Getting rid of the exhaust fumes is not only important, its also a crutial step to increasing your cars performance. The less pressure there is in the exhaust system the higher the BHP, so polishing the inside of the exhaust will aid the fast exit of gases. Anything around or under a 1.6l engine, stick with a maximum 1.5-2 inch pipe (the smaller the engine the smaller the bore size). Anything under 2.5l engines go with 2-2.5 inches and over this go with a 3 inch exhaust (this bore size applies throughout the length of the exhaust up to the back box - the back box & exit pipe can be a little wider but this will not release more power, it will just give a deeper sound.). Fitting 4 4 inch pipes out the back of your corsa may sort of look cool, but it will slow you down, and the noise will drive you insane!

Gases flow better from a large pipe to a small pipe if the join is cone shaped. Steps will cause "turbulence" and this can interfere with the airflow and affect any increase in performance fitting the exhaust originally gained.

The shape of the exhaust manifold is also critical as is the primary and secondary length. This is something best left to the experts to calculate, most manufacturers do a reasonable job but to improve a standard one; Smooth out the inside with a grinding wheel on a drill or buy a purpose built stainless steel unit for your car. The best systems are called 4-2-1 which means they go from 4 pipes into 2 pipes and then into 1. Some standard exhausts are 4-1 where all the pipes merge at one point lower down the manifold.

A larger diameter exhaust means that as the pressure needed to expel the air is lower the whole process will be quicker with a lower push needed. Cats suck power by slowing up the airflow but are unfortunatley a requirement on modern modified cars. A typical reduction of 1-4 bhp is expected from a cat, so if you find a high flow sports cat or use an off road only de cat pipe you will notice an improvement on most applications although this is illegal, and will be very minimal on smaller engine'd cars.

Another option is to go for a 4-2-2 exhaust where you have twin exit pipes with 1 for 2 exhaust ports from the engine. 

Most sports silencers add a deep roar to the exhaust but add little to the overall experience. One of the best exhaust's I have ever seen was a standard one (from a larger engine'd car) with the rear silencer removed – the centre expansion section was larger to help muffle the sound but it really improved things a lot.

Just as induction length is critical, exhaust length is also vital. You should also aim to have the same exhaust length from each exhaust port.

Most standard exhausts are cast and have many imperfections on their internal surface and often have marks and ridges where surfaces are joined. Grinding the inside of the exhaust smooth will aid airflow and improve things and while you are at it you could polish it to completely remove any resistance. Performance exhausts are made of stainless steel which has a much smoother internal surface and is also much lighter.

Under bonnet temperature is another one of your biggest performance killers. Wrapping the exhaust manifold with a heat resistant wrap can yield a dramatic increase in this, and will help the cat to reach its preferred operating temperature faster, therefore prolonging its life and efficiency.(don't use cloth/material bandages as this becomes a fire hazard!)

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