changing cylinder heads without family change..
so as it stands - using the same block, but having a different head
for instance, Honda early D series SOHC heads are interchangable mostly, 1.5 block with a 1.6 SOHC VTEC head? no problem..
idea as it is - you've built a 2 valve SOHC setup, but as years pass, you want to update it with 4 valves, now you need to build a completely new engine, almost from scratch, why not just let update the head?
same goes for DOHC setups as well and of course VVL