How do you calculate HP from data logs?
04-22-2007, 12:09 PM
#41
Originally Posted by plumpzz
^ +1 
, almost. Since you know the change in speed over the RPM, drive train loss, and friction is already factored into the time. The only problems is that I cant do enough calculations without setting up a program in excel to do it for me. One would have to do every calculation and graph it, and find the max, just like the dyno does. Im doing the psuedo math that a dyno does to get that number.
, almost. Since you know the change in speed over the RPM, drive train loss, and friction is already factored into the time. The only problems is that I cant do enough calculations without setting up a program in excel to do it for me. One would have to do every calculation and graph it, and find the max, just like the dyno does. Im doing the psuedo math that a dyno does to get that number.So, it comes down to: what are those equations that you're using? The dyno is doing the same, what you call "pseudo-math" (which it isn't), in order to calculate torque and HP at specific RPMs. The only real difference is that the mass of the roller (or the amount of resistence, hence "Brake Horsepower") is known on the dyno, and it is actually the car and air resistence that you need to know to calculate identical results.
And sabrefanpc, I know that it can be done, it would be nice for all of us if we could get the formulas up on the site for other people to access, especially since there are so many people who have access to data acquisition now.
04-22-2007, 02:45 PM
#42
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I call it pseudomath because that OP is challanging what Im doing. Im pretty sure after 5 semesters of calculus and 3 semesters of physics and a few semesters of chemical Engineering things I can figure it out. Im pretty sure I'm right, but Im not sure how accurate the raw data itself is. You realize how little data it is because the change in time is not all that much.
Last edited by plumpzz; 04-22-2007 at 02:52 PM.
04-22-2007, 03:21 PM
#43
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Force due to drag on our cars should be = 0.5*p*C*A*(v^2), right?
I found that the C for the coupe is 0.29 and for the roadster 0.34.
p is air density and 1.293 kg/m3 at 0°C and 1 atmosphere.
A should be easy to calculate and is constant.
I did tried subtracting this force from my WORK METHOD calculations, but it didn't do much (unless I did the calculation wrong).
Also, if I would just want to convert to power instead --> Power = 0.5*p*C*A*(v^3)?
h8bumps, see if you can set Cipher at a higher sampling rate.
I think we are almost there guys...
Late,
Nick
I found that the C for the coupe is 0.29 and for the roadster 0.34.
p is air density and 1.293 kg/m3 at 0°C and 1 atmosphere.
A should be easy to calculate and is constant.
I did tried subtracting this force from my WORK METHOD calculations, but it didn't do much (unless I did the calculation wrong).
Also, if I would just want to convert to power instead --> Power = 0.5*p*C*A*(v^3)?
h8bumps, see if you can set Cipher at a higher sampling rate.
I think we are almost there guys...
Late,
Nick
Last edited by 350zfred5283; 04-22-2007 at 04:12 PM.
04-22-2007, 06:07 PM
#44
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You can't just subtract it like that. Usualy at higher speeds the fluid friction is ^2 or ^3 depending on more than one thing (too complicated for this model). What gear was that in. For this situation, I'd say anything over 80 mph is concidered to be squared. Theres no real solid definition for that formula. Also, if you're taking the speed values from ur cipher, all forms of drag are already included in the time difference. If you subtract out frictions, you'll end up getting something rediculous like 80hp. Which is pretty damn unreasonable even in 4th gear.
You also have to remember that even though A is constant, the cross sectional A isnt. Also, remember that there is almost always a vortex in the rear of the car which at higher speeds also has a prodound impact. None-the-less, you shouldnt need to add air drag into your model since ALL frictions are already valued into the time.
You also have to remember that even though A is constant, the cross sectional A isnt. Also, remember that there is almost always a vortex in the rear of the car which at higher speeds also has a prodound impact. None-the-less, you shouldnt need to add air drag into your model since ALL frictions are already valued into the time.
Last edited by plumpzz; 04-22-2007 at 06:09 PM.
04-25-2007, 09:33 AM
#48
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Originally Posted by sabrefanpc
If you will notice, I was responding to the post above where a device was linked. If you care to look at the device, it does infact ask for exactly what I stated. I made up nothing, only read and reported what others like yourself obvious were too busy to look into. Here is specifically where to look:
http://www.bahntech.com/products/Aut...er/default.asp
Please, no BS flag for myself, as I am simply reading and responding, not asserting anything without considering, like how "a few tens of pounds" will not make much of a difference ( 30 lbs in a 3000lb car would make about 1% difference in your power numbers, small to some I guess but still something to be considered).
http://www.bahntech.com/products/Aut...er/default.asp
Please, no BS flag for myself, as I am simply reading and responding, not asserting anything without considering, like how "a few tens of pounds" will not make much of a difference ( 30 lbs in a 3000lb car would make about 1% difference in your power numbers, small to some I guess but still something to be considered).
Math:
A vehicles HP is the force accelerating your vehicle F = M*A, where the mass is the weight of your vehicle, and the acceleration is your change in speed.
This formula nets the HP required to accelerate your vehicle, but it does not take into account the HP required to overcome rolling resistance or to overcome aerodynamic drag.
To measure the force or HP required to overcome rolling resistance and aerodynamic drag you will need the Frontal Area of your vehicle, its Cd [Coefficient of Drag], and Weight
Rolling Resistance (LBS)=Weight * (Rolling Coefficient + (Speed Effect Coefficient*3.24*(Speed/100)^2.5)
Aero Drag = 0.5 * Air Density[kgm3*kgm3_to_slugs[slugs]*Frontal Area*DragCoeff*(speed*1.467)^2
Rolling Resistance [LBS] + Aero Drag [LBS] = Total Drag in LBS
Convert Total Drag [LBS] to HP
HP = Speed * 1.467 * Total Drag / 550
The constant 550 is derived from 1 HP = 550 LBS moving one foot in one second.
The math is actually far more complicated, this is just a brief explanation of the mechanics
OK, the math may be more complicated, but probably not much more for our simple calculations (I did miss a lot though
)...basically, all I was missing is:Rolling Coefficient
Calculating WHP or Calculating BHP
mechanical loss (which can be estimated)
Aerodynamic Information
Climate Information
SAE Correct HP/TQ Data (if wanted)
I'll run the equations/numbers again. If I come up with something good, I'll make sure I share with the people who want it and not all the HATERS!
Late,
Nick
04-25-2007, 12:14 PM
#49
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Keep on trying, I used to have a program that did the calculations for me. It gives you a great representation of "street" horsepower if you can figure it out.
07-23-2007, 05:43 AM
#53
Originally Posted by Nietzsche
Keep on trying, I used to have a program that did the calculations for me. It gives you a great representation of "street" horsepower if you can figure it out.
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