Hooker 31/2 diameter header extensions. Never used. Come with weld-on tabs for mounting, or use clamps. Hooker 18" header extensions are popular bolt-on exhaust components designed to let racers tune their engine's torque curve for different track conditions by adjusting exhaust scavenging. They're a common way to extend the effective length of the collector for optimal performance on drag/street-strip vehicles, with the 18-inch length being a well-regarded standard for open exhaust setups.