Looking up a book by R-R on supercharging, and the Merlin XX in particular, MS gear ratio was 8.15:1 and used for t/o, SHP of 1318, BHP(at prop) 1132 at 50 insHg(+9lbs) at 3000 rpm.
Running the same boost and rpm in FS (9.49:1) would only give1236SHP, and 1020 BHP(prop), figures for sea level. Changing to FS gear when the boost began to fall would restore the power to peak at around 20000ft.
Although the higher gear ratio restores the power , this is due to improved intake efficiency, and reduced exhaust back-pressure
So , changing to FS gear is speeding up the supercharger from 24000 rpm to 28000 rpm, if you do it too early , more power is used from the SHP to drive it resulting in an overall loss, at low level. The reason is that it is actually like going into overdrive/ top gear too early, as the engine is running at a constant speed, and cannot change to accomodate the extra load.
Hope that explains it!!
For you Varsity/ Valetta guys, memory says after t/o , one reduced RPM first and then boost, against normal practice of boost ,then RPM??
Baron, thanks, your corrected post answered my question- just shows the grey-cells still have some life!!