units (Newton-Meters and Kilowatts), but it's just an exersize in math.
Bonus question: What are the units for rpm in the metric system?
There are 2(PI) radians per revolution, and 33000 lb-ft/minute/hp. I'll leave it up to the math majors to get the 5252.1131... constant.
Yes, @ 5252.1131... rpm the values of torque and horsepower are equal in the English system of units. Below that, the torque value is higher, above that the hp value is higher. That's obvious from the equation for HP.
You could figure things in metric units (Newton-Meters and Kilowatts), but it's just an exersize in math.
Bonus question: What are the units for rpm in the metric system?
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TECH Fanatic
Joined: Jun 2002
Posts: 1,979
Likes: 3
From: Upstate NY
("π"is a close as I could get to "pi")
Most folks probably think of engine speed in revs/min vs. rev/second.
Easy Bonus question: If engine speed is expressed in rev/sec (rps), find the constant "K" in the horsepower equation: HP = Torque x rps / K
Now do you see why we generally use rev/min, not rev/sec?
No it's not because our tachs read in rpm, not rps.
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Verdad Gallardo That's the one. I have to write about recovery techniques used in some Mayan city to get crafts made by people in the 900s. I think I just might break into an essay on why Mustangs suck. We'll see.
TECH Fanatic
Joined: Jun 2002
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From: Upstate NY
Hmmmmmm
Let's assume your engine makes 360 lb-ft @ 6000 rev/min on the dyno.
HP = Torque (T) x (Rev/min) / 5252
HP = 360 x 6000 /5252 = 411.3 HP
So if HP = T x (rev/sec) / K
and:
K = 60(5252) or 315120 as proposed by 2 folks who shall remain nameless, and 6000 rev/min = 6000/60 = 100 rev/sec (doesn't it?)
then:
HP = 360 x 100 / 315120 = .114242193 HP
so your engine lost about 99.97% of it's power just by your math?
Does that seem reasonable to you two (nameless) guys?
Thoughts:
1) Always try your new answer back in the formula to see if it works. The engine output hasn't changed, has it?
2) Shortcut: You already knew the HP, so solve the formula for K, then "plug and crank" to get the value of K and then figure out how it should be derived. That's what I want: the reasoning behind the (correct) value of K.
I already ate the cookies.
Both anthro majors, right? I hope not math or science majors.
Please, no help from the peanut gallery!
Let's assume your engine makes 360 lb-ft @ 6000 rev/min on the dyno.
HP = Torque (T) x (Rev/min) / 5252
HP = 360 x 6000 /5252 = 411.3 HP
So if HP = T x (rev/sec) / K
and:
K = 60(5252) or 315120 as proposed by 2 folks who shall remain nameless, and 6000 rev/min = 6000/60 = 100 rev/sec (doesn't it?)
then:
HP = 360 x 100 / 315120 = .114242193 HP
so your engine lost about 99.97% of it's power just by your math?
Does that seem reasonable to you two (nameless) guys?
Thoughts:
1) Always try your new answer back in the formula to see if it works. The engine output hasn't changed, has it?
2) Shortcut: You already knew the HP, so solve the formula for K, then "plug and crank" to get the value of K and then figure out how it should be derived. That's what I want: the reasoning behind the (correct) value of K.
I already ate the cookies.
Both anthro majors, right? I hope not math or science majors.
Please, no help from the peanut gallery!
obviously we didnt check. And I'm a mech major, who has to take a perspective anthro class.
What I looked at is the fact that you wanted rps instead of rpm. So you would divide by 60. I'm sleepy...
EDIT- WAIT WAIT just looked at what you did. We KEPT RPM in the equation, but took 60 out of the denom to change the equation. So you changed it to rps, AND put 60 in the denom. So you divided by 3600, not 60.
we both did 5252*60, when it should be 5252/60. Lesson learned.
What I looked at is the fact that you wanted rps instead of rpm. So you would divide by 60. I'm sleepy...
