A volatile fuel/air mixture can be a very useful thing. But after you burn it down to carbon monoxide and a collection of other lesser gasses it becomes little more than a liability. What do you do with this liability? Get rid of it. Fast.
Such is the principle of a performance exhaust system.
A performance exhaust system is the embodiment of a pursuit towards one goal – waste extraction with close-to-zero resistance. Restriction in an exhaust system places a load upon the engine, bleeding productive effort away to force exhaust out of the engine and through these obstructions. This effort could much more effectively be funneled in building revs, producing more power at the point where rubber meets the road.
So what do you want?
You want an exhaust system where restrictions are minimised to the maximal practical extent. Simple as that.
How?
There are four main factors which cause restriction in an exhaust, consequently harming exhaust efficiency. These are catalytic converters, mufflers, pipe bends and pipe diameter.
CATALYTIC CONVERTERS
A catalytic converter, or cat, is a small device placed near the start of an exhaust system. Essentially a small muffler-like skin containing a honeycomb-like material, a cat converter is employed in all Australian cars running ULP. The cat’s function is to convert hazardous exhaust gasses to more environmentally friendly matter before venting to atmosphere.
All this is well and good, however in the performance game this is akin to cramming a thick sponge in your exhaust pipe – a catalytic converter is a major restriction.
Thankfully, there are a variety of ‘high-flow’ cats available from most exhaust shops. While not as good as no cat at all, these are the best legal option…
…unless you live in New Zealand! If this is the case I understand that you have the option of either entirely removing your cat or alternatively simply disemboweling it – a hammer and chisel work wonders here. Check your local laws first, as the removal of a cat in my home nation of Australia carries with it a liability to large EPA fines.
Mufflers
A muffler does just that – it muffles exhaust noise.
A factory exhaust system is commonly designed with a primary emphasis on silencing the noise created by and engine’s fuel-air burn reaction. This silencing is most financially achieved through the use of a restrictive exhaust system. These items are effective silencers, but give huge restriction and back pressure as a result.
So buy straight-thru mufflers.
These items won’t have a maximal silencing effect; depending upon your attitude this may be a good or a bad thing. The goal of these designs is to reduce exhaust noise by an acceptable amount while still providing low- or zero-restriction flow properties. For example;
Pipe Bends
Generally when steel pipe is bent the interior diameter of that pipe is compromised. This reduction in diameter, combined with the redirection of exhaust flow, causes a restriction. Think… the object is to remove exhaust from the engine as quickly and effectively as possible. Imagine you’re running down a 2m wide corridor abreast of another person, thus filling the corridor. Suddenly, the corridor narrows to 1m in width and turns a 45ยบ corner! What happens? You slow down. Your mate runs into the back of you and has to wait – you can’t squeeze 2m worth of people through 1m worth of space at once. What if people are running behind you and your mate? They back up. This is restriction and back pressure.
So what do you do about this?
Firstly, you minimise the bends you use. Simple. Don’t bend an exhaust more than you need to.
Secondly, use mandrel bends. A mandrel bent pipe maintains its internal diameter through a bend – you still have to turn that corner, but the corridor is still 2m wide. Your mate doesn’t have to wait…as long, anyway.
Pipe Diameter
This is a contentious issue. An undersize exhaust pipe will cause a restriction as too much exhaust is forced through a space simply too small to accommodate it. Remember, restrictions will place an unnecessary load on your engine during the exhaust stroke, robbing you of power. On the other hand, exhaust gas is hot, and thus it is also light. This is desirable as light hot gas is easier to force through an exhaust than heavy cool gas. If an exhaust system uses oversized piping you can find that exhaust gas will find room to expand and cool before it clears the exhaust tip. While this last is a minor concern, it should be considered.
Generally 2 1/2″ is seen as perfect for an E15ET system.
The Front End – Dump Pipe
The point at which exhaust is extracted from the turbocharger exhaust housing is the most important component in an E15ET exhaust system, and it is here that the stock system is simply not up to the job.
The standard post-turbine exhaust housing violates two of the principles discussed already, in that it is a narrow-diameter system which is bent at a ridiculous 90degree angle. This is in our favor, however, as the replacement of this exhaust elbow with a custom fabricated large-volume exhaust dump pipe will increase throttle response markedly as turbine back-pressure is reduced and the spool-up rate is improved.
3″ mandrel bends should be used here.
How Much? Where?
There are a multitude of exhaust shops scattered throughout most metropolitan areas who could perform work such as this. Prices available between these shops varies considerably.
For a cheap quote on a press bent mild steel system, $200 is possible.
The use of stainless steel will increase longevity of the exhaust, but will bump up the price by 150-300%.
Mandrel bent exhaust systems will invoke a premium.
A dump pipe fabricated to suit the front end of an E15ET exhaust could cost anywhere from $100 to $400. Workmanship is more an issue here than in any other exhaust component, as a clean tight seal with the off-round exhaust housing outlet is vital.