How can i calculate the mass air flow rate

okay so i need to calculate the mass air flow rate so i can calculate the volumetric efficienty ,so am using a gas engine for the tests, the fuel "OCTANE 87", the equation of fuel combustion is, C8H18 +12.5 O2 + 3.76(12.5) N2>>8CO2+9H2O+3.76(12.5)N2
so i calculated the mass rate of carbon dioxide per second that formed from fuel combustion, i got 6.844 grams of CO2 per second
okay here're additional info incase
the engine is 2.4 liter, assume a constant speed 60 mph, constant RPM 3145, straight road, constant pressure of the car"empty" etc..
thanks in advance guys

Given mass flow rate of CO2, convert it to a molar flow rate by dividing by the molar mass of CO2, of value 44 g/mol.

N_dot_co2 = 0.155545 mol/sec

Now use the molar coefficients of the equation to get the molar flow rates of BOTH O2 and N2, by multiplying the CO2 molar flow rate by the stoichiometric ratios of 12.5/8 and (12.5*3.76)/8.

Results:
N_dot_o2 = 0.243039773 mol/sec
N_dot_n2 = 0.913829545 mol/sec

Convert back to mass flow rates by multiplying respectively by the molar masses of O2 and N2, which respectively are 32 g/mol and 28 g/mol.

m_dot_o2 = 7.77727273 g/s
m_dot_n2 = 25.5872273 g/s

Add it up, and state the answer with 4 significant digits:
m_dot_air = 33.36 grams/s