No need for you to know why, those of us whom have flown the engine types I specified previously, would know for sure,
I can only assume from that response that you don't know why yourself.
We have examples cited from people who have flown supercharged Griffons on Shackletons that categorically state that once the status quo has been resumed,ie the prop is now driving the engine, there is no loss of manifold pressure.
In the absence of any information from your supposed font of knowledge I can only assume that the American engines that you rave about suffered excessive back pressure such as Werbil describes, and there lies the problem!
Usually when people puff and blow without any real substance to their argument It's due to lack of knowledge!!!!
