I have used both water-heated dual-carb intake manifolds and exhaust-heated dual-carb intake manifolds.
First: the Clifford and the Offenhauser intake manifold supposedly allow coolant transfer from end-to-end of the intake manifold. Fenton intakes do not allow end-to-end circulation.
The
Offenhauser intake is designed to direct the flow coolant from one side to the other -
the Clifford intake directs coolant to both sides at the same time. In order for the Offenhauser intake to direct coolant properly, a special bottom plate is needed, and a proper gasket must seal across the middle of the bottom of the manifold. So, except for properly plumbed/sealed Offenhauser intakes, only simple heat transfer takes place at the center base, even though there is an inflow and and an outflow port/line hose for coolant.
The exhaust heat plate/tube set-up (one or two tubes) is quicker in heating, and also only works by heat transfer to the base (not by circulation from end-to-end of the intake manifold).
Plumbing with coolant adds an extra concern regarding coolant leak.
Heating with exhaust air is less expensive, less complicated, and has no risk of damage if a leak occurs (if you get a coolant leak, you must deal with it immediately.
As William of Ockham might have wondered: why not use the simpler (and in this case, safer) solution - exhaust heat?
