How Do Stepped Head­ers Work?

Hot Rod - - Contents -

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.

HHOTROD. COM/ Mar­lan-Davis

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...

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