I believe you guys are thinking to much into it...

If your manifold is ported to *match* the larger TB (which has the same opening pattern as a small one.....horizontal at WOT) then you will get better airflow, your engine won't struggle nearly as much as a small orifice, and it will get plenty of oxygen.

While if your manifold is not ported, when the 60mm TB is opened air will slam into the manifold walls, disrupt the flow, cause turbulant air, and not flow evenly. While if you matched the opening w/ a smaller one, you would be getting air a greater velocity, but at the expense of the engine having to use more power to 'suck' it in.

In other words, you will get increased performance on the 60mm if your manifold matches (and probably more than the stock TB if your not ported, do to less restriction), but if your not porting it get the 56mm (I believe this is the max size for the stock intake mannifold).

Joel Baldridge, ASE Certified Master Technician, Audi Certified Expert Technician