For designing an exhaust system you first need to know, whether it is a turbo or non turbo application. There is distinctive differences between the two types when designing an exhaust system.
Looking at the non turbo engines (including kompressor) the designes in principle differ between the number of cylinders as well as the configuration of the cylinders.
To start this this means that the design of a V12 exhaust is not only different from any 8 cylinder, but also from any straight six. To make things even worse the Volkswagen group threw the W-configuration in.
Also between the straight six and V6 exhaust designs are differences that are quitye substantial, which are not related to the extra length of the engine than rather because of the different firing angles. Although physically a bit more tricky to build right the straight six exhaust system is technically easier to optimise than a V6 configuration.
On a V8 engine at a 90 degree configuration a balancer pipe between the two secondary pipes is required, which is in most other angles not necessarily required and in some angles at relevant applications not even wanted.
5 cylinder engines are very hard to do and again Volkswagen was able to top it up with the VR5 engine, which is an absolute pig of an exhaust to design.
Coming to the 4 cylinder engines, the worst design to do is the one with siamese ports, which we fortunately do not have to deal in Fiats with. A rather easy one to deal with is the good old Alfa boxer engines. Unfortunately they are history.
V4 engines haven't been in production for a long time, but proove a bit more complex problem on exhaust design then a straight four.
Now being confronted with any of the normally aspirated Fiat engines, exhaust design gets comparatively easy. This should not lead to the conclusion it is just a walk in the park.
Before the design of the actual system can be started it needs to be determined, whether the car is being used for peak power or a more torque oriented use. This affects the the manifold. If you wish to go for peak power but are willing to sacrifice some torque the 4-1 design will be your choice. The more road oriented version would be the 4-2-1 design giving you a much wider torque band.
Having decided to go for the slightly more complex solution (technically, but not necessarily physically), which is the 4-2-1 manifold two more things then on what engine it is going are needed. This is what cam design is the one that is finally in the engine and where the peak power point is supposed to be. From the engine we have we know the individual cylinder volume is known and the show can begin.
I am not going to bore you with a string of formulae for the principle design now.
More important is to know that there is an effect on the hole system when altering lengths and diameters of the individual pipes. By making the primaries of a different length you can widen the torque delivery but will loose a bit more peak power. Also you can achieve it by altering the pipe diameter of the primaries. Once the primaries are calculated you gon on to do the same with the secondaries and finally with the collector pipe. The collector pipe will have to be stepped down if it is not fairly short to help scavenging.
By altering any of the dimensions of the system when not done in a professional manner the result will be a loss of power due to getting the hole system out of balance. By increasing the pipe sizes you always will increase the peak power (when based on proper calculation). The only problem that will hit most of the applications is that because the hole torque curve shifts downwards that the usable torque is inferior to the standard setup or a fast road setup and the peak power delivery is coming in so high that it will be well above the rev limiter.
Additionaly there is quite a few more points to look at. The exhaust ports shape, the optimal step between the port and manifold, the junctions, the leading edge of the junctions, the bends (quality, shap and angle), the back box design, the cat design and the exit.
Welds and joints are important too, which is often neglected by a lot of the so called exhaust specialists.
If you ever have the opportunity to see an F1 exhaust in real life, take a very close look at the detailed work done on it. Then you will find out why Supersprint as an off the shelve provider and a lot of the real specialist places are not as cheap as you wish them to be.