Most stock PCV valves have a poppet weight and aspring acting on it, at the beginning of it's travel. The vacuum will lift it off it's seat at about 2" and it gives some flow at 4-6" of vacuum. At 6" vacuum the poppet contacts the spring and now it is fighting both it's own weight and the spring as it rises. At the top of it's travel it begins to close off, not completely, but to a flow of 1.3-1.7 cfm or so. With the oem spring it reaches that point at 14" or so.
The lighter spring and adjustable ME Wagner will allow less vacuum to pull it to it's full travel, whereas the oem spring won't.
TSM chart pulled from a AMC forum. here is the flow rating in cfm:
