Aluminium cylinder heads are rubbish, apparently?

[Alan]

It’s true. However I’d qualify it by saying all other things being equal. The reason aluminium heads are around is that aluminium is a lot easier to cast than iron, so anyone wishing to mess around with combustion chamber shapes would find it a lot easier to use aluminium. So, it might not follow that an iron head always develops more power, but an iron Ulster head (say) will develop more than the same shape cast in aluminium.