[Bitza]

Have just found an exhaust calculator on-line, based on the formula from A.Graham Bell's book. Having fed in the capacity and cam(Vee-Two 300) details, it tells me that a 45mm dia header would be optimum at around 6000rpm and therefore I guess becoming progressively more restrictive as revs rise. A 50mm dia is optimum at around 7750rpm. On which basis I conclude that the advice to use 50mm is sound.

Thanks for the feedback people have given, but don't let this post stop anyone from saying their piece. Bitza