Recommended procedure for assessing power output of a car based on race data
The Race Technology data logging products are capable of accurately measuring power output at the wheels of a vehicle. This data has been used in several applications to classify cars by their power output in various race series.
For high accuracy power measurements some care is needed, otherwise measurements will be inaccurate and variable. In particular the power measurements are sensitive to errors in the accelerometer reading, particularly at high speed. For example an offset of 0.01g at 100mph in a typical saloon racing car would result in an error of about 8hp.
For accurate power measurement, correct mounting of the logger is absolutely essential, at the very least make sure that:
- The logger MUST be mounted squarely in the car, i.e. the front of the unit must point directly towards the back of the car.
- The logger should be mounted as flat as possible. Although the software will correct for errors up to 20°, such large offsets are not recommended.
- The logger must be relatively free from vibration. It's very difficult for the end user to actually measure vibration, but a good guideline is if it looks like it is vibrating then some form of isolation would be beneficial.
- Avoid extremes of temperature
To measure the power of a vehicle based on logged race data, we recommend the following procedure:
- Make sure that the car's mass, rolling resistance and CdA have been set correctly in the car configuration.
- Check that a realistic filter time constant is set for power - normally 1 second is adequate.
- Load in the data.
- Make absolutely sure that the accelerometers are zeroed correctly to remove the remaining acceleration offsets, and make sure you enable "Tune the longitudinal accelerometer offset using GPS data".
- Enter the correct ambient conditions into the run manager.
- Make sure you have selected the correct power units and correction standard under variable manager.
- Check the track map and add a lap marker on a suitable straight.
- In the measurements table add a measurement to show the maximum 95th or 98th percentile of the power.
The results will be shown as the peak power per lap. It is best practice to take the power from the fastest lap, these are typical (real) results :
|Lap 1, 59.06s
|Lap 2, 57.61s
|Lap 3, 57.66s
|Lap 4, 57.53s (f)
|Lap 5, 57.77s
|Lap 6, 57.78s
|Lap 7, 58.58s
|Lap 8, 58.09s
|Lap 9, 58.64s
In this case you can see that the first few laps have a higher power than the rest - this is entirely typical and occurs because during the first few laps the vehicles are very closely bunched, and subsequently the cars aren't being subjected to the full aerodynamic drag over the whole lap. It is normally most sensible to take the power reading from the fastest lap - so in this case the power will be 204.6hp.
There is now a tool that automates this procedure, details are available on this page: Power Measurements
To processing save time it maybe desireable to predefine all the car files so they are automatically loaded. Information on how to do this is available here.