I understand the X-wedge engine has an anti-knock sensor which automatically retards ignition to stop possible pinking (S & S documentation confirms there is one fitted to rear cylinder).
If the issue occurs around an indicated 5200rpm and given inaccuracy of tacho, it could well be the rev limiter cutting in which I can best describe as a misfire sound with associated power hesitancy. Little point in holding onto revs above 5000rpm anyway. Just viewing power/torque curves will confirm that both are tailing off then.
With the stock exhaust the pinking is due to the restricted airflow exiting the cylinders, not the timing.
Hence why the headers glow red after a few minutes and using higher octane fuel or an octane improver makes no difference.
