Have been thinking about this for a few nights lying in bed, can't sleep.

Basically in Fluid Mechanics, there is a law stating that what goes into a closed system (pipe) must come out, if the flow is incompressible! I am sure Everyone knows that one.
However the rate of flow i.e M (Mass flow rate)in = M (mass flow rate)out - (M = p (Density) X v (fluid velocity) X A (cross sectional area)), now since the flow is incompressible we can cross out p (density) on both sides of the Mass flow rate formula ^, as density remains constant.
When you cross out Density you are left with, v (fluid Velocity) X A (cross sectional area) = V which is volumetric flow rate.
If you think about it Vol rate in must = Vol rate out same as M(in) = M(out), so to keep the Vol rate constant, you must either increase the cross sectional area which will slow down the flow or do the opposite which will increase the velocity of the flow. Same reason why people tapper the openings of their Throttle bodies!

Also when you increase velocity at that point the pressure is lower than any other point in the system, causing the gasses or fluid behind to get sucked towards the velocity direction creating a chain reaction, that is why 4-1 headers work well at high RPM compared to 4-2-1.

Knowing that you can experiment with different diameter exhaust pipping to reduce Head losses, which is caused by pressure loss due to loss in velocity, and increase efficiency = more power!

Gonna try this with my Build after my uni exams.
Any thoughts?