In Part One of "Out the Exhaust" we discussed the different approaches to post-turbo and post-extractor/manifold exhaust systems. Here we'll take you through the specifics - how to select components and design your own custom system...
Muffler Types
At present there are two main types of mufflers in the aftermarket - straight-through and reverse flow. The once common baffle type mufflers are becoming rare as people catch on to the fact they flow miserably.
As its name implies, a straight-through muffler has a straight perforated pipe between the inlet and outlet fittings. The perforated pipe - like in any non-baffle style muffler - allows exhaust gasses to expand through to the muffler packing. Note that the term 'straight-through' also includes mufflers that have their inlet and outlet fittings at left and right sides of the casing. These are known as 'offset' straight-through mufflers.
From a low restriction point of view, straight-though mufflers are the pick of the bunch. Testing has revealed a quality straight-through muffler has more than 90 percent the flow of a straight length of exhaust pipe; in other words, you only lose less than 10 percent exhaust flow compared to having no muffler at all.
Steer clear of reverse flow mufflers at all times - testing has shown they have merely 60 - 70 percent the flow of a straight-through muffler. This isn't surprising when you consider the internal layout of a reverse flow muffler - the exhaust gasses are forced to turn back against their initial flow direction (hence the 'reverse flow' name), before turning around once again and exiting the opposite end of the muffler. So there are two 180-degree direction changes that contribute to the considerable overall restriction.
Furthermore, within each category of muffler, there are a few manufacturing techniques to be wary of...
Cheap mufflers are often pretty horrible when you look inside them. Many have non-flush pipe ends protruding into the exhaust flow, as well as numerous welding dags. These defects can impede flow massively and - on the road - can cost a noticeable amount of power.
Become familiar with the different types of perforations used across different mufflers. Many mufflers have their perforated pipe(s) formed by an external punch, which creates 'louvers' that protrude into the internal pipe cross-sectional area. The idea behind these louvers is that - when installed in the correct flow direction - the exhaust gasses are drawn into the low pressure area directly behind each louver and enter the muffler packing. This may be an effective approach to noise reduction, but there's a definite penalty to be paid in flow restriction.
Higher quality mufflers using perforations punched cleanly through the pipe do not protrude into the flow area and - therefore - have reduced flow restriction. We have not seen any noise comparisons between cleanly punched perforations and the aforementioned louvre type.
When it comes to noise suppression, picking mufflers is a lucky dip - a given muffler design may perform well on one car, but not so well on another. Unfortunately, there is no way of predicting a muffler's effect on noise - the only approach is trial and error.
The price of mufflers varies hugely. A fairly low grade straight-through muffler can be an extremely cost-effective purchase at around AUS$70, but - if you're after a brand name or something shiny - you can spend up to around AUS$600. Much of this extra cost comes from the quality materials used, such as 304 and 409 grade stainless steel, but oftentimes you're also paying a lot just for the name. Both of these stainless grades last longer than a conventional mild steel muffler, and can be polished to a sparkle.
Catalytic Converters
Catalytic converters are an essential part of the exhaust system on any modern road car.
The function of a cat converter is to reduce tailpipe emissions. Inside the cat housing you'll find a dense honeycomb core, which - although you can't see it - is coated with precious metals such as platinum and rhodium. These metals serve to reduce the toxic content of exhaust gasses, transforming it into carbon dioxide and water.
While the main function of a cat converter is to improve emissions, we have never seen an aftermarket unit with quoted emissions performance. Instead, the aftermarket focus is on minimal flow restriction, where - as a rule - larger inlet and outlet diameters perform better. Bear in mind that even a high quality aftermarket cat converter has about the same flow restriction as a low-grade muffler of the same diameter.
In addition, the flow restriction of a cat converter worsens, depending on how long it's been fitted to a car and the conditions it's endured. As you can imagine, a cat converter with 40,000 kilometres of poorly tuned running behind it is likely to be coked in carbon. To some extent this will impede exhaust flow.
