Asking for technical headers informations
 

I ask to the experts, changing diameter and lenght in primaries and secondaries pipe what does effect?
I made a lot of researches on the rest of the exhaust, but headers remain a mystery to me...

Its voodoo. Show me the dyno. lol

It affects power but we don't know how lol. You're getting into some serious engineering shit here

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I'm certainly no expert, just few youtube vids here and there ..

Changing diameter changes exhaust gas speed, bigger - slower, smaller - faster (to later if too small also extra resistance to flow?).
Length together with diameter, both affect how fast exhaust gases reach runner merge, from that one can get "free gains" by enhancing exhaust scavenging .. there was something like when pulse from one cyl reaches merge, it should help scavenging from "paired" cylinder, enhancing scavenging & fill efficiencies.
Well tuned for use type exhaust & intake manifold tube lengths & diameters is common way to get extra performance, especially on NA engines. Matters a bit less for forced induction, where one can just up boost.
Unfortunately, just as with intake tube diameters/lengths, as gas speed is linked also to engine rpms, intake & exhaust tube length & diameter usually can be tuned just for some specific rpm range, which is usually selected by intended engine/car use type, eg. high-rpm race, low-rpm daily driving, or compromise inbetween. Good examples - PTuning short runner header with high absolute gains in top but relatively little in mid & low. Longtube headers for twins seem doing well in "mid"/overall, also UEL headers, that compromise length "tuning" for cylinder pairs in fashion of "jack of all trades, master of none" .. but also easing clearance, lessening material costs (and for some also providing wishing specific sound change from "subaru burble").
Most advanced (but also most expensive and difficult to design/make) intake and exhaust bits are of variable length (via valve-switching between two sets of different tubing length for example, or changing length of tubing itself). Due cost/complicate variable length intakes/exhausts design rarely used (maybe in some racecars? supercars? motorcycles?).
When best tube length & diameter gets known, be it by some calculations/prior statistical data or simple experiments by selecting different length tubes to mock up test headers, there unfortunately comes in packaging problem, more pronounced in today's cramped engine bays. This may strike out some most effective configurations or add some other issues (eg. how with long tube headers Ace may get some overpipe clearance/rubbing issues, or Nameless some heat radiation/melting some parts issues).
So at the end it's often about choosing best compromise for use type within budget and design/manufacturing/spacing limitations.

my understanding is the length of primaries effects the top end most and secondries are to build midrange. The diameter has more to do with gas flow. Larger the diameter the the more volume of gas you can flow but it also effects the gas velocity. If the velocity is too low you lose the scavenging effect you are looking for with the tube lengths, so it is a balancing act. Then you have different shapes of collectors but I haven't read enough about that to really comment.

This is all a generalisation and specifics will change for different motors. Especially ones with different cylinder counts and configurations. For example I know that due to the the timing between cylinders of a straight six you really need to have a 6-2-1 setup as a 6-1 wont actually scavenge properly. Or that a cross plane V8 have poor scavenging due to the firing order but a flat plane V8 has the potential for good scavenging which is why it is used in a lot of high end race engines.

KNO3+C+S and add forum contents. Now light and chant. Oh hold on this thread is full of it already

The trend seems to be decatted 17+ header and tune. A place in Chicago does a nice job of it. Can't remember the name though.

Do any of you play a wind instrument? I played tuba in HS but we also had a guitar and parlor grand piano in the house. I came to really live in the world of vibration and harmonics early in life.

I could regurgitate the math but that is a worthless abstraction without first some real feel for the idea of standing waves in the context of acoustics.

I haven't picked it up in more then a decade, but I used to have a book on building two stroke engines and exhaust systems. A lot of the examples were given as thinking of sound waves since a sound wave is essentially a pressure wave. It made it more relatable. I have forgotten most of what was in it, I should find it and read it again some day.

Once I commented here that ACE longtube 4>2>1 headers, due to very good scavenging, are less noisy than other headers and been told is non relative... although I know sound and gases travel the same way, that's why a saxophone sounds completely different in ice cold winters than in hot summers!

Ah yes trying to bring the 2s up to volumetric efficiency similar to a 4s.
I might try flying an aircraft in a fluid next.

:clap:

Just make sure it is Newtonian fluid, otherwise you are going to have a bad time.

Lengths of the internal standing waves are shorter in the cold. ...and longer at high altitude.