Using OBD2 Data to Calculate Tq/Hp

[Gary in NJ]

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.

[jamesm]

Originally Posted by Gary in NJ

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.

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.

[Gary in NJ]

Thanks for the input. I'm thinking that there should be a relationship between MAF (air consumed) and fuel flow. Since fuel has a known energy value and I know the AFR, there must be a a formula to connect this information. The only guess would be volumetric efficiency, but at WOT it must be only a 30% loss.

[stugray]

The torqe app calculates it for you.