I've never examined the chuff data in the serial data, might be interesting some day.
The Super-Chuffer varies the chuff duration based on spacing of previous chuffs. It's also edge triggered, it doesn't care about the duration of the chuff signal once it sees an edge. I do have a maximum length of chuff that's active at very low speeds, and also when you reach a speed of 20 chuff/second, the fan runs continuously.