why is a vacuum pump needed with a 5 stage dry sump pump bbc

[badbug3]

i see a lot of dry sump engines with a separate vacuum pump. Why is that? Why wont 4 scavange outlets create enough vacuum in a bbc engine?

[edvancedengines]

nope surprises me too and surprises customers when I tell them to add vacuum pump system.

with big motor and 4 stage suction you may get a negative atmospheic pressure in crank case of 5 to 6 inches. not enough to help anything except oil return and it helps a bunch if you have one stage plubmed for drain back from on top. . ed