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Additional performance can be obtained, with installation of freer-flowing exhaust headers (or "Sprint" manifold from 1975+ Opel), and a wider exhaust system towards the rear of the car. Be sure that any modifications are custom-made for the car, as the GT's underbody tunnel areas are narrow and mismatched components can crack from vibration and uneven harmonics.
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Diagnosis of source of exhaust smell in GT, 2 pages Mentions as possible causes: Cracked head pipe connection at manifold flange, and worn or missing rubber seals/gaskets on rear of body. Also suggests checking for a gas tank vent hose leaks, and using small amount of oil or diesel in carb at idle to spot leak (when car smokes).
Exhaust systems modifications, 1 page Joe B. describes a higher performance fabricated GT exhaust system of 2" pipe and Walker DynoMax muffler, custom bent from the 2-into-1 "Y" on the stock head pipe, with chrome end tips from ANSA. Discussion of pros and cons, of larger 2.5" pipe also.
Pacesetter Header Source, 1 page (illustrated) Exhaust Headers by Pacesetter of Phoenix, AZ listed, part numbers are #70-1057 for the 1.9 liter GT and #70-1058 for the 1971-75 Manta. Note: Not known if these are still currently available.
"Performance Exhaust Systems for Your Opel," 2 pages (illustrated) Bob L. describes upgrades for Opel exhaust systems, including installing the 1975 "sprint" exhaust model, followed by larger 2" diameter headpipes, tri-y headers, and other modifications.
Aluminization of Exhaust Headers, 1 page Rick G. notes a source, for a process where molten aluminum is sprayed onto headers to resist corrosion; QuickSand in Stanton, CA.