UK vs USA piston engines

[virgo]

Are there any old-timers who can sort this out ??? (For more details see Tech Log, Windmilling Propellers)

I am absolutely correct in saying that on a Rolls-Royce Griffon engine the boost (manifold pressure) will be maintained if the engine rpm is unchanged.

411A and a few others insist that on Pratt and Whitney supercharged engines the manifold pressure will REDUCE even if the rpm is unchanged.
(Both cases assume the is no change to throttle (power lever) position)

To summarise,
1. Aircraft straight and level in cruise
2. Propeller in the constant-speeding range (On speed)
3. Throttle (Thrust lever) set to cruise boost - say +6 lbs (42 " Hg)
4. Power is lost due to either total fuel starvation or both magnetos being switched off

Most people agree that engine rpm will be maintained by the CSU (propeller governor) moving the propeller to a finer (lower) pitch to maintain the selected engine rpm.

Question...........What happens to engine boost (manifold pressure ) and WHY

Is there anyone out there, who understands the question and knows what they're talking about, who can give an explanation for the alleged differences between British and USA engines ?

[Lancman]

A supercharger is a pump.

If it's RPM are constant, as in one mechanically connected to a windmilling engine that is being driven by a constant speed propellor, it's output will remain constant

If, however it's being driven solely by the residual pressure in exhaust gasses that are released from a windmilling engine in which no combustion is taking place then the supercharger RPM will fall, together with it's output pressure.

[oxenos]

If the supercharger is being driven by the residual pressure of the exhaust gases, it is a Turbocharger. The original question ( on Tech Log ) specifically referred to a charger driven mechanically by the engine, which is a Supercharger.