v6 Heads are over-ported from the factory. You see the ports are shaped around the american bunch of bananas manifold, and the junction between head and manifold is the area in which the air stream turns from a vertical up and down type flow, into the back of the valve.
In essence - the port doesn't cut straight across the designed air path, but at a diagonal.
Holden australia re-worked the Inlet manifold to build torque - quite successfully I might add. No Alloytec has ever had the torque of the ecotech.
In the re-work, the existing port ended up too big for new air path. which as long as you don't machine out the manifold, doesn't affect airflow much, and seems to impart a bit of tumble which helps atomisation at the injector.
If you machine out the manifold to match the port, you end up with a dirty big chamber at the end of the manifold runner, that is almost twice the cross sectional area of the inlet runner. What this does is slow down the airflow, reducing torque.
From what I have been able to test, there actually looks to be some gains available (as much as 25kw) from welding up the ports in the head, and grinding them back to match the manifold. It seems if you add some material to the floor of the port in the head, flattening out the curve in the floor, and match it up to the inlet runner, you get a much much better flow velocity.
I've been doing some flow bench work and at 26inHG the flow figure goes from 121 CFM up to 138CFM and airspeed 88% of inlet to 104% on inlet - inother words flow is increasing and air is accelerating as it travels down the runner instead of slowing.
But atomisation is reduced due to less tumble. Careful shaping may see me able to add some swirl back in though. Lots more work to do yet.