As mentioned in Part One, a cat converter doubles as a noise reduction device. Depending on the specifics of your selected aftermarket cat and the rest of the exhaust system, you can expect a 4dB(A) noise drop during a 3000 rpm free-rev compared to having no cat at all. This magnitude of noise reduction is superior to many aftermarket mufflers!
In terms of cost, an aftermarket catalytic converter is no more expensive than a brand name polished muffler. A 3-inch converter can be bought from less than AUS$200 up to about AUS$500. The longevity of aftermarket cat converters depends largely on engine tuning, but expect an effective lifespan about the same as an OE converter - about 60,000 kilometres.
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Making a Catalytic Converter Perform...
The placement of the cat converter in the exhaust system is critical in determining emission performance. The internals of a cat converter need to build to around 400 degrees Celsius before it starts effectively reducing emissions. As a rule, moving the cat converter closer to the engine helps it build up to operating temperature only a short while after start-up. The only downside is - again, as a rule - moving the cat closer to the engine reduces power slightly. Another approach to improve the effectiveness of the cat converter is to apply thermal insulation to the pipe immediately before the cat - this helps to maintain exhaust gas temperature, therefore, warming the cat converter. More on exhaust wraps and coatings will be presented in Part Three of this series. Note that, while it's best to warm the cat as quickly as possible, it is possible to overheat the converter core. This occurs when raw fuel passes through the engine and is burnt inside the cat body. Under these extremes, the delicate core is prone to melt.
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Be warned - fitting an aftermarket catalytic converter as part of your exhaust system does not automatically make your car emission law compliant! Only a complex 5-gas exhaust analysis can determine emission levels.
Pipes and Bends
Exhaust piping is available in two common materials - mild steel and stainless steel.
Mild steel is the cheaper variant and - in most cases - it's what you'll receive unless you specifically ask for stainless steel. We've had mild steel exhaust systems fitted to many of our cars - and have had no problems with them lasting nearly ten years.
Stainless steel pipe, on the other hand, offers superior rust resistance and so is said to last even longer. The precise lifespan of a stainless exhaust system depends largely on the grade of stainless pipe used.
The exhaust pipe diameter you should chose is related to the power output of the engine - a very powerful engine flows a lot of exhaust gas and, therefore, requires a large diameter exhaust pipe. The big question, however, is what specific pipe diameter is required for a given power output?
Unfortunately there are no concrete answers to this question, but we can look to current aftermarket exhaust trends for some guidance. It's now common practice to install 3-inch exhausts to V8s and turbo engines up to about 500 horsepower, 2½-inch exhausts on 200-odd horsepower naturally aspirated engines and 2¼-inch exhausts to naturally aspirated fours with about 150 horsepower. These are guides only.
Twin exhaust systems should be large enough that - when combined - their cross-sectional area corresponds approximately to the area of an appropriate single pipe system.
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Flex Pipes...
Most transverse engine'd vehicles require the fitment of a flex pipe. A flex pipe is a short length of braided tube, which allows the engine to rock on its mounts without forcing the exhaust system into the vehicle's undercarriage and - in severe cases - cracking it. Braided flex pipes are quite expensive, depending on diameter costing up to a couple of hundred dollars. Don't be tempted by the cheaper spiral flex that's commonly used on trucks - it is very thin and prone to splitting within a short time.
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There are two types of pipe bends commonly used in exhaust fabrication - press bends and mandrel bends.
Press bends are the most common variety, and are formed by a machine that acts only on the outside of the pipe wall. As a result, some of the pipe's internal diameter is crushed from the outside during the bending action.
Mandrel bends are made by the insertion of a mandrel inside the pipe while it's being bent. The mandrel holds the pipe diameter to nearly its maximum, giving the greatest possible cross section for exhaust gasses to flow through.
