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Autor Tema: COMO: Medir caballos de fuerza y torque con el G-tech  (Leído 382 veces)

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  • Fernando Gamboa
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COMO: Medir caballos de fuerza y torque con el G-tech
« : enero 17, 2008, 09:30:03 am »
COMO: Medir caballos de fuerza y torque con el G-tech

http://www.caranddriver.com/features/9380/do-inexpensive-performance-meters-work.html


Introducción
El uso del G-Tech por Tesla Electronics es una forma facil y util de obtener una indicacion precisa de tus resultados en los ajustes a tu motor. Este dispositivo ya tiene algunos años en el mercado, pero las ediciones mas nuevas tienen mas informacion y ventajas que sus predecesores.

El G-Tech utiliza acelerometros para medir la aceleracion. También puede calcular la velocidad y la distancia mediante formulas matematicas avanzadas basado en la informacion de la aceleracion. Los acelerometros de hoy en dia estan contruidos con circuitos integrados de gran precision. Esto significa que existe una gran precision asociada al G-Tech. Si ustedes siguen los consejos indicados aqui podran ver que el G-Tech ofrece muy buenos resultados y repetibles tan buenos como utilizar un dinamometro.

No crean que los dinamometros tienen la ultima palabra en cuanto a precision. Si alguna vez han ido a un dinamometro o visto varios resultados de mediciones podran ver que existen variaciones de un dinamometro a otro, en algunos casos con enormes diferencias. Los dinamometros son convenientes para los talleres para trabajar y hacer ajustes al mismo tiempo, para los clientes son buenos porque pueden medir sus motores sin necesidad de pasarse de la velocidad permitida en las calles.

El G-Tech siempre muestra numeros inferiores a los obtenidos en los dinamometros de tal forma que debes corregir los numeros si quieres normalizar los resultados con los de un dinamometro de roles. Si deseas obtener resultados lo mas cercano al SAE necesitaras hacer aun mas correcciones. Aqui es cuando una hoja de calculo es muy util.

La razon por la que los numeros de los HP's salen bajos, es porque el G-Tech no te mide los HP's al motor ni a las ruedas, mide la potencia neta real, es decir, resta la potencia perdida por la transmision, por la resistencia del viento, la fricción de las llantas contra el piso, etc. Todas esas fuerzas son mayores entre mayor sea la velocidad, por todo eso el motor pierde potencia mucho antes de lo que muestra un dinamometro de rodillos.

Por que utilizar un G-Tech?
  • Un G-Tech es preciso y los resultados medidos pueden ser calculados y comparados con otros resultados facilmente.
  • Un G-Tech es conveniente. Cuando haces algun cambio en tu vehiculo puedes inmediatamente utilizarlo para medir los resultados. No necesitas esperar o ir al dinamometro o esperar a ver si mejoras tiempos en el autodromo.
  • Un G-Tech tiene una buena relacion Costo/Beneficio. Nuevos cuestan al rededor de $300 dlls y quiza mucho menos usados. Puedes hacer mediciones tantas veces como quieras, en los vehiculos que quieras y no necesitas estar pagando dinamometros cada vez que hagas un cambio al motor.
  • Un G-Tech tiene opciones avanzadas. Puedes medir los tiempos de 0 a 100 kms/hr, ¼ de milla, octavo de milla, 20 metros, con sus respectivas velocidades, en los modelos RR puedes medir y grabar los circuitos de pista con su velocidad, aceleracion, distancia, fuerzas G laterales.
  • Un G-Tech puede grabar lo sucedido lo cual te da la suficiente informacion que puedes utilizar para conocer el estado de tu motor. Ademas la informacion que obtienes es representativa del desempeño real en calle/pista y no de una superficie artificial como la de los roles en un dinamometro.

Yo empeze a utilizar el G-Tech para ajustar el motor de mi Shadow GTS 91 (Arcangel) con muy buenos resultados.

