It takes energy to push disorganized exhaust gases out through a vehicle's stock exhaust manifold. Headers don't make horsepower; they relieve the engine from the task of straining itself while extruding gasses through stock exhaust manifolds. Installing properly designed headers will relieve the energy previously wasted by pushing the exhaust gases. This increase in efficiency of installing headers can be thought of as a free horsepower gain.
Jamie Joyce, manager of Doug Thorley Headers, will walk us through the Doug Thorley Headers design and manufacturing process as these first prototype headers were fitted for the LS1 V-8 5.7L, PN324Y-C, that were used for the '67 Chevy C10 project Nightmare.
Doug Thorley Headers foundation are the 3/8-inch thick flame-cut flanges and 1-5/8-inch-diameter 14-gauge mild steel primary J-tubes.
The foundation of the Doug Thorley Tri-Y headers is the 3/8-inch-thick header flange. The 1-5/8-inch-diameter 14-gauge mild steel primary tubes were mocked up using 14-gauge J-bend tubes. After the flange was bolted to the LS1 cylinder head exhaust port face, the J-tubes were cut to provide exact angles to avoid clearance issues, with such obstructions as the steering shaft, framerails, crossmember, and starter. After the header's four primary tubes were cut, they were divided into pairs: two tubes that merged into two separate collectors. A single tube emerging from each of the primary collectors merged into the third and final collector. With the pieces cut and fitted, they were then tack-welded together. A fixture was made to support and hold the cut pieces in place while the multiple pieces were welded. After the header was welded into the fixture, it was removed, then each weld was carefully inspected.
After the prototype headers were made, they could be traced and duplicated into a CAD program. The automated CAD tube benders then automatically rotate, index, and bend each tube to its correct and precise configuration. The pieces were then laid into the fixture and welded. After the welds were inspected visually, they were submerged and pressure tested for any leaks. After everything checked out, the headers were sent to the polisher before they were chromed or ceramic coated.
Having the '67 Chevy C10 cab, front fenders, core support, and inner fenders mounted into place allowed Jamie to achieve the proper J-tube bend angles and fitment of the primary tubes of the prototype headers for both passenger and driver sides. | 
The cut-to-fit pieces were tack-welded into place. |
Each of the four primary tubes were extruded from the header flange, cut, fitted, rotated, and welded to avoid any clearance issues. | |