here you go i spell it out simply
502 divide by 8=62.75 thats one cylinder
every revolution 4 cylinders draw air so multiply by 4
=251 divide by1728 to covert to cubic feet = 0.145254 then multiply by max rpm intended 5000 grand total 726 cfm.
so no point going larger as it can only draw that amount.
that figure is with 100% volumatic efficiency,due to marine cams,intakes,exhausts etc your more likely to have about 85%
now do you still need 850cfm i will sell you one but dont complain when its no better than what you have already,steve