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Smaller plenums, like those in a dual-plane manifold, do well in the lower rpm range while larger plenums, like the single plane manifolds, tend to boost the top end of the rpm range. Plenum size also has a dramatic effect on power production.
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While longer runners tend to work well at the lower rpm range, shorter runners favor the upper range of the rpm band. Longer intake runners take advantage of the natural pressure-wave pulses to provide a greater atomized air-fuel charge to the cylinder, which boosts low-end torque, improves idle quality and also part-throttle response. This is an automatic challenge for single-plane manifold manufacturers because the flow paths to each of the cylinders are different. In most dual-plane manifolds, the two separate plenums lead to individual intake runners that are longer and closer to the same size. In a typical V8 engine layout, the cylinders on the far corners are further away from the plenum than the cylinders in the middle of the engine. “Dual-plane manifolds do a better job at balancing the air from cylinder to cylinder to keep them more balanced throughout the rpm range because of the 180-degree design.”ĭistribution is also an important consideration in manifold design. “Dual-plane manifolds are often called 180-degree manifolds because of this,” says Smith. Each plenum and set of runners connect to every other cylinder in the firing order therefore, each side of the manifold is subjected to pulses from every other cylinder in the firing order. Unlike the single-plane manifold that has overlapping pulses every 90 degrees of crankshaft rotation, the dual-plane manifold only sees an induction pulse every 180 degrees of crank rotation. The dual-plane design separates the manifold into two plenum sections. “Then, when the rpm increases, the effect of valve overlap and velocity of the airflow do not matter as much as meeting the demands of the engine’s airflow requirements,” says Smith. This can draw exhaust gasses into the cylinder during the overlap phase, which results in rough running at low rpm and less torque production.
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However, the overlap can be a disadvantage at part throttle and low speeds, especially with big camshafts when another cylinder in the overlap phase also opens to the common plenum. In many engine combinations, this turbulence helps keep fuel suspended and the air tumbling as it enters the combustion chamber. A single-plane manifold feeds all eight cylinders from a common plenum.