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      11-11-2008, 04:43 AM   #96
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Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

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Originally Posted by lucid View Post
Also, do you know what kind of accelerometer the DashDyno unit is using to estimate torque?
Originally Posted by exdos View Post
The DashDynoSPD doesn't use an accelerometer to compute torque.
To do a Power/torque calculation you first need to set up a profile of the car, where you need the frontal area of the vehicle, weight, Cd, tyre diameter, gear ratio, air temperature, altitude and humidity. The DashDyno software then makes its calculations based on vehicle RPM sensor. The figures that are given are "at the wheels" and are corrected for all the variables of environment (altitude, temperature and humidity) so that the figures are "standardised" and thus comparable for the same vehicle driven in widely differing conditions.
To more fully answer lucids question...

I looked at this unit a bit more and really like it. Definitely a great tuners tool as exdos has shown us.

Basically the unit gets around having to use an acclerometer by reading vehicle speed from the OBD. You can get from velocity to power with

P = F * v = m * dv/dt * v

dv/dt can come from numerically differentiating the veolcity vs. time curve. I'm not 100% sure this unit does this exact calculation but as you know by numerically integrating or differentiating can take you from a acceleration to speed to position or in the other direction. I'd pretty much be willing to bet this is the basics of the method it uses.

Now what would be really cool is a function that accounted for the vehicles changing mass as the tank emptied. An empty vs. full tank in our car does give rise to a 12 hp difference. I guess the lesson of that calculation is if you want really accurate and repeatable numbers with this system (as with a Gtech or any meter that requires vehicle mass) make sure you know it accurately and/or always test at the same fuel level.