A good porter can get the standard 42mm valves to flow as much air (or more) than the "oversize" valves. The gains in airflow come from propper port resizing and carefull shaping in the right areas. The cvh doesnt suffer from valves too small, but highly restrictive ports. However, if a standard inlet manifold is used, the porting and big valves is largely a waste.

The larger valve in most cases is not required. Oversize valves is a sales tactic used by some shops. Everyone likes to tell thier friends that they have a "stage 3 head" whoopeee!!!

lol

Cheers