Common Plenum Manifolds
This is where all the manifold runners connect to a common chamber that is fed by a carburetor or carburetors. The stock single carb layout is an example. Provided the carb and the runners are big enough bulk flow shouldn't be a problem, but maintaining good distribution can be a challenge. In the old days it was common to fit dual or triple Stromberg downdrafts to the old Holdens, and I suspect the improvements these brought were as much a result of better distribution as the increase in flow. They have the advantage of simplicity, and because the carburetor isn't subject to the violent pulsing that can occur in an IR setup they can be a bit easier to jet for clean running over a wide rev range. On the downside it's almost impossible to make the manifold runners the same length, and curves in the runners are unavoidable. Fuel drop-out is a problem, and this is the reason most factory manifolds are heated. The heat does wonders for smoothness and mileage, but it can reduce the power output - more on this a bit later.
In summary they are relatively cheap and simple, but with limitations regarding runner lengths and distribution.
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| Current single four-barrel manifolds can provide reasonable performance without the complexity of triples. |