thats a dificult one. what size engine hp a rough guide without knowing is make a temp jet up 3mm bore. if it holds at 90psi its probs between 14 and 28 cfm . using a 6mm bore jet if it holds at 90 psi its probs between 60 and 90 cfm . my 90cfm pump has a 20hp 3 cylinder perkins on it . its not exactly a scientific way but will give u a cheap quick clue