The flow capacity of a mandrel bend is always superior to a bend that's press formed - however, the extra flow of the mandrel bend varies depending on the angle of the bend. Flow bench testing has shown that a good quality 45-degree press bend (one that isn't overly squashed) flows only around one percent worse than an equivalent mandrel bend. Once the bend is tighter, however, the mandrel pipe's advantage grows - across a 90-degree bend, the airflow of the press bend trails by 2.5 percent and a across a 120-degree bend it's down by nearly 4 percent.
Note that the quality of press bends does vary from workshop to workshop - some of the pipes are crushed more than others. The sole attraction of press bends is cost - there's only a small amount labour involved, the bender is cheap, and the length of pipe is typically worth less than a dollar.
Mandrel bends, however, are usually bought pre-formed and are welded to the ends of straight lengths of pipe to create the system. As a guide, a 3-inch 90-degree mandrel bend in mild steel costs AUS$30 - 35, while the same in stainless blows out to around AUS$80.
To summarise: where budget permits, it's always best to use mandrel bends in preference to press bends. On the other hand, press bends can be a cost effective alternative - especially if they're used only for small angle changes or the pipe size is specified one size larger than would normally be necessary.
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Tips...
Exhaust tips are all-important for aesthetics, but be wary of exhaust tips that impede exhaust flow. Any tip smaller than the diameter of the main exhaust pipe will reduce flow. Furthermore, tips that are either down or up-swept introduce an unnecessary bend and - therefore - increase restriction. Oh, and don't be tempted by tips with rolled in edge - these can cause massive in-pipe turbulence and drones.
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Designing a New System from Scratch
When designing an all-new exhaust, you should start by considering 'the big picture'. The key considerations are backpressure, noise, durability and - of course - cost.
If you're chasing performance more than anything else, your approach is simple. Go for the largest possible pipe diameter (after the tuned length part of the system in a NA car) and step up to stainless tube if you plan keeping the car for more than ten years or want a show-level appearance.
Try to keep the number of bends in the system to a minimum. Also, where there's enough space, aim to give each bend a large radius - tight bends cause considerable restriction.
As mentioned, mandrel bends are the pick and are usually bought pre-formed; a few workshops, however, are equipped with their own in-house mandrel bending machine. If you have access to one of these workshops, it's preferable to have them custom bend a length of pipe rather than weld individual bends to the ends of straight sections of pipe. Welds often penetrate through to the inside of the pipe, causing flow restriction.
Cat converter selection is relatively unimportant - all of the big aftermarket units flow fairly similar amounts of exhaust gas.
When it comes to mufflers, however, go for only straight-through mufflers and try to keep the number used to a minimum. On a turbocar you can often get away with just a cat converter and single rear muffler, but on a naturally aspirated vehicle you're likely to need two mufflers to keep noise to a semi-bearable level.
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Turbo Dump Pipe...
The pipe that bolts to the exit of a turbo car's turbine housing is commonly referred to as the 'dump pipe'. This pipe is arguably the most important section of a turbo car's exhaust - it's critical in helping to determine turbine response and the overall flow of the exhaust system. There are a couple of approaches to fabricating a high-flow dump pipe. The most common is to shape the end of a large diameter pipe to match the turbine flange. In the case of a turbocharger with an internal wastegate, the flange-matched dump pipe gives plenty of space for turbine and wastegate gasses to merge. The most recent approach to a free-flow dump pipe is to use a separate pipe to accept the gas from the turbine, and other pipe to accept the gas from the wastegate bypass. The wastegate pipe is usually merged into the main pipe about 50 cm downstream, but some owners chose to simply vent wastegate gasses directly to atmosphere. These so-called screamer pipes are highly illegal. We have not seen conclusive testing showing which approach is better. Neither, however, are cheap - budget at least AUS$300 for the dump pipe alone.
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Follow the above route and we guarantee you'll piece together the least restrictive exhaust system possible for a streetcar.
If, however, you place an emphasis on low exhaust noise, we can deviate from the above approach without sacrificing too much flow. One of the increasingly common methods of reducing noise is to taper the pipe diameter towards the later part