Calculation of exhaust system.
Main differences are; do you want torque or top end power?
As an example in these calculations, I use my 1776 cc Volkswagen engine, it sends a whopping 130 Hp at 5.800 rpm. That results in 1/8 mile in 9.7 sec. And 0 to 100 km/h in 9 sec.
All this is fine, in the end is more important: What space is available? Ground clearance can be crucial, cause crushed pipes dont perform!!! A warmer engine room will reduce power cause cool intake air gives you power for free.
Also, think balance. Short intake manifolds works with short exhaust pipes and mega-lift/ mega-duration cams for top-end power.
P=A x S x D2 / 1400 x d2
P = length (foot)
A = exhaust valve opening period (degrees)
S = cylinder stroke (inch)
D = cylinder bore (inch)
d = valve seat diameter (inch)
This formula results in an even flow in primary pipes.
A=280 ; S=2.72 ; D=3.56 ; d=1.18
==> P=4.97 foot = 1.51 meters
T = L / 6 x V
T = rpm x L / V
T = time for sound to go back and forth (seconds)
L = length (foot)
V = speed of sound inside gasmixture
This formula is based on standing wave in the tube.
My beetle received approx. 850 mm primary tubes, all equal length.
That resulted in a big power peak at 2500 -> 2800 rpm and an additional second peak at 4500 rpm. It became accentuated with a stinger pipe (conic megaphone!!!!!) and 312 degree cam.
When joining primary pipes, you can use a collector, however an exhaust box is better. This depends on what space there is available.
Length = 50% more than intake plate diameter, more length is better.
Connect the primary pipes in ignition order, so it creates a swirling gas motion that actually helps to suck out gases from neighbouring pipes.
Is there to flatten out pulses, so that no disturbance of pulses occur when entering this chamber.
Volume should be at least 12 times a cylinder volume, 15 times is preferred.
Dont use a large diameter, it will create noise and vibration in walls. I have seen 1.5 mm sheetmetal crack because of this, totally fatigued.
Very short tube between exhaust box and silencer. Preferrably 150 mm.
Much better to have pipes after silencer.
Diameter shall be so that area is 50% larger than one primary pipe.
Same diameter for intake and outlet.
Here you make adjustment to torque.
The longer pipe you have, the more number of standing waves you will have, which results in broad torque range.
On my beetle, this is only 200 mm because of practical limitations with rear-engined cars.
Front-engined car which deserves top-end power, one can make a side exit in front of rear wheel to have a shorter tube.