fortmyerspolice

iTrader: (7)

Why do they meet eachother on the dyno on almost every engine at 5250?

Jpr5690

iTrader: (13)

Hp = (tq* Rpms) /5252

So Its Kinda Circular Reasoning That It Must

3.4camaro

iTrader: (1)

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.

Old SStroker

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?

Jpr5690

iTrader: (13)

Bonus question: What are the units for rpm in the metric system?

IM GONNA SAY YEN

Jpr5690

iTrader: (13)

Just Kidding

3.4camaro

iTrader: (1)

radians per second?

or

revs per second?

It's gotta be seconds. Do I get a cookie?

Old SStroker

Half a cookie. It would be 2π radians per sec (or minute)

("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.

3.4camaro

iTrader: (1)

k = 60(5252)

I need the other half. AND MILK!

EDIT- because there is more detail in a per minute system.

VIP1

315120

VIP1

dammit
i was typing
beat me my a millisecond.

3.4camaro

iTrader: (1)

lol...

3.4camaro

iTrader: (1)

This anthopology paper i have to write SUCKS, so I'm on here.

VIP1

Do you mean anthropology?

3.4camaro

iTrader: (1)

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.

Old SStroker

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!

3.4camaro

iTrader: (1)

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.

Old SStroker

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!

VIP1

Nope.

Close. 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.

I've often been told I'm nuts.

VIP1

..... 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.