Smokebox Physics

This months CLASSIC TRAINS had an article about the PRR S-2 which indicated the locomotive had four seperate stacks that worked sequentially from lower to higher throttle settings to maintain a proper draft. This drives ther question: if one had a manometer or vaccum gauge, what readings would be expected under low & full throttle settings in the smoke box of a modern (designed after 1936) Northern Type locomotive?

I have no idea whether "The Ultimate Steam Page" is still up, but they did, a few years ago, have an article about a (then new-ish!) book on steam locomotive drafting.

Good news! "The Ultimate Steam Page"still exists (and is still regularly updated). It's page on drafting and the theory behind it,
http://www.trainweb.org/tusp/exhaust.html
has many links: I haven't followed them, and I don't know if one will give the answers to your question about the physical conditions inside a locomotive smoke-box, but it might be a starting place for finding out.

Did it really say at partial throttle steam was only passing thru two or three of the four stacks? So one or two stacks would be allowing outside air into the smokebox? Which would destroy the draft? Or did each stack have a plate to close it off? How well sealed would such a plate have to be?