The original poster needs to clarify the question.
"Nominal displacement" (converted to the units of your choice) x half (because of 4 stroke engine) x revolutions per minute x volumetric efficiency under the specific conditions involved = volume rate of flow.
The trouble is that the volumetric efficiency depends on a TON of variables and can often only be estimated. How accurate the estimate needs to be, and how much effort and calculation and simulation needs to be put into producing that estimate, depends on what you are doing with the outcome, and THAT is something you haven't told us.