As discussed last month, exhaust-header primary-tube length is the primary determinant for establishing the engine speed at which the header system tunes. But what about header primary-tube diameter? Basically, tube diameter establishes the velocity of the exhaust gas slug as it travels down the pipe. Too small a diameter and backpressure becomes a problem; too large a diameter and the inertia of the incoming gas will be insufficient to scavenge the cylinder of exhaust gas. Gas velocity within a tube is inversely proportional to the tube’s cross-sectional area. In other words, gas velocity is high through a small tube and low through a large tube. Taking this into account, most applications use the smallest-diameter tube that flows enough air to handle the engine’s displacement and maximum engine speed. The smaller diameter will generate the velocity needed to improve torque at low engine speeds. If you’re building a header set from scratch, the accompanying graph should serve as a rough guideline for primary tube diameter. Next month: header collectors.
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