intake manifold heat riser

I actually don't know of any factory engines that did NOT have a heat riser or heated intake.

The cooling on a 6 manifold with no exhaust heat, as when headers are added, does NOT come mostly from airflow around the outside, but rather via what is known as the "refrigeration effect" which is the evaporation/ pressure change as the fuel atomises in the manifold.

Anyone who has every seen stuff like Corvair / aftermarket intakes on dune buggies has seen ICE forming on the intake even on a hot day. This is why aircraft engines used to be a big problem, and in fact has actually caused crashes and deaths.

Having said all this, my own 67 (when using Edelbrock carb) has no carb heat and no heat riser. But I don't drive it in very cold/ wet weather. The other thing is, if you use an aluminum manifold (V8) the aluminum conducts heat way faster than cast iron. This means that the manifold heats up much quicker from a cold start, and once the engine is warm, hot oil is hitting the bottom of the manifold. Many people incorrectly think that aluminum manifolds run cooler, but this is not the case. The high conductivity of aluminum carries heat FROM the heads (the heat SOURCE) TO the manifold, which is trying to cool things because of the carb.