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Those people who are saying we want back pressure are just very confused. It is not that people argue for the smaller exhaust because it creates more back pressure, it is because of the velocities, timing, and strength of the low pressure pulses, which is the technical part of scavenging.
But first an explanation of the exhaust basics so everybody has an idea what we are talking about because a lot of confusion is people having no idea what they are reading and then pass on their god awful interpretation of what they just read.
There are mainly two ways overall, through the theories and purposes of the exhaust, in which incomplete removal of exhaust gases reduce performance.
One is a little thing called volumetric efficiency. This little wonder decreases when residual gases take up space in the cylinder. The other is the higher cylinder pressures from incomplete exhaust blowdown will force burnt gasses into the intake port when it opens.
Blowdown occurs during the period between the exhaust valve opening and bottom dead center. This is where the exhaust gases are expelled from the cylinder, from higher cylinder pressure than the pressure in the exhaust, before the actual exhaust stroke begins (piston starts moving up). The left over gases not discharged during blowdown must be physically "pumped" out of the cylinder during the exhaust stroke, which lowers power due to pumping work.
The pressure wave of the exhaust leaving the cylinder is known as a compression wave. When this wave reaches the end of the header primary tube (the collector) it then expands, creating a suction wave (low pressure). This wave travels back up the primary tube and if tuned right it should just reach the exhaust valve just as the overlap is finished. What this does with the low pressure is evacuates all the spent gases and in turn will also create an effect known as inertia supercharging where it increases the velocity of the intake charge and actually creates higher intake charge pressure in the cylinder than it would be normally be and allows the ability to reach VE over 100% if done right. That, my friends, is scavenging.
Now all this explains how the exhaust system works. If you understand what you just read you should realize why we go after velocity (NOT DIRECTLY VOLUME). Just bigger pipes (more volume) reduces the velocity and velocity values below whatever the particular motor needs will not have strong enough suction wave to completely scavenge the cylinder which hinders the blowdown process and decreases volumetric efficiency.
Now finding the right header length and diameter is how you "tune" your headers. Having the right length times the suction pulse (at its optimum velocity) to reach the exhaust valve at its closing and the diameter which determines velocity and must be correctly sized to utilize all the strength of the suction pulse.
With all of this i mentioned NOTHING about obtaining back pressure and which few before have correctly stated WE DO NOT WANT.
This idea also goes along with the ports which is why if you want a truly good job on your ports you should leave it to the pros. NO exhaust can correct a poor port job.
I tend to explain things in which i understand but i think i did a decent job on this so hopefully this will clear some things up. Any further questions or if you do not understand part of this ask and i will try to explain it in another way or hopefully someone can.
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