Following on from my basic 'What you need to know about' Induction kits I thought I shed alittle light on the minefield that is the World of Exhaust systems - Like I said before this is just what I know and what I have learnt.
Exhaust systems
Before I go into a tuned exhaust system its important to understand a few things.
Why do cars have exhaust systems? Well . . . there is a number of reasons that have changed over the years, the first reason is to get exhaust gases out of the cyclinder and away from the induction system so that dirty air or exhausts don't re-enter the system.
But why dont cars have exhausts like aircraft engines - short? The simple reason is that aircraft 'prop' engines spend most of their time at a fixed rmp normally quite high, the same as say drag cars, where low rmp performance isnt important but a road car need to perform well across the whole rmp range and a exhaust system helps add torque to the engine using 'back pressure.
If you want to learn more about exhaust back pressure read
here
One last thing before we start the good bits is it is also important to understand all cars from sub £6,000 to £100,000+ cars are all built to a) a budget, b) Noise regulations and c) Emission controlled tests so a manufactor could build a better exhaust for performance but might fail a noise test or might push the budget up and out cost the car to a rival car.
Ok now on to tuning
Back Boxes
On all cars the first and only part of the exhaust you will see unless you open the bonnet is the back box or just the tailpipes. Its only really been over the last 10yrs or so that manufacturers have added style to their tailpipes before they would just have been a pee shooter that exited out under the bumper. But now your be hard pressed to find a car without a shinny tailpipe. As for improving performance because most of the restrictions in a exhaust system are more to do with either a manifold or cat and middle box the final box is just to reduce noise.
By changing to a sports or custom back box you may free a pony or two but it will be more for looks and sound rather than performance.
Custom Jap style can can be had for as little as £30, branded sports backboxes will set you back more like £100+
Centre boxes/Full Sports systems and Single Box systems
This is where the next best option to tuned exhaust are found. Sports exhausts use more free flowing boxes increasing gas flow and adding a sportier tone to the exhaust system. Good exhaust can free upto 5bhp and more on some cars and make the car more free reving as your are increasing the cars breathing.
Some people will say for best results for for a single box system which removes the centre box from the system, and athough this might add a few more ponies you push the torque further up the rev range. It basically works like this:
Twin box system = low to mid range power gain
Single box system = mid to high range power gain
On sub 250bhp cars I would say it wouldn't make much difference but on a 300 na engine or turbo charged engine its different.
Sports Cats
These again have only really appeared over the last 5-10yrs as cats have become a legal requirement to pass a MoT. Sports cats have less cells which allow better gas flow but still removing harmful elements from the exhaust gases. These are quite expensive and on a sub 150bhp engine I wouldn't bother as the cost out weighs the gain.
Now some people will advise you to run cat less using a decat or cat bypass pipe as people say cars performed better before cats. Now that is a fair point but its important to remember that cats where just added to early pre cat engines but these days engines and ecu's are tuned to run WITH the car. I have read about Civic Type R's running better with their cats than without. Dont forget that your need to replace the cat to pass a MoT
Manifolds
In old school tuning one of the best ways of increasing performance was to change your exhaust manifold to a 4branch design in either a 4-2-1 or 4-1 layout. This basically means 4pipes (one per cycinder) then in to two then into one which enters the start of the exhaust system. Some cars will still have a cast iron two part or even three part manifold which is very restrictive. The best pound for pound bhp gains are to be found here, If you can change your manifold. If you already have a 4-2-1 then I wouldn't bother looking to change it. I personally was very happy and suprised to see a 4-2-1 on my TwinSpark 156.
In real terms the best setup for a car without spending a fortune is just a exhaust system that replaces the cat back metal work. If you go for a Jap style or large bore back box which doesnt have many baffles ie you can see straight through then be prepared to lose abit of back pressure and low down torque if you go for a single box setup with that style of back box.
Single box systems car be great fun around town and on B-road drives but on motorways they can tend to boom alittle - Stainless steel also has a habit of resonating where as mild steel wont.
If you can't find a system for your car there is nothing stopping you having a custom system build by a custom exhaust shop, and custom doesnt mean a baked bean can like on Corsa's or Saxos, custom means you get what you want where want and how your want it to look.
Like I said I'm not a expert but I've heard enough and read enough to know the basics and asked the questions myself - hope this helps.