Headers vs. Exhaust Manifolds ~ 'Let The Debate Begin'
'Exhaust Restriction 101'
Back-Pressure > Will negatively effect Engine Performance.
Increased 'back-pressure' levels, will effect the internal combustion.
The Engine has to compress the Exhaust Gasses to a higher pressure,
which involves 'mechanical work', which equates to more fuel consumption
and higher exhaust temperatures.
Even the 'slightest' improvement in reducing Back-Pressure, will increase
Horsepower {2% to 3%} in a 'stock' Mopar 340.
Using the general factor of a 'stock' Mopar 340 {290 Horsepower @ 5150 RPM's}.
That comes to {5.8 to 7.7} in Horsepower gain.