Good stuff
But just to correct your assumption on the way the exhaust works, there should NEVER EVER be any back pressure in a 4-stoke engines exhaust, ever, lol
EGR valve SUCKS in exhaust when it is opened, because of the lower air pressure(vacuum) in the intake manifold, there is no pushing
Factory exhaust manifolds actually lower the pressure in exhaust in the mid-RPM range, best torque RPM
Its called a scavenging exhaust, thats what 3rd party "headers" do to change power band, but usually at a lower or higher RPM
Most factory exhausts have used this since the 1960s
And that's where the MYTH of back pressure came from
People would put on "headers" with larger pipes(free flow) and then go for a drive
"WTF, I lost power!!!???"
"This engine must need back pressure", and the MYTH begins, lol
They lost power because they removed a scavenging exhaust manifolds so lost the extra mid-range power it provided
Stock size Cat converters and mufflers should never ever restrict exhaust flow, if they do then they are broken inside
How scavenging exhausts work
If you have a smaller pipe that runs into a larger pipe and you pump air(or a fluid) thru it, when the smaller pipe dumps into the larger pipe there will be a Pressure Drop
That's the science part it's based on
The art and science part is the velocity of the air flowing in the smaller pipe, and the size of the smaller pipe and then the size of the larger pipe(collector), these all decided the amount of the pressure drop and at what RPM it occurs, in the case of exhaust
With multiple smaller pipes like 4cyl exhaust manifold or V6 or V8, dumping into the same collector, this pressure drop in the larger pipe causes pressure drop in all the smaller pipes
So when an exhaust valve opens the exhausts it PULLED out of the cylinder, the piston doesn't need to PUSH it out, and that leaves more power on the crank for the rear wheels
Free power................by design
Most factory exhaust manifolds are tuned for mid-RPM
3rd party are almost always tuned for lower RPM, off the line power, but you lose the mid-range
Racing headers go for high RPM scavenged power, no low or mid range
Even engines with short stack exhaust, one open pipe per cylinder use it, the diameter and length of that one pipe can be tuned to give best power(lower pressure) at a specific RPM