the displacement of the motor is not increased with increased air/fuel mixture. The dismplacement is the measurement of how much the pistons are moved... area of the cylinder X the stroke of the crank X number of cylinders. Increasing air/fuel mixture merely makes the combustion process harder and thus the pistons are pushed downward spinning the crankshaft harder which is measured as HP. Because you increase the air does not mean the displacement has increased. Yes you are producing more exhaust... but still not nearly enough to max out a 2.5 inch pipe. Even if your argument,which does make some sense, did hold true then you could not explain how other cars like Grand Nationals are producing 1000+ HP with a larger motor (3.8L turbocharged V6) with only a single 3 inch exhaust. Do the math... pi X radius squared. A single 3 inch exhaust has 7.065 area. A 2.5 inch exhaust has 4.906 area... but that's for each pipe... so a normal dual 2.5 inch exhaust on a 300ZX has 9.812 total area for a still smaller engine.
