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Gas flow dynamics are complex, but simply put, a short restriction is better than a long one. The gas "drags" on the walls of the pipe, and a smaller pipe flowing the same amount of air means the air travels at a higher speed, causing more drag.
A good example is your exhaust. The paths inside the turbocharger, the wastegate and turbine itself are tiny, perhaps in the region of 40mm, but a 45mm long exhaust all the way to the back of the car would seriously choke the engine, so you go 2.5" or larger.
Similarly the turbocharger boost exit pipe is 50mm, but going up to 57 or 63mm for the long pipe run round to the the manifold will certainly help when power climbs. The S3 for instance has bigger charge pipework than the normal A3 models.
A good example is your exhaust. The paths inside the turbocharger, the wastegate and turbine itself are tiny, perhaps in the region of 40mm, but a 45mm long exhaust all the way to the back of the car would seriously choke the engine, so you go 2.5" or larger.
Similarly the turbocharger boost exit pipe is 50mm, but going up to 57 or 63mm for the long pipe run round to the the manifold will certainly help when power climbs. The S3 for instance has bigger charge pipework than the normal A3 models.