Lo que necesitas:
  • G-Tech Pro Competition o un modelo mas nuevo como el G-Tech RR (el modelo SS tambien es bueno pero sin posibilidad de pasar la informacion a una PC)
  • PC o Laptop
  • Microsoft Excel u otra hoja de calculo
  • Una calle plana de 400 mts preferentemente sin imperfecciones en el pavimento y de doble sentido o bien una pista de ¼ de milla

Nota: Si tienes o consigues un G-Tech Competition Pro, existen nuevas versiones del software PASS disponible para descargarse asi como tambien un firmware mas nuevo para actualizarlo a la version RR.

Set-up:
You will need to read the Gtech owners manual carefully. Set-up is critical to obtaining the best accuracy possible. I’ll point out a few areas in particular you will want to pay attention to.

Your loaded vehicle weight is critical to know if you are going go get accurate results. The Gtech measures the acceleration accurately, but the wheel torque it calculates requires the weight of the vehicle to be known. Do not use factory specified curb weight. Do not forget to take into account your body weight and the weight of the gas in the tank.

Recommendations:
* Measure your car’s weight with a set level of gas and you in it at a local waste management facility or vehicle scale

* When you log your car's performance with the Gtech use the same driver and same level of gas you used when you weighed your car

There is no wiring on a Gtech. The unit senses the RPMs from the cigarette lighter/power adapter. Firings of the sparkplugs produce sharp current spikes and these small but detectable signals traverse through most of the vehicle’s wiring, which are detected by the unit. It is important to calibrate your RPMS because the Gtech will take RPM data gathered during a torque measurement and calculate effecting wheel horsepower. Do not use the vehicle’s tachometer to calibrate your Gtech. Use an automobile analyzer quality gauge, Apexi hand controller (if you have a Power FC), or something digital that has a crystal reference clock and is accurate. These are avaliable at most automobile part supply stores. Do not use analog RPM gauges.

Most cars are sensitive to air temperature, so if you measure performance on a 100 deg F day you will get different readings than a 70 deg F day. Colder air is denser and reduces knock, which provides more power. This is the case whether you use a Gtech or dyno – so this applies to all cases. Do your Gtech runs on days that have consistent temperatures, or use SAE correction factors to normalize the readings to a SAE specified temperature. I recommend doing your runs on 73-75 degree F days, to eliminate the need to do SAE corrections for temperature.

Avoid doing runs on windy days for obvious reasons.

For those living at high altitudes, your measured results are likely to be lower than someone who lives on a flat coastal plain. Again, it doesn’t matter if you are using a Gtech or a dyno, but higher altitude has less dense air and the associated air pressure is less. SAE recommends referencing all measurements to a set barometric pressure which is indicative of a sea level condition. Equations exist to correct your measure data. I do not have an accurate barometer and although I live at a nearly 1000 ft elevation I did not correct for barometric pressure. If I did, it is likely I would get slightly higher numbers for my measured horsepower and torque.

Choose your road to be flat, straight, smooth, and as little grade as possible. It should have (2) lanes with the same characteristics going in opposite directions. Bumpy roads will provide measured results with a bumpy appearance. If the road has a grade, even one that is hardly noticeable, it will skew your numbers. The reason is gravity has a constant acceleration that will influence Gtech measurements when the runs are not perpendicular to the force of gravity. If you take your Gtech and rotate it 90 degrees from it’s normal operating position you can actually measure the 1G gravitational pull. I’ve done runs on roads I thought were perfectly flat and they weren’t. Don’t worry, by using the techniques outlined below this won’t be a problem.

Recommendations:
* When you do a run, use your Gtech to log in both directions on the road. By doing this and averaging the runs, you will be able to cancel out the effects of road grade. If you do not do this you are likely to produce inaccurate results.

* Find a road that will allow you to hit top speed in either 2nd or 3rd gears. Note that 3rd gear places you over the speed limit, so if you decide to do this or for that matter any Gtech measurement on a public road it is entirely your decision and responsibility. It is likely the road will need to be close to ¼ mile long, and of course devoid of traffic, pedestrians and road hazards. Doing 4th gear runs is dangerous, and not practical because to do a horsepower/torque measurement requires you to shift from 1st gear and land in 4th gear - the car will likely bog. Also a 4th gear run will require a long stretch of roadway which also limits practicality.

