Do filters affect the power readings?

Category: Dyno Usage

Sure!. The general rule for the Low-Pass-Filter (the one that is set in seconds) is that the filter size shouldn’t be bigger than the test duration / 10 (the acceleration part).

As a general rule an inertial dynamometer shouldn’t produce acceleration tests of less than 5 seconds (at the suitable gear), then the minimum filter you should be able to use always is 0.5 s.

For braked dynos as you are able to control the test duration with the brake, our recomendation is having tests between 7 and 10 seconds (or more), then the filter could be 0.7 to 1.0.

On the other hand the filter named “HF” normally is used with fixed values. Actually all filters are low pass type, but the “HF” attenuates specially higher frequencies, (actually it is a Low Pass Filter that focus on higher frequencies), and not so on slow vibrations, for that reason the “Low Pass Filter” was provided to reach lower and lower mechanical frequencies that are present specially in car dynamometers, also the LPF works worse for the very high frequencies, for this reason it is good to use both: “HF” with a fixed value, and LPF depending on the average test size

The recommended values for the “HF” are 5 and 7. The software provides a feedback of the cut frequency, but it is not very intuitive to understand how the test works as frequencies (for instance a “round” HP graph that last 10 seconds has a main componen of a 0.25 Hz wave), for that reason we suggest to use fixed low values (5 to 7)

The problem with the “HF” filter is that the bigger it gets the more delays and distortion it adds, so it is better to use it only for high frequencies (from digital noise and gear teeth irregularities) and not trying to go beyond on filtering, and then using the “LPF” for the mechanical vibrations

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