I've seen this around alot on a couple of the threads from a while back. Someone gives thier RPM along with the gear they were in and at what speed and someone else would calculate thier estimated hp.

How does it work? I know you have to use some kind of equation. Just intrested in the logic.

 

No such animal buddy.....

 

power is dependant on the first derivitive of speed (acceleration)... top speed is usually power limited however, and given the drag coeff. of the car (Cd = .30 with the lights down, .34 with them up) approximate power at the stall rpm could be calculated.

i built a computerized system for my celica a while back that took samples from the digital speed sensor and calculated the rate change of kinetic energy of the chassis
d(mass x velocity^2 / 2)/dt
which was the wheel power output in joules/s (yay metric)... got some great, reasonably accurate numbers, but my "rolling dyno" would always read high on a low tank of fuel, and read low whenever i tried to show it off to fat people.

 

Great logic. I know I[ll get into this in 2 years in school but seeing as how I'm only a freshman don't both d's cancel out here ? d(mass x velocity^2 / 2)/dt

btw I had no Idea the coef. of drag even played a minor role here.

 

dt is determinant. they stay together, im laymans terms.

 

the expression d(function)/dt represents the first deritive of "function" with respect to the varible t, which in my case would be time. Calculus is my friend, i swear.

the d's cancelling out would be true is calculus was like ordinary math, but its not.

Cd isnt mentioned in my calculations cause thats a wholly different approach.. i forget most of my fluid dynamics, so id have to look up the equations again... one thing to consider: using a cars terminal velocity in conjuction with Cd to calculate power output will give one power number only valid at that particular RPM, which may not be the peak number, which is what you tend to advertise. Ideally, vehicles should specify their torque curve, as measured on a dynometer.

(btw: power is directly related to torque and RPM, thus Power = TQxRPM... not exactly, but thats only cause im not using the metric system)

 

you really want a way to roughly estimate your power? G-Tech baby. ebay
If you want something more complicated I believe theres one for $200 that measures torque as well as some other additional things and gives you a pretty little graph like a dyno.

 

yeah, my system gave me the neato graphs, and i had a laptop in my car ("danger to manifold - you're too fast and too furious!")

how does the g-unit (nyuk!) work? do you have to program it with your cars mass, or specify your vehicle in some sort of setup mode? i know they use silicon accelerometers to capture the two dimensional forces on the vehicle... but ive never used one.

 

Thanks for the help guys. If anytyhing I'll hopefully get it dynoed soon. When calc. becomes my friend then I'll hopefully understand this a lot better, though I do understand. It is possible to calculate but I doubt the average person would. (I would lol)

 

for the cheap one you have to enter in the mass of the vechicle + you every time you decide to use it. if you leave it on you dont have to change it, just every time you turn it off and on.

 

Wow, I feel smarter by reading this thread. But I have no clue what all these numbers and letters are, I am barely passing college algebra.

But i'm curious, what was the final verdict, could it be done? Yay or Nay?

 

the subject of the thread is quite simply impractical, if not impossible.
even when used in conjunction with the wind resistance calculation, you need to assume this is TOP speed, and the number calculated wont be the peak (from a statistical point of view, its highly improbable)
your number may say your car is a 10 hp beastie if you dont work it right.

the calculus methods and majik black boxes DO exist and DO work, good as a matter of fact.. its two steps up from "butt dyno" (first step up being 0 -> 100 and a stopwatch)

and yes, reading does make you smarter.. my project had software engineered for it by a fellow student (at another university, a COMPETING university.. boo!) who posted some of the code and screenshots on the web and some software engineering related design notes. my hardware is also there but its rather elementry.

 

http://jayee.dyndns.org/projects/492project/index.html


there we go.. kinda wordy and such, but you might enjoy the read

 

you can calculate the hp using rpm, gear ratio and mph. it requires some system modeling and approximations for effective inertia

KE=(1/2)(I1 + (I2/N^2)w^2

KE=kinetic energy
I1=Inertia 1
I2=Inertia 2
N=gear ratio
w=angular velocity

take dKE/dt like SparkienSuggah said and you got powa