Exhaust sizing

First,please tell us what engine/displacement your running, and what you intend to do
with it.Then we can give accurate advice.
Second,some backpressure is NOT needed,that is an old wives tale perpetuated by
those unaware or unwilling to re-jet their carbs accordingly.The only exception is running
open headers,where a collector extension is absolutely needed to maintain the pulse signal
and directional flow velocity.
If you go to Burns exhaust tubing's website, I believe they state that anything past
5ft. beyond the collector results in a power loss,regardless of sizing.The exhaust "cools as it
goes" theory is fact,but only applies in a non racing type situation,where the tail end of the
system has lots of low flow time to cool.
For reference, I did some heads on an AAR 340,cc'd 9.7:1,292 purple shaft,9.5"converter
heddman of the shelf 1 5/8" units,OE port matched 6pk intake w/ heat blocked,carbs jetted up
drilling the outboard plates.Runs 3" thru the muffs then dumps B4 the axle.
On pump premium as driven w/a set of slicks the 'cuda has run a best of 12.35@109mph.
I believe the avg. times were around 12.45-12.50,not bad for a .030 over 340 in an E-body.