Cylinder head change between engine variants

as the title states..
why not?
why not?
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Time taken to develop a block: a engineer-hours
Time taken to develop a SOHC head: b engineer-hours
TIme taken to test & validate complete engine: c engineer-hours
Time to develop New Engine 1: a+b+c
Time taken to develop DOHC head: d engineer-hours
Time taken to test & validate the new combination: e engineer-hours
Time required to develop a DOHC "variant" of New Engine 1: a+b+c+d+e
nialloftara wrote:Honda's a very bad example sohc dohc and vtech all in the same famiy. I'd say GM is better, many sizes of the same engines and Valve train changes only happened after family changes.