choppersafari
My understanding of it, stated as simply as I can put it (and correct me if I'm wrong anyone please) is that it's not that more manifold pressure is 'available' but that more manifold pressure is 'required' in order to obtain your 5 minute maximum (131hp). Cooler, more dense conditions - less MAP required. Hotter, less dense conditions - more MAP req'd.
Lets see what the more technically minded and more experienced amongst us have to say...