I am told by a head machinist / porter that there is a variation between I6 heads on AU's. In fact something like 13 different variants. Some are better flowing than others. Has anyone heard of this and if so how do you identify them. Building new engine for speedway Super Sedan.

Richie.

All I can think of is the three different models:
Standard
XR6
XR6 VCT

The difference between the standard and XR6 is the size of the exaust ports, i think its 38 vs 41. The valves should be the same size on all.