How Do Stepped Headers Work?
A tuned-length primary exhaust header pipe creates a rarefaction (negative low-pressure wave) that’s reflected back up the pipe. On a standard header, the negative wave occurs at the primary tube’s open or collector end due to sudden gas expansion into the larger collector and consequent velocity decrease. With the proper tuned-length for the combo, the wave arrives 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 residual gases.
Incrementally increasing primary tube diameter before the collector—aka a “stepped header” design—creates additional rarefactions. The steps’ location on the pipe and the amount of increase between each step can be used to tune the strength of the various reflected waves. The greater the stepup amount, the stronger the negative pulse. On the other hand, too many or too much step-up weakens the “primary” rarefaction of the main wave at the collector. A typical stepped header has one or two steps ahead of the collector, and each step is progressively 1⁄8- inch larger.
Stepped primary tubes can be used to create multiple peaks in the power and torque curves and to dial-in at what rpm they occur. One step may be adjusted to enhance the torque peak and the second for peak power. On the other hand, multiple rarefactions may trade off peak power in favor of more area under the curve. Stepped headers tend to be very finicky, and to actually see gains over a conventional primary tube requires closely coordinating the steps with the camshaft and induction system. In fact, a tuned induction system with controlled plenum volume and runner lengths acts very much like a header, but on the induction side, giving the tuner yet another tool for careful crafting of the power and torque curves.
Rob McGaffin [ This custom header with stepped 1¾ to 17⁄ 8- inch primary tubes was built by Chuck Keech (KTC Engines) in 2014 for a 5.7L Gen III Chrysler Hemi stroked to deliver 397 ci. It appeared in our sister pub Mopar Muscle. Power came in at 529...