The basics
All cars are standard fitted with an exhaust (duh). These exhausts are not designed for the best performance, therefore it is possible to fit your car with another exhaust which gives just that bit extra power and that sound of a fast engine.

The backbox  
In a sport exhaust, the fumes are directed in a straight line, in the backbox there are perforated holes and around it there is a soft and heat-resistant material. Exhaust fumes can be directed without any obstructions (best performance), these exhaust need not make any more noise then an original exhaust. It is wrong to think that car that produce a lot of noise will perform better.

Single round DTM-style Rallye-style Twin square



Regarding manifolds, it is an area a lot of people, including manufacturers seem to neglect, as most exhaust manifolds resemble those they fitted 30 years ago. The main reason here appears to be cost. It is far easier and cheaper to produce a compact cast iron manifold than it is to fabricate a tubular 4-branch manifold that has to be hand built. Quite useful gains can be made with a custom-built exhaust manifold, even on a standard road car.

All modern cars are emission controlled, one part of equipment to do this being a catalyctic converter, which is a box on the exhaust, similar in appearance to a normal silencer. This consists of a honeycomb of small passageways through which the exhaust gases pass, which forms a chemical reaction with the coating on the inside of honeycomb, to clean up the emissions. This method of forcing the exhaust gas through small passageways causes backpressure and restricts the engines performance. The solution is obvious, you remove the CAT and insert a plain pipe to bridge the gap between the pipes the CAT connected. On most cars removing the CAT will improve power by between 7% to 12%, and the cost for such a solution is usually about 60, where else can you get such an improvement for so little spent.
The only problem that will occur is that it will not pass an M.O.T. To get around this you just keep the CAT in your garage and refit it for the test, which is simple and at most only takes about half an hour to change.

Looks are massively important - who wants last year's design? Not us. Stainless steel square-ovals are trick now, as they make your car look like it's got a bigger than stock motor. But beware, if you've got a bodykit room can be limited.

As a rule of thumb the size of the silencer box, not the tailpipe, governs the sound of the exhaust. A larger silencer will probaly be quiter than a smaller one, although the internal design makes a major difference.
Most back boxes come with a new correct-sized clamp. If not, you've got the hassle and the extra cost (a few pounds) of getting them from a car spare shop.

  Things to check

Welds (A)
Neat welds around the tailpipes and hangers are what you want. Untidy welds can point to poor construction, which in turn can lead to a short life (even on stainless steel boxes)

Hangers (B)
These must be in the same position as the factory box for easy fitting. Beefy welds mean the hangers aren't likely to come loose in time

  The jargon explained

Stainless steel

Doesn't rust but is more brittle than mild steel. Lowgrade stainless steel exhaust systems can crack easily due to the constant engine vibration.
De-cat pipe A simple piece of pipe that replaces the catalytic converter. Fitting this device can increase power.
Downpipe The pipe connecting the manifold to the exhaust centre section.
MIG welding Metal Inert Gas welding is a relatively simple process where a wire fed from a reel is electrically charged and creates a join between two sections of steel while gas is pumped around to keep the join clean.
TIG welding Tungsten Inert Gas welding is a high quality yet more tricky process than MIG welding. An electrical arc is used to heat the metal unit it melts and creates a permanent join. TIG welds often look like fish scaling.
Flexi-joint Used to connect stainless steel systems, which have a tendacy to be brittle, flexi-joints help soak up knock and bumps which could otherwise cause cracks.