Gtechs accelerometers are self-leveling, so the main issue with mounting your Gtech is to find a solid mounting location and to make sure the mount itself is solid. I use the windshield mount that comes with the Gtech and mount it low and centered on my windshield.

Gtech Logging:
The key to getting good runs is being consistent. Your launch and steadiness is steering are contributors to having good runs. I’ll describe the steps I use to achieve consistent runs.

It is best to do a 3rd gear run. 3rd gear runs are more consisten to the 3rd and 4th gear runs done on dynos, so this will provide better correlation if that is once of your goals. Also, 2nd gear appears to not be as efficient in transferring power to the ground as you can see by this picture.



You can see the comparisons between the 3rd gear run (magenta) and the various 2nd gear runs done with different PFC basemaps (untuned).

Here are 4 steps to follow to log some good runs:

Step 1
Before you begin a run, you will need to look at what RPM range you want measured. That’s because the GTech triggers it’s torque measurement once you make a gear shift, so you need to know in advance at what RPMs you will want to shift at to land at a particular RPM and start the measurement. To do a 3rd gear run with measured RPMs starting just below 2500 RPM, you will want to rev 1st gear to just below 4000 RPM then shift to 3rd gear. This should give you measured results from 2500 RPM to redline (or what ever you chose your cut off to be).

Step 2
Start by launching in 1st gear. You want a brisk launch to trigger the Gtech, but you do not want to spin the tires. A rapidly feathered clutch launch at 2500-3000 RPM will work good. Shift to 3rd gear and keep you foot planted on the accelerator until you reach the upper RPM you are looking to measure, upon which you release the accelerator which triggers the Gtech unit to stop measuring.

Check your run to make sure it looks like what you expect. There is a menu to check the torque and horsepower curves.



Step 3
If the run is good repeat the process on the exact same stretch of road, but on the opposite direction. These two runs should be averaged together and will form a "composite run." Do not mix pairings of runs. You will find that if you look at the averages of various pairings they are very consistent, which is why I call these composite runs. If they are not consistent you may have blown a launch or messed up the measurement.

Step 4
Repeat step 3 at least 3 more times. This will give you a total of 8 runs (4 composite runs).

Here are some hints to achieve the best possible measured power:
* On a 2Z engine, you will likely get a good initial run if your car was run when it just hit operating temperature, but you will find that for the next several runs the power will slightly taper off. However, the power comes back once the oil has warmed up and thins out. I find that running the car 30-35 minutes before doing runs achieves the best results.

* Most engines hate heat. Avoid heatsoak. Also, a high water temperature usually will result in the ECU retarding timing and the intake charge being less dense. To help this situation, I run the heater full blast – I get a little better power when I do this.

Post Processing:
The Gtech PASS software is a good logging tool but I found Microsoft Excel is a better tool to first average the runs then do the needed corrections. The raw averaged results you measure using the Gtech will be low relative to dyno results. This is primarily due to air resistance which rises as the cube of the velocity. On a 7th generation Toyota Celica, Air resistance accounts for a parasitic loss of 20 ft-lbs of torque at 8000 RPM in 3rd gear! A secondary factor is rolling resistance which contributes less to parasitic loss, but impacts the reading nonetheless. On a road “dyno” the ground does not move and provides added friction which shows up as a parasitic loss. On a dyno that uses rollers, the rollers spin and offer much less frictional resistance.

For both air resistance and rolling resistance the correction factors are much larger for 3rd gear coreections than 2nd gear corrections because the speed in 3rd gear is much higher. Here are the 3 steps needed to provide corrected horespower and torque measurements.

Step 1
In your Excel spreadsheet start first by calculating the appropriate correction factors for the appropriate speed and RPM. These will be your correction factor tables. Calculate the correction factors starting at 2000 RPMs and go to the desired upper limit in 2nd or 3rd gear, depending on what gear you want to measure. I use 100 RPM increments.

