I'm a newbie here... Just ordered my Genboard & hoping to contribute to making new stuff.
I have written a street dyno before (for old shape Impreza WRX)
It was based on sampling RPM very quickly through the engine diagnostic port and time stamping the data...
- If you know the gear ratios and tyre size you can work out a road speed for each data point.
- You can then numerically differentiate the speed to get acceleration.
- Then use F=ma to get force and thus estimate accelerating power.
Sensors that can be used
- RPM - most common and simple
- gearbox switch
- not needed if we have vehicle speed
- vehicle speed - several options, see below:
Vehicle speed - note that climb is also interesting, not just horizontal component
- wheelspeed (no info about climb)
- G-meter (both for acceleration and climb)
- analog output (relatively unexpensive)
- G sensor from ABS system. Maybe we can talk to it over CANBus?
- GPS (both for speed and height -> climb)
To deal with climb, I did 2 runs (one there and one back) and took an average. This also helps with wind.
I have found that GPS altitude is not all that accurate as it seems a lot of handhelds use pressure to work out altitude (even if they claim to do it using the satalites!)
RPM-only dyno: problems
The main problem was the quantization in RPM - this limited useful data points and made the answer very "noisy" at times. (Road speed has worse quantization in these cars!!)
Other problems were accounting for clutch slip and wheelspin. (Not too bad to do)
It is also possible to measure free-revving, which eliminates vehicle-speed measurements altogether. However,
- it is more sensitive to RPM measurement
- while good for comparison and tuning, absolute power results are not accurate
- most engine management systems make this kind of tuning impossible, because they don't calculate/actuate ignition advance (and maybe fuel) precisely (fast enough) in extremely-fast revving situations.