Resident NZPC tech-head Rob Dawson dispels the myths on performance exhaust systems

Next on the list in our series of basic tech features is the exhaust system. Yep, that fruitful little mod that’s just about always first on the list when it comes to buying a high performance car ripe for tuning. It has to be said that in terms of bang-for-buck, a new, free-flowing exhaust system gives the best results. Unless you’re driving something really modern or something super rare, you can usually get a custom system installed for no more than $1000. For that money you’re almost guaranteed a good 5 to 10 per cent bump in horsepower, assuming your engine’s in decent nick. Not bad, right? But what if your dreams of a 4-inch exhaust are shattered by the guy installing it, and you end up with something a little weedier than what you hoped for? Don’t despair, because contrary to popular belief, an inch or two can make all the difference in the wrong way.

There are plenty of theories about what sizes and materials are best to use, but a lot of that is based on people’s personal experiences, not necessarily the facts. The truth is that just because you own a car with a two-litre turbocharged engine, you do not need to go and get a 3-inch exhaust fitted. Nor do you need to go out and have a 4-inch exhaust fitted to your GT-R to allow it to make 9,000 horsepower. I know it’s an over-used example, but WRC cars get by with a basic 2.5-inch system, and although they are only allowed to make do with 300hp/225kW, their engines develop masses of torque, usually upwards of 550Nm. Since it’s torque that actually pushes a car along and gives you that punch in the back, it’s the torque that we’re interested in. Outright power is great for boasting about down at the pub and on the back straight of the circuit at 240kph, but on the street what you really want are the Newton-metres. Granted, WRC cars have ultra-sophisticated computers and NASA-spec boost controllers on their side, but you can still mimick their power curve, albeit on a slightly smaller scale.

Firstly, it’s important to understand the key functions of an exhaust system. Up until recently, when the authorities and greenies started sticking their beaks in, an engine exhaust really only had two purposes. First, they were designed to remove spent, hot, noxious gases. Second, they were designed to lower the sound output from the engine. On top of those two functions, exhausts are now required to significantly reduce emissions as well.

In terms of construction, an exhaust system is actually pretty simple and consists of three main components: the manifold, the piping and the mufflers. The manifold is really where the science of understanding the way an engine expels its exhaust gases comes into play. It’s not a case of making a manifold to fit in a space, to look a certain way or even sound a certain way. It’s about allowing the gas flow from, for argument’s sake, four exhaust ports in the cylinder head to converge in one common area, then flow out through either one or two primary pipes and finally into the exhaust pipe as smoothly and quickly as possible without losing heat and thus velocity.

While manufacturers usually do a decent job with exhaust manifolds, as with any component on a car, they are obliged to meet the demands of everyone who might buy it including the person who wants a quiet car, the person who wants the car to be cheap, the person who wants the most power, the emissions authorities, and everyone else under the sun. Because of this, they have to come to a compromise with the design. This means that stock exhaust manifolds are usually made of cast iron since it’s cheaper, and are relatively restrictive in internal diameter so as to keep the noise levels down. With all the compromising factors, it is usually the performance that suffers.

Much of the key to realising power by fitting a new exhaust is based around the manifold design. Ultimately, if you are not concerned with noise or expense, you can have a set of tuned-length tubular extractors fitted. Rather than the heavy, bulky cast iron manifold with restricitve internals, a set of tubular extractors will be lighter and allow a greater volume of gas to pass through without causing a loss in velocity. The internal construction is such that the flow of gas is much smoother, and when the gas from each exhaust port converges into two or one, that progression is smoother and faster. See, we want more gas to pass through but we don’t want it to slow down as it exits the engine. Some of you might have worked out why by now. As a gas cools it becomes denser and thus heavier.

Hot gas that is pushed out of an engine is not as dense or heavy and therefore retains its speed as it goes through the manifold and down the exhaust pipe. And some of you may also know that a body of gas travelling at speed leaves an area of low pressure behind it almost a vacuum, in fact. The reason this is important is that the exhaust from a car engine does not come out in a constant stream, it comes out in pulses. As each pulse leaves the manifold and goes down the pipe, another follows in its path. If each pulse leaves a nice area of low pressure behind it, the following pulse will have less resistance to push through, and therefore will retain its own velocity, and so on.

These two reasons are why it is not always best to have a massive exhaust diameter. They might look sweet but they don’t always do the job as well as something a little smaller.

Having said all that, an increase in pipe diameter of both the manifold and the exhaust will still be required if proper gains are to be made. But there is a point at which you can go too big. If you have a massive turbo running big boost (and thus a much more powerful expulsion of exhaust gases from the ports to the point where the existing diameter just can’t cope and your engine suffers from too much back pressure), then you can go for a larger diameter system than normal, but someone in the know needs to advise you on that. Just because your mate with a TD-06 has a 3-inch pipe doesn’t mean you should get one because you had a 50-trim wheel put on your TD-05. Get the drift? I’m sure you knew that already though. On top of that, you don’t want to get an expensive set of extractors installed only to find out that they are super-thin walled and lose too much of the heat from the exhaust gas. Remember, you want the heat to stay inside the exhaust system, not escape out into the engine bay!

When it comes to the material, many people will swear by stainless steel, whereas others maintain mild steel is adequate. This simply comes down to opinion. Stainless steel is certainly more expensive and in some cases it will cost you twice as much to have your pipework made. However, it is more resistant to corrosion, which is a definite benefit. Mild steel, on the other hand, is cheaper and is also easier to weld back up if you happen to gash your pipework open on a driveway. Stainless steel also tends to be more brittle than mild steel, and this can lead to cracking if the exhaust is forced to flex a lot (always remember a flexi-joint). Some also say there is a difference in sound between the two, but again, if that’s true then it just comes down to what it is you actually want.

The diameter itself has always been a bone of contention and always will be. Most camps agree that a slight sacrifice in size usually helps drivability and low-mid-range performance, while a larger-sized option helps things out at the top end. Bearing in mind that 95 per cent of your driving will be conducted in the former, the sensible choice would probably be in the case of say a 350hp Evo with mild turbo work, say a 2.5-inch setup. But if that same car spends its time above 5,000rpm then the 3-inch would be the better choice. If it were my choice I would follow the WRC route any day of the week: opt for the 2.5-inch pipe and focus on the computer and boost control to maximise both midrange and top end at the same time. That way you won’t end up with a gutless wonder below 4,000rpm and a noisy overboosting pig at the top end.

The main thing to remember is that you usually get what you pay for, and this is especially so with mufflers. Pay for the best quality mufflers you can afford (usually larger means quieter), and don’t be put off by having two mufflers either. Most of the major brand Japanese mufflers barely hinder exhaust flow at all. But they are damn helpful when you’re spending nine hours driving and don’t want to hear that annoying drone from under the car.

Words: Rob Dawson