EDIT- WAIT WAIT just looked at what you did. We KEPT RPM in the equation, but took 60 out of the denom to change the equation. So you changed it to rps, AND put 60 in the denom. So you divided by 3600, not 60.
we both did 5252*60, when it should be 5252/60. Lesson learned.
Last edited by 3.4camaro; Nov 9, 2007 at 05:40 PM.
TECH Fanatic
Joined: Jun 2002
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From: Upstate NY
obviously we didnt check. And I'm a mech major, who has to take a perspective anthro class.
What I looked at is the fact that you wanted rps instead of rpm. So you would divide by 60. I'm sleepy...
EDIT- WAIT WAIT just looked at what you did. We KEPT RPM in the equation, but took 60 out of the denom to change the equation. So you changed it to rps, AND put 60 in the denom. So you divided by 3600, not 60.
we both did 5252*60, when it should be 5252/60. Lesson learned.
What I looked at is the fact that you wanted rps instead of rpm. So you would divide by 60. I'm sleepy...
EDIT- WAIT WAIT just looked at what you did. We KEPT RPM in the equation, but took 60 out of the denom to change the equation. So you changed it to rps, AND put 60 in the denom. So you divided by 3600, not 60.
we both did 5252*60, when it should be 5252/60. Lesson learned.
Duh!
OK so if 5252... comes from 33000 lb-ft/min[Watt's definition of a horsepower] / 2 π [2 pi], how about just dividing the 33000 by 60 sec to get 550 lb-ft/sec [the other way Watt defined a hp] and divide that by 2 π ?
That's the concept I wanted you to see. The concept is more important than remembering the number (87.535...) If you remember the concept of 2π radians = 1 rev you can always derive the formula. Understanding is the key to knowledge, not rote memorization. You also won't make such horrendous errors!
My soapbox speech for the day.
Hey, all of this is even On Topic!
Last edited by Old SStroker; Nov 9, 2007 at 08:19 PM.
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Joined: Apr 2002
Posts: 9,764
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From: Eastern MA
Originally Posted by Old SStroker
Both anthro majors, right?
Originally Posted by Old SStroker
I hope not math or science majors.
Comp Sci.I actually slept in Calculus and still got straight A's.
I liked it and could do it quite quickly/accurately.
(By hand and in my head, not using a graphing calculator...thats cheating.)
Took me 1min to do a 10min quiz with 94+% accuracy.
Its been a few years... I can't do that anymore.
I was just lazy with my answer.
Originally Posted by Old SStroker
Please, no help from the peanut gallery!
Last edited by VIP1; Nov 9, 2007 at 10:52 PM.
Copy & Paste Moderator
Joined: Apr 2002
Posts: 9,764
Likes: 208
From: Eastern MA
..... JUST SKIP TO MY NEXT POST OR READ THIS ONE FOR HUMOR....
Wait a minute.... you goofed and we are right.
We converted the entire formula into seconds (since you wanted to use seconds insteasd of minutes) and you didn't. And you compounded that error by misusing our answer. Funny math there.
Here is the correct formulation:
360 lb-ft @ 6000 RPM
360 lb-ft @ 360000 RPS
HP = ((TQ @ RPS) X RPS) / RPS Crossover
HP = (360 X 360000) / 315120
HP = 411.2718964
Where are my cookies. 
Wait a minute.... you goofed and we are right.
We converted the entire formula into seconds (since you wanted to use seconds insteasd of minutes) and you didn't. And you compounded that error by misusing our answer. Funny math there.
Here is the correct formulation:
360 lb-ft @ 6000 RPM
360 lb-ft @ 360000 RPS
HP = ((TQ @ RPS) X RPS) / RPS Crossover
HP = (360 X 360000) / 315120
HP = 411.2718964
Where are my cookies.
Last edited by VIP1; Nov 9, 2007 at 10:48 PM.