Where did this 'cold air will warp the valves' thing come from?
A tube header holds no heat. Cast iron was not chosen for exhaust manifolds because of some mythical 'keep the valves warm longer' reason, but because cast iron is easy to pour into complex forms (like an exhaust manifold) and machine.
I'd like to see some sort of verified information (SAE or something) that talks about this issue.. most of what I see is rumour and 'I hear that'...
Like you should not lay a crankshaft on its side or it will warp and you need to add ZDDP to an old motor.
