As discussed last month, exhaust-header primary-tube length is the main 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 outgoing 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…
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Hot Rod (Digital) - 1 Issue, December 2017