The pipe the takes the exhaust gas away from the engine and lets them loose at the back of the car so the occupants don’t breath it in. Normally it has mufflers (silencers) to reduce the very high sound levels that the engine produces, without some sound reduction the cars occupants would end up deaf very quickly.
Usually the exhaust comes in several parts, the bit attached to the engine is the ‘Manifold’, this is connected to the ‘System’ which goes all the way under the car to the back. The system starts with the ‘Down pipe’ coming from the manifold down under the front bulkhead, then there may be a front section with catalysts, a mid section with a larger silencer and possibly a separate rear section with a smaller silencer and finishing with a ‘Tail pipe’ showing at the back, although there are many other arrangements too.
As well as transporting the waste gasses safely away and muffling the noise down to acceptable levels, the exhaust also effects the engine performance, its has to be big enough so the flow is not restricted. But also the gas speed needs to be preserved for high speed power, so making the exhaust to big can actually reduce power. As with all tuning its a fine balance to get the best performance, and there is no one perfect solution.
The bit that bolts to the engine is the Manifold, it has a tube for each one of the cylinders which join together. The exact way they join together and the length of the tubes makes a big difference to the tune of the engine, they can improve low end torque or sacrifice that for peak power. Its important to get the shape and size of the manifold ports to match up with the exhaust ports on the engine, any mismatch can restrict area or leave a step which causes turbulence and reduces flow.
There are two main types of muffler, one uses absorptive rock wool matting and the other type sends the exhaust gases through a sort of maze which breaks up the sound pulses. Generally the absorptive type removes high frequencies and the labyrinth type removes the basey boomy noises.
Most standard systems have a mixture of both, but for a more sporty sound they can be replaced with simpler ones that have less noise reduction and slightly more flow.
Twin pipes are still popular, factory fitted to most V engines which have two exhaust manifolds, they run an exhaust pipe on each side of the car floor pan and finish with two tail pipes.
On V6 and V12 engines these can be two totally separates systems, but on V8 engines they often have a balance pipe between the two systems close to the engine in order to run smoothly because of the way the firing order overlaps, giving an uneven sequence of exhaust pulses on each bank and that distinctive burble.
The least important part for performance is the tail pipe, usually finished of with a decorative trim.
Many systems run twin tail pipes running from the back muffler, although some systems try to get the twin pipe look by fitting a Y piece close to the back.
Catalysts (cats) convert partially burnt fuel and fumes into carbon dioxide, water and nitrogen. They do this by passing the exhaust gas over an immense area coated with an incredibly small layer of precious metals such as platinum which do the actual catalysing bit.
And it really does need a huge surface area to work, this is archived by folding the surface into a honey comb and by giving it a microscopically rough surface. In fact a typical catalyst can have the same surface area as a football pitch, all folded up into something the size of a 3 litre pop bottle, amazing.
It only works when its hot, at least 300C and preferably 600C, so it is usually put as close to the engine as possible so as not to loose any heat. In order for it to heat up quickly cats are usually made of ceramic which makes them fragile, so the catalyst brick is supported in the can by a soft fibre mat.
So if the cats hit a bump in the road there is a fair chance they will shatter. Also if the engine is tuned badly then un-burnt fuel will burn on the cat face and melt it.
When cats were first fitted back in the ’70s they were too small for the job and would restrict flow, modern cats are usually very good at flowing and can even cope with mild tuning, but for big power gains usually a bigger sports cat is needed. Racing cats use a metal brick instead of fragile ceramic, it takes longer to warm up but can take more abuse.
Exhaust systems can be either mild steel that has been coated in an aluminium based protective layer making it look dull silver, or made of stainless steel which lasts much longer and looks shinier. Stainless is a harder metal and so when it vibrates it makes a higher pitched noise, some people claim stainless exhausts sound ‘tinnier’ than mild steel ones.
The difference between quality brands and budget options is often in the grade of metal, cheap stainless will start to rot nearly as fast as quality mild steel. Also cheaper systems can end up with rusty welds, mild steel systems should have been coated after welding and stainless systems should be welded with stainless wire, not the cheaper mild wire. If the welds on a new system look rusty then it was a cheap one.
Sound affects our mood and generates strong feelings, so the exhaust sets the tone for the whole car. Get it right and the car sounds strong and purposeful, get it wrong and it sounds like a fart in a tin can.