Next up is a performance exhaust. Whereas induction kits and replacement performance filters are to do with reducing pressure drops on the way into the engine, performance exhausts are to do with minimising pressure restrictions on the way out of the engine. Some performance systems have an increased bore to reduce pressure losses but most retain the standard bore and have more freely flowing exhaust boxes.

However, just opening up the exhaust is not necessarily the answer. The extreme would be to remove the exhaust system altogether but, for those of you who have experienced it, if the exhaust breaks near the manifold, all power is lost. For a normaly aspirated (non-turbo) engine in particular it is important to retain a certain amount of back pressure. It can also be important where the first exhaust box is situated. The reasons for this are all to do with exhaust scavenging which is to do with negative pressure pulses reflected back up the exhaust manifold to remove the last of the exhaust waste from the engine. All a bit technical I know - a new page describing all this will be posted soon.

Having said all that, the situation with turbocharged cars is different. The turbocharger is positioned as close to the engine as possible in order to maximise its efficiency because it needs to be as close to the hot source as possible. And because the turbocharger is spinning at such a high speed compared to the engine this means that any pressure pulses from the engine are evened out and exhaust scavenging is not possible to the same extent as it is in normally aspirated engines. Therefore turbocharged cars can increase their power much more easily by simply increasing the bore of the exhaust system and removing exhaust boxes.

So the outcome of all this is that typical turbocharged cars such as Imprezas and EVOs can be producing more power as they get mega loud due to reduced back-pressure (i.e larger bore systems and free flowing boxes) but a normally aspirated car will actually reach the point where it becomes less powerfull as it gets louder. This is where the performance exhaust manufacturers come in. They know what is best for each kind of car.

The biggest brand names in the UK are Magnex, Mongoose and Scorpion who all produce a large range of stainless steel performance exhausts for the most popular cars for tuning in the UK market. Other popular names are Piper, Sebring and Supersprint with Sebring and Supersprint manufacturing a huge range of part numbers and Piper (of the cams fame) manufacturing an impressive range of both mild and stainless systems and backboxes with a vast range of tailpipe options - all with a 2-3 day delivery! Other well known names include Janspeed and Ashley Competition Exhausts from the 70's who still produce a good range of performance exhausts and manifolds.

Ah, manifolds.....that is another subject unto itself. For normally aspirated cars in particular, the design of the exhaust manifold is all important. The standard manifold supplied on a car is a compromise between cost, power and space. However, a well designed aftermarket manifold will fit in the same space but be tuned to produce significantly more power. This is achieved through carefully choosing the correct length for each of the manifold branches, thus maximising the cylinder purging through pressure waves. Again, more details coming soon...! There are not actually that many manifolds available in the market. Magnex do some as do Supersprint but there are more available from Janspeed and Ashley Competition Exhausts although they do major on older applications like the Mk2 Escort.

Prices for a stainless steel performance exhausts start at around £350. Top of the range exhausts can release around 8-10bhp.