Wilf ,
I think of it this way; I am running a four barrell with mechanical secondaries (primary 2 on each carb with progressive link) each of those 2 bbl's has its own filter, this is fine for most driving.
THEN if at higher throttle openings (wot) my engine says "how bout a little more " those precisly tuned vaccuum secondaries open just the correct amount to accommodate breathing and fuel charge at high rpms, I doubt the effective cfm used at 6000rpm exeeds 850.
and they are really shiney
KK