[wilf leek]

The truth of the matter is this - it is FAR easier to cast aluminium than iron. And there are FAR more foundries working in ally than iron.

Therefore, any aftermarket supplier is automatically going to offer ally heads, and in whatever way, will try to market them as somehow "better" than iron. They usually are better than the OE iron heads, but only because of modifications to valve sizes and combustion chamber and port profiles leading to better breathing.

The truth is that if you could get these new designs in iron, they would be better, from a thermodynamic perspective, than when in ally. I am discounting the weight advantage of ally here.