Originally Posted by
Onceapilot
Lumps, you are correct that the very coarse fixed pitch prop would hold back the RPM at low speed, and so reduce the supercharger output, but the Merlin was fitted with a boost controller which throttled the supercharger to prevent excessive manifold pressure. This could be overridden in emergency, with risk of engine damage/failure. Unfortunately, I do not know if the override completely removed control of boost or, if it allowed a higher controlled level of boost. The latter would seem more likely as the amount of extra boost available at low level was considerably more than rated boost.
Cheers
There is an excellent explanation of how the Merlin boost control works
here
The use of the term 'throttle lever' on a Merlin (and other similarly controlled engines) is a a bit of a misnomer, since it does not have complete authority over throttle position. It's more of a 'power lever' working in conjunction with the propeller pitch (rpm) lever.