The equation to use for air resistance is:
[Cd *area(sq ft)*(3rd gear mph^3)/15000]*5240/RPM

The equation to use for rolling resistance is:
((0.008+3.24*0.0018*(3rd gear MPH/100)^2.5)*load weight)*(3rd gear MPH*(5280/3600)/550)

When you are done the correction factor table should look like this:



Step 2
Construct a data processing section in your spreadsheet. Make room to enter the raw data with RPMs on the vertical axis and torque/HP on the horizontal axis. Take the data from the Gtech and enter it into the spreadsheet. Use the closest RPM data point the Gtech offers to the RPM data point in the spreadsheet. A composite run (pair of runs) should look something like this:



After looking at the pairs of data you will find the average of each pair is consistent to other pairs. For even more consistency, I toss the highest reading pair and the lowest reading pair of data. I look at the overall values and not the absolute peaks to gauge which set of data should be tossed. (If you log the data carefully there really should not be much difference) After tossing the data there will leave a minimum of 2 pairs of composite data. The more data you have taken the better, but I have found 2 sets of composite runs is the minimum set of data you will want to run to achieve consistent results.

Step 3
Average the remaining data together and apply the correction factors calculated in the correction tables. The data will represent your corrected torque and horsepower and look something like this:



If you want to correct for air pressure (altitude) or temperature (if you did not log at the recommended temperatures), you can also perform SAE corrections. I did all my runs at consistent temperatures, but did not correct for my higher than sea level altitude, so my torque and HP is likely to be still a slightly lower than SAE corrected numbers.

At this point you've got data! Use the graphing functions in Excel to display your measured torque or horsepower.



or compare tuning maps or other tuning progress.

-Chris (Torqued)

G-Tech Template: http://users.ameritech.net/trdcelica/Gtech_Template.xls

There is a guy from The Netherlands who entirly tuned Is Subaru WRX with a gtech and he did an Excel Spreadsheet wich he updated a lot of times (last june 2006)

There is a lot of info you can enter on the sheet to correct the HP/torque numbers

-You can import the .csv from gtech.
-You can switch from metric and US unit.
-You can apply drivetrain loss correction.
-You can apply atmospheric correction (there is also a weblink where you can find the temperature in your region. You can also go back in time).
-You can apply aerodynamic correction.
-There is a toolbar with some option and much more

You'll need to lower your macro security to enable them and be warn the the file will start in Dutch language, you need to switch to english from the TAAL button in the tool bar

Here is the file --> http://members.chello.nl/~r.herweyer/software/gtech.zip

Here is his website --> http://members.chello.nl/~r.herweyer/
« Última Modificación: febrero 06, 2008, 04:09:40 pm by starman »
Fernando Gamboa
Monterrey

Neon SRT-4 2005 Record: 11.525 Stock: 14.54
LZT6AIX-13, STS, Insertos Prothane Race, Termostato 180°F, Escape 3" MaxxFab, 750cc, 3bar, DSTrinity TDSTuned, MPxFMIC, LM-2, N2MB-Boostbox, Clutch ACT DN4-HDSS, Turbo AGP Delta 55

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Re: COMO: Medir caballos de fuerza y torque con el G-tech
« Respuesta #1 : septiembre 18, 2009, 04:26:21 pm »
Pues ya hace tiempo estuve pasando esta informacion en el foro, pero se me habia pasado ponerla disponible y terminar de traducirla, de todas formas aqui se las dejo para ver si les pude ser de utilidad ;)
Fernando Gamboa
Monterrey

Neon SRT-4 2005 Record: 11.525 Stock: 14.54
LZT6AIX-13, STS, Insertos Prothane Race, Termostato 180°F, Escape 3" MaxxFab, 750cc, 3bar, DSTrinity TDSTuned, MPxFMIC, LM-2, N2MB-Boostbox, Clutch ACT DN4-HDSS, Turbo AGP Delta 55