You never want backpressure, why would you? What you want is the exhaust sucking out the spent gases and drawing in the fresh gas/air mixture. But this takes "tuned exhaust", the correct cam overlap and right sized carb/TBI.
Real world, the EPA DOES NOT want ANY unburn't fuel leaving the combustion chamber.
So you get cams with little overlap and restrictive exhaust to keep some exhaust in the CC.
Its easy to slap some pipes on, RE-cal fuel and go. But, you still need the correct cam overlap, correct exhaust scavaging and right sized carb/TBI to be right.
As far as reversion goes, theres pages upon pages on this. Best reading is from Buddy Reher of Reher-Morrison Pro Stock fame. Their engines actually get 105-110% V.E.
wilks3
:USA
finally.... A+++