On my W-9 engine(405ci) I cant say what the gain would be because I built it with the dry sump from the get-go but with just the dry sump I would make 12"hg at RPM and maybe 1"hg at idle.. if you use both a dry sump and a vac pump you will most likely need a check valve to limit the vac level... myself, I dont like to go above 14"hg