Myth, been proven otherwise a few times, here’s the deal.
Headers or not, the amount of pipe after the collector or the manifold or header should be dyno tested to provide the best possible length to provide the best possible amount of torque possible. After this, a pressure wave cancellation box equal to a minimum of the C.I.D. of the engine should be installed on each pipe.
From here, if one should choose to use a muffler or not, the engine is now making the best low end torque and will retain maximum power throughout the rest of the dyno run.
Any back pressure is just performance limiting. Period. It is exactly the same as me limiting you exhaling while you’re running. You can breathe in as deep as possible, no problem. But I’ll only allow you to exhale so much. Just by this alone, not exhaling all that you can, there is now a limitation on the amount of air you can breathe in. Your overall performance in running is now curtailed. Apply this to the engine.
Silencing an engine after the pressure wave cancellation box is another topic since the OP wants maximum sound.
To this end, after your header extension length is found, cut your cars exhaust there and let it run like that. Running a full length exhaust pipe to the rear of the car/bumper could very well be reducing power since the pressure wave tubing is nullified and engine breathing is cut short as described above.
Adding a pressure wave box for maximum torque and continuing the exhaust pipe without a muffler may indeed possibly reduce sound. I don’t know for sure. I’ve never done that myself. Nor do I know of anyone that has.
The header extension pipe should be the same size as the pipe diameter of the header collector where all 4 pipes come together. Normally this is 3 inches on small blocks. Keep that size throughout the rest of the exhaust system. This should produce maximum sound. Stepping up to a 3-1/2 or larger pipe to run the length of the car may help in more obnoxious sounds.
