alright, first you take the displacement of you engine and get it into cubic inches, not liters. a 1.8 is 110 ci, the 1.9 is 116 ci, the 2.0 is 122 ci. Now take your redline, in the case of the 1.8 7000rpm, divide by 2 - you have 3500 times the cubic inches 110 in this case. 3500x110 = 385,000 ci cubic inches. divide by 1728 to get CFM. 222.8 CFM in this case for the 1.8L. the 2.0 ztec with its larger displacement but lower redline requires only 229.5 CFM just a slight increase. and since the whole idea is to get air into an engine to mix with fuel, the 2.0 just barely edges out the 1.8 in power despite of its 10% displacement advantage.