The supercharger on the DB series only have 1 speed until; I believe was, 1 km in height. Then the variable speed started as the second hydraulic pump started to kick in to start adding speed to the supercharger. This was a problem with the supercharger system of the DB series of motors as they consumed power without a way to increase boost/power down low. You can only turn the motor at X number of RPM.

Now you're going to make me find that info. to make sure I have it right. Well I was off on the height. It's 4,900 ft. instead of 3,000 ft.

OK here's one explanation of how the supercharger operates.

"A two-stage pump supplies the coupling from the main pressure oil filter, each pump stage being of the normal gear design. The first stage has no control on it and supplies the whole of its output
to the coupling, so that there is always some oil present in the spaces. The output of the second stage is governed by a capsule which responds to intake pressure, so controlling a piston valve
which divides the oil between the coupling and the by-pass back to the engine sump.

The second stage commences delivery to the coupling at a barometric height of 4,900ft. and full delivery occurs at 11,500ft. approximately. In this way is the desired result of a variable-speed drive
between engine and supercharger obtained."

https://drive.google.com/open?id=0B-ehX ... EsyVllhTmM