I will try to dig up the info i had been reading about it, but in a basic nutshell ill focus on the Helmholtz resonance effect. The inlet manifold is the reason you have a peak tourque range(but its to do with your cam aswell, the two work in tandem) Basically when the valve slams shut a pressure wave is formed in the runner due to the high velocity of the airflow in the runner suddenly stopping and creating a high pressure point at the valve. This high pressure point tries to equalise out, but due to its high speed nature it oscillates up and down the runner. The idea is that you re-open the valve when the pressure wave bounces back up the runner and is close to the valve again. The pressure then at the valve will be greater then atmospheric pressure and as such the air tends to "Burst" into the cylinder chamber when the valve is next opened(reaserch volumetric efficiency in manifolds), this sort of act like a micro turbo charger feeding positive air pressure. This is all caculated in the factory in the engine design stage. The strength and speed of the wave will be determined with some simple mechanical maths, and the wave whereabouts is simply some speed equasions in relation to runner diameter and length. The factory designates the runner legnth to catch this pressure wave at a certain point so that the cam opens the valve at the right point. One thing they cant do is make this pressure wave dynamic to the engine speed, that is "it is the speed that it is". So the pressure wave is only caught properly at a particular RPM, such as about 3500-4000rpm on the 5ltr i think, which is maxium tourque. As you go above this point power becomes difficult to generate(which any stock 5ltr owner will tell you)
If you chop the mainifold up then you simply throw out all this work and theres a fair chance your gonna catch the pressure wave in the wrong direction and loses alot of power. If you change the Cam then you risk opening the valve to early/late and missing this wave and basically have a negative affect on the engine. So really the runner legnth of the manifold needs to be optimized to suit the cam and at the level you want maximum tourque, usually anwhere from 20-100mm adjustment. Same in reverse, if you adjust the runner legnth then you need to adjust you cam to suit. If you whack a gas axe through the middle of the manifold and cut off 2/3rds of the total runner legnth, then you can see how stoopid that is
Somthing like that anyway, someone with a bit of smarts might correct me on this a bit.
Also it should be moderatly clear by now as to why with turbos and supercharges all this theory goes out the window. So basically with them, the shorter the runner the easier it is to just pump bucket loads of positive pressure into the chambers.
