Induction InvestigationThe induction system has a very important role in providing for the air and fuel mixture, and delivering it to the cylinder heads. At the top is the carburetor. Here, the primary consideration is the size, typically in CFM of airflow. Various manufactures have published charts in their catalogues making sizing selection a relatively simple matter. Carb size, to a certain point, can be edged up from these base recommendations for a small increase in top-end power, but the danger is in losing sharp throttle response, or potentially having the engine fall over at lower rpm. Here a little judgment is called for. If the car is a deeply geared drag machine with a loose converter, far more carb size can be tolerated before any negative effects become apparent. The fact is the engine never really sees loading at an rpm range low enough to bring the loss of drivability into evidence. Two plane manifolds with true divided plenums can also tolerate more carb size, owing to the 180-degree division of the intake pulses entering each half of the plenum, a factor which increases booster signal. The design of the carb also plays a role. Modern, highly streamlined carbs, such as the Demons or HP-series Holley's have much higher airflow than earlier carbs of similar orifice size, achieving higher flow through efficiency rather than by an increase in venturi and bore size alone. For instance, the Holley 950 HP carries the same critical dimensions as an old 750, but flows 200 more cfm of air. Chances are if it didn't bog or lay down with the 750, the new 950 HP will boost top-end power without giving anything up to the old 750 down low.
Manifold choice is also vital. The two things to consider here are the layout and the airflow capacity. The airflow of the manifold needs to be sufficient to sustain the peak flow potential of the cylinder heads for the combination to work most efficiently. The layout of the manifold is equally important. Here the basic choices are single plane or two plane. Two-plane manifolds will produce more torque lower in the rpm range primarily due to the 180-degree pulse separation inherent in the layout, with a secondary effect favoring the low-end stemming from the longer runner length and reduced plenum volume. A two-plane manifold virtually always favors engines built for power under 5,500 rpm. Up to engine speeds of 6,500 with street-type cylinder heads, the latest crop of high-performance two planes (such as the Edelbrock Performer RPM AirGap) will generally provide top-end output on par with a single plane, while retaining the two plane's inherent low-end torque advantage. The advantage of the two plane is typically 20-25 lb-ft at the low-to-mid range in comparison to a single plane. Higher engine speeds or race-type cylinder heads will require the airflow capacity of a single plane for optimal high-rpm output. Carefully consider the intended rpm range and airflow requirements when selecting an intake manifold.