Toyota GR86, 86, FR-S and Subaru BRZ Forum & Owners Community - FT86CLUB

Quote:

Originally Posted by Gary in NJ (Post 1296502)

Is there a formula I can use to infer Torque or Horsepower from the OBD data? I know it's not completely accurate (but then again neither is a inertia dyno). I'd like to be able to calculate torque to asses changes to my engine.

here is a rough translation of the most basic version of the formula that innovate uses, which doesn't require an accelerometer. if you have an accelerometer you can get better results.

Code:

values in () are reasonable defaults (not necessarily correct for the 86)



Vehicle weight in kg = m (580)

Tire diameter in cm = td (53)

Gear ratio of run = gear_ratio (1.5)

Final drive ratio = finaldrive (4.375)

i Name of RPM channel = rpm (RPM)

Frontal area of vehicle (sqm) = Ar (2)

Drag coefficient = Cd (0.45)

Current atmospheric pressure (millibar) = atmp (1013.2)

Current temperature (degC) = temp (15)



#SAE correction factor

SAEcf = (1.18 * ((990/atmp) * sqrt((temp + 273.15)/298)) )- 0.18



#air density

density = 0.0412236 * atmp/ (temp + 273.15)



R = td / 200 #tire radius in m



a = slope(RPM) * pi * td/(6000 *gear_ratio * finaldrive)

#acceleration in m/sec^2



# speed in m/sec

v = (RPM * R * pi ) / ( gear_ratio * finaldrive * 30)



# aerodynamic draq in Newton

Fa = 0.5 * density * (v^2) * Ar * Cd



F = m * a + Fa



trq = F * R / (gear_ratio * finaldrive) # in nm

hp = trq * rpm / 7121

not saying it works or is perfect, just that is what innovate uses.

you can calculate all of this realtime or afterwards from logs using the ot-2, which uses the accelerometer in your iphone in addition to the mts obd2 stream.