Can anyone think of another reason why mild or stainless steel isn't commonly used for aftermarket inlet manifolds?It's cheaper and easier for the homebuilder to work.Alloys and plastics are easier and better to mould and weld.If you have the tools to hand.If youwant to put throttle bodies or bike carbs on you're looking at £100 ish just for the engine inlet stubs(new) as a starting point....Bit of 60x6 flat bar and 16g 50mm dia tubing and your away,aren't you?

As with most things manufactured - I guess it is usually a cost and/or weight issue. My twin 45's manifold tapers down to the inlet size on the head which would be tricky to mass produce and is also quite light for its size, I think a steel one would be heavier.

I wouldn't like to try and make a manifold for a single carb - it would be quite tricky I think.

Jacko made an inlet manifold for his pinto - I think it is steel. Mind you he probably had to as no-one makes a SU manifold for the pinto

its all about heat transfer,a steel manifold would ice up and cause running probs unless you run hot water round it

Very common material for full race combined inlet & exhaust manifolds for DCOE carbs Janspeed made some nice Imp ones that tapered beautifully into the port.

The main reason road manifolds are not made this was is I guess economics and the fact that a longer road manifold might run into vibration resonance resulting in fatigue failure.

A friend of mine made his own inlet manifold when he installed a Toyota 4A-GE into his toyota. Although he used ally. Flat plate and tube, just as you mention.

I was amazed - thought it was a work of art. If you're used to doing this type of thing though, I guess you just get on with it and do it.
He also fitted a Ford Dizzy onto his 4A-GE.
In his shed - with an ancient lathe to turn up an adapter collar.