So….more header talk….this had me scratching my head

MOPARMAGA said

Because you lost exhaust velocity, it built a low pressure wave at or near the collector which did not get evacuated completely, it just sat there until and was partially sucked back into your intake charge.
Not enough fresh air paired with all that fuel caused rich mixture

Click to expand...

Yep. The scavenge effect.
As was said by mattax about statements of truth becoming untrue when taken out of context , people hear "no exhaust/pipes, "straight header" "bigger be better". "High rpm need no back pressure" look at those imporks,hear the sound of torque loss..lol
Same for gasket matching exhaust ports and turning them into a trumpet at the exit where it all expands too much too soon and loses velocity aka loses scavenge/pulse.
Like a train or big rig blowing by you on the side of the road...it pulls you in its direction with its passing....nascar 'draft'. Etc.
Many versions of that vacuum.