compressor housing Qs

[iTurbo]

How does the size and A/R of the turbo compressor housing affect the airflow characteristics of a turbocharger?

Are there different A/R housings available for the T3?

When upgrading the compressor wheel, does the housing have to be upgraded or is it machined somehow to accept a larger wheel?

What is the difference between a Garrett T3 .42 A/R housing and a T4 .42 housing? Is the T4 bigger even though the A/R is the same?

[shelbyglhs222]

My stock compressor housing has a big t4 46 trim wheel stuffed in it(75 mm exducer). All turbonetics did was machine it to accept the bigger wheel.