Check this link out. It gives a very good explanation of "backpressure".
At the same time, the reverse pulses that allow more air to be sucked into the combustion chamber, thus causing the PCM to change the fuel curve, does provide more torque. The more you open the exhaust, the reverse pulses diminish. That'* why most go with and "H" pipe when going with dual exhaust.
Saying that backpressure is exhaust fumes reversing into the chamber is not actually true. It'* "pressure", not back flow.
Very different things here. You can have pressure without flow. That being said, you can have flow one direction, and still have opposing pressure.
Use this to determine exhause flow:
Exhaust flow rate =
D (in3) x full load rpm x E x (7* x 460*)
C x 941,760
D = displacement in in3
E = 0.85 efficiency for naturally
= 1.2 for engines with scavenging blower (supercharger)
= 1.4 for turbo-charged engines
T = exhaust temperature; if unknown,
use 900°F for diesel and 1,200°F for
C = 1.0 (two-cycle engine)
or 2.0 (four-cycle engine)
Once you have that info, use this to determine backpressure:
The silencer is, of course, the muffler, and/or any resonators. By using these formulas, you'll see that on any given stock or lightly modded engine, the backflow rarely exceeds 1" Hg. So, there'* no problem.
The problem arises when you modify the exhaust without changing other functions, such as intake, valve duration, fuel/air ratio, etc.
That'* not what we do here, nor do we condone changing only one thing. We warn plenty of individuals not to open the exhaust, or produce more boost, without also changing the breathability of the engine in its entirety.
Just do a search, you'll see.