Can somebody explain the effect of the diameter
of the exhaust. You often hear that the proportion of
the diameter and the lenght of the exhaust is quite important.
To the lenght you are able to find a few infos.
And you can think for yourself a bit acording to waves, frequency
speed of sound and so on.
But I don't know anythink about the diametre of the exhaust.
And what is, if the diametre changes?
I think i read somewhere that the shorter exaust is better for the RPM (it will produce more) Now I´m not sure about the diametar but after a litle bit of thinking i think that wider will be better because more gasses can be sucked into wider pipe... just a tought.
Its like a trombone that changes its tune as it changes length. There are certain frequencies that are more favorable at certain rpms that enhance cylinder scavenging. They tune the trombone to operate at the rpm range that they have targeted.
Innovation over refinement is the prefered path to performance. -- Get rid of the dopey regs in F1
In two words
Length influences the frequency of the waves.
Cross sectional area influences the intensity of the waves. The smaller the area, the more intense the wave. Then obviously for a given mass flow the smaller the area and the higher the speed.
What you want are waves as intense as possible to increase the mass flow but you can’t reduce too much the area due to friction and, most important, reach of sonic conditions inside the duct. So basically there’s a minimum section area you can use to maximise volumetric efficiency.
As for area variation, the thing is more complex, I’ll give you a few basics concepts hoping to not generate confusion but if you really want the details of it my suggestion is to forget about internet and look for a good book. Carlos already suggested one, I could suggest you a couple but they are in Italian, I don’t know if you would find them useful.
Every time there’s a variation of area the single original wave splits in two waves, going in the opposite directions, one continuing with the same direction as the singe original wave (usually called the transmitted wave) and another one going in opposite direction (the reflected wave).
If there’s an enlargement of section the reflected wave is of the opposite sign compared with the single original wave (ie. if the original one was a pressure wave the reflected one will be a rarefaction wave and viceversa, if the original one was a rarefaction wave the reflected one will be a pressure wave) while the transmitted one keeps the same sign.
In term of module of the wave, after the area increment both waves (the original one and the reflected one) will have smaller module that than the original wave.
If there’s a reduction of section both waves (reflected and transmitted) will have the same sign as the original one (if the original one was a pressure wave both reflected and transmitted will be pressure waves, if the original one was a rarefaction wave both reflected and transmitted will be rarefaction waves).
In term of module, after the area reduction the reflected wave will have smaller module than the original one while the transmitted one will have higher module.
Extreme situations are the end of a tube.
With a open end you have basically an infinite area increment. So you have a reflected wave of sign opposite to the original one.
With a closed end (as for example with closed throttle) you have the infinite area reduction so you have a reflected wave of same sign as the original one.
