A tuned exhaust header's primary-tube length has a direct influence on engine performance. It controls where in the engine's speed range the header will add performance. Longer tube lengths pull the torque down to a lower speed. Shorter lengths move the powerband up into a higher speed range. In an extreme example, engines that run to 10,000 rpm might need the primary tubes to be as short as 26 inches. A more typical mild, street hot rod engine with a maximum speed of 5,500 rpm needs a length in the 35- to 38-inch range. The accompanying graph shows the approximate collector lengths needed as a function of engine speed (rpm) and the type of fuel used in the motor. As for the influence of primary-tube diameter—that's for next month. Stay tuned.