This post was in reply from a thread below. I re-produce here in case there is interest from other PuristS on the effort MJLC went through to maintain the accuracy and stability of the movements
Other factors being equal, the higher the frequency, the better the timing accuracy will be over the 8 Days running of the watch.
From the very beginning of Product Development (The 8 days movements as used in Septantieme, Reverso Grande Date, Reverso Grande Reserve, Master 8-Days and Master Antoinne), the original idea of the project team is to decide on the 4 Hz frequency. see our previous write-up here : click here
At 4 Hz, more energy will also be needed to maintain these oscillations, hence more difficult to ensure sufficient power reserve.
The size of the balance is also crucial which must be taken into account to ensure sufficient supplied energy. Too big a balance wheel would consume too much energy without providing additional benefit in modern watchmaking (In the old days, a huge balance wheel is necessary to maintain accuracy)
This internal JLC (much simplified and re-plotted with the exact same shape to protect the innocent
) chart shows the force curves of the two barrel springs which were tested on the first prototypes.
In red : A watch with the spring at a frequency of 3 Hz has a good power reserve of around 10 days but the force and amplitude are too low (MJLC keeps this confidential, but a technician told me that this was measured from an actual 10 days watch. I will not disclose the brand other than to hint to fellow PuristS that this Geneva brand is THE No 1 haute horlogerie brand. And their 10 days implementations still had a big marketing impact this year)
In blue : Initial JLC prototype, for the 4 Hz spring, the amplitude is correct at the outset but falls off very rapidly and the watch stops after 8 days. In this initial 4 Hz version, There is no safety margin in terms of power reserve.
MJLC did not wish to sell an 8 day watch without being certain that the watch would always go on working after 8 days. At the same time, they did not want to lose the benefits of the 4 Hz system.
The ideal solution was to keep the 4 Hz frequency, while trying to lengthen the power reserve.
The team recalculated the spring (shown in black on the chart) which is thinner, hence permitting more turns in the barrel and extending the power reserve to 9 days.
To maintain the 4Hz frequency, the initial amplitude is slightly lower but has the advantage of being much more stable over a period of time.