How Tuned, Equal-Length Headers Improve Performance

When an engine’s exhaust valve opens, there are two distinct movements within the exhaust pipe. The first is the pressure movement of the shock wave that is generated by the violent expansion of hot exhaust gas past the valve. This shock wave propagates through the gas in the pipe at a speed of 1,300–2,000 ft/sec. The second movement is that of the exhaust-gas slug itself as it travels from the cylinder to the end of the pipe. The speed at which this slug travels is determined by the exhaust pipe’s cross-sectional area and the engine’s piston speed, but is usually about 200–300 ft/sec at the engine speed where maximum power is produced.

When the shock wave reaches the pipe’s open end and passes into the atmosphere, a rarefaction, or low-pressure wave, is reflected back up the pipe. If the pipe length is correctly adjusted, at a given engine speed this reflected low-pressure wave will arrive back at the exhaust valve during the valve overlap period when both the intake and exhaust valves are open, in theory completely scavenging the cylinder of any residual gasses.

Formulas exist to figure out the proper dimensions for a given application, but they’re based on laboratory work under ideal conditions. For example, it’s possible to determine theoretically the proper primary-tube length to develop max torque at a given engine rpm, but such an optimized configuration for any given speed may not produce the best overall results at other speeds (that is, yield the most area under the curve). Nevertheless, there are generally accepted header design principles; we’ll get into them next month.

 

Whatever the books and formulas may tell you, the exhaust pipes still need to fit in the chassis! This 180-degree header set is under fabrication for a 383ci Ford Cleveland-based small-block with Yates heads installed in a 1972 mid-engined Pantera.

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