Components of the Exhaust System
The exhaust system on 4×4 vehicles is more than just a way to reduce noise; a properly designed and installed system creates a way for exhaust gasses to be extracted from the engine, rather than merely being a conduit through which exhaust gas is pushed out by each piston on its exhaust stroke.
Even experienced mechanics, who should know better, often get the basics behind the design and functioning of the exhaust system wrong, and in particular, about back pressure. In the off-road context, this issue is the single most important aspect of exhaust gas extraction from the engine, so what is back pressure and all the other considerations that go into making an efficient exhaust system?
Exhaust gas is not produced in a smooth flow; each cylinder produces a pulse, with the more pulses, even at high RPM’s, the more continuous the gas flow. Back pressure can be seen as the resistance to the positive flow of gas that is built into the system.
• Back pressure and exhaust gas velocity
Even among mechanics there exists a belief that the bigger diameter the exhaust tubing, the more efficient the system is at clearing exhaust gas from the combustion chamber. This is simply not the case, since the efficient working of an exhaust system depends on the correct balance between pipe diameter and gas velocity.
However, while it is true that gas through a 75mm pipe will flow easier, the flow rate may so slow that the next pulse may arrive while the first is still in the pipe; and this will lead to excessive back pressure, (a heaping up of exhaust pulses as it were), seriously reducing the system’s ability to clear the exhaust gas. On the other hand, a 50 mm pipe wil speed up the pulse, but may not allow its total expulsion before the next pulse arrives, also leading to a pressure build up. For this reason, the intelligent exhaust system designer will select a pipe diameter say, 55mm or 60 mm, that allows sufficient gas velocity while at the same time allowing for sufficient volume to prevent the build-up of pressure in the system.
Manifolds, usually made from cast iron, are designed to collect exhaust gas and to channel it into the exhaust tubing. However, a badly designed manifold will allow some gas to escape past the collection point back to other cylinder ports, so to minimise this, designers always use very narrow junction angles and other methods such as making all the channels in the manifold to be the same length, to prevent a build-up of pressure inside the manifold that seriously effects engine performance.
• Scavenging manifolds
The debates surrounding scavenging manifolds, or headers, are as old as the car itself, however, there is no simple one-size-fits-all answer to the question of whether they work or not. The problem lays in the fact that there are two main types of scavenger header; one that works only on high performance engines that run at 6000 RPM or more, and the other type, known as an “interference” header, that works at low RPM’s, but then only on petrol engines, and non turbo engines at that.
Turbo fitted diesel 4×4 engines do not allow the fitment of anything else but a manifold that houses the turbo. In these engines, the build-up of pressure in the exhaust manifold is a necessity in order to drive the turbo, and any potential power loss because of manifold pressure must be weighed against the demands of the turbo, with the needs of the turbo winning every time.
For 4×4 or off-road applications, the interference header exhaust system does seem to offer some advantages since the primary pipes are joined to the secondary pipes in the order in which cylinders fire. This produces a lower pressure in the other pipes, which scavenges, or sucks, the gas from the engine more efficiently that a standard manifold.
However, too much scavenging can have the effect of sucking some of the air/fuel mixture from the inlet manifold during the brief period of valve overlap, during which both valves are open. To prevent this, the fitment of long duration camshafts may be necessary, but this could potentially affect other operational parameters such as injection timing and/or duration or ignition mapping.
There is no clear advice to be offered on whether to fit scavenging headers or not; considering the relatively small gains, the exercise might just be more trouble than it is worth. Having a reliable off-road vehicle is crucial to the success of an overland expedition through say Africa, or the Outback, so, use your best judgement and gather as much information on your particular 4×4 vehicle before embarking on this route.
Although having a catalytic converter fitted to the exhaust system of any vehicle is a legal requirement in many parts of the world, in the 4×4 context, apart from cleaning up the exhaust gas, they do nothing to enhance performance or power delivery. On the contrary, a catalytic converter can become clogged, which has the effect of blocking off the flow of exhaust gas, which in turn can cause your off-road vehicle not to work at all.
• Diesel particulate filter
While the theory behind these devices seems to be sound, the truth is that they reduce particulate matter in diesel exhaust by burning the collected soot through the addition of raw diesel or in some cases, a special chemical, and then only when they are hot. On balance, they contribute nothing to engine efficiency; modern diesel engines are already clean with very little particulate matter in their exhaust system, so by adding raw fuel to burn the little there is seems like a waste, not to mention a contributing factor in global warming by burning fuel that could have been used for conversion into movement.
Free flow exhaust system
In theory, a free flow exhaust system increase the flow rate of exhaust gas through the system, however, not all exhaust systems are created the same. Many aftermarket suppliers will sell you a so-called free flow muffler with promises of huge power gains and even bigger fuel savings, but the truth is somewhat different.
By itself, a free flow muffler will do nothing for you; a proper exhaust system generally consists of slightly larger than standard pipes, a free flow muffler matched to the piping, and a resonator box which acts as a muffler, but without the restrictions of a muffler. An exhaust system like this will not offer huge fuel savings, but by removing the exhaust gas more efficiently, more torque is available sooner without significant changes to the power/torque curve. A proper exhaust system installation may also significantly reduce turbo lag due the increased efficiency of exhaust gas extraction. While in the 4×4 context, a free flow exhaust system may be worth the cost when your existing standard system becomes due for replacement, the many variables that could potentially affect their performance make this a choice that should be approached with some circumspection. The advertising claims made by free flow exhaust system vendors rarely live up to their hype; the sensible way to approach this issue is to compare the relatively small gains (but gains nonetheless) with the potentially huge cost of installing a system that may or may not deliver on the promises of its maker.
Many die-hard supporter of the free flow exhaust system, especially in the 4×4 fraternity, will swear by a free flow system while just as many others could tell stories of money, time and effort wasted. On balance, a free flow exhaust could save you some money in fuel, while giving you a little extra torque, but it will not do so in massive figures and amounts. Use your best judgement when considering a free flow exhaust.
Stainless steel vs. mild steel exhaust system
If you are going to go to the trouble of making you 4×4 as tough and reliable as you can, consider replacing your mild steel exhaust with a stainless steel system. Off-road road driving is a rough business and is particularly hard on exhausts.
A stainless steel exhaust system may be more expensive, but its corrosion resistance means it will outlast a mild steel system several times over. Apart from that, a stainless steel system also makes good sense from the recovery point of view; a mild steel system may look good from the outside, but unseen rust spots could blow out if you try using a gas bag to lift your 4×4 out of thick sand or mud. Even a tiny blow hole could render a gas bag useless, so, if having a hassle free off-road road trip is your main priority; consider the tougher and more durable stainless steel exhaust.
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