These exhaust header inserts improve torque, throttle response and power. At low engine speeds, the taper of the Tork Valve increases back pressure resulting in increased low rpm power and torque. At high engine speeds, the venturi created by the Tork Valve increases exhaust gas velocity. This maximizes engine power by increasing the rate at which the exhaust gases are drawn from the exhaust ports. A horsepower increase of up to 2 hp over the entire rpm range were measured during Dyno runs. Tork Valves fit all OEM and most aftermarket pipes on both Evolution Big Twin and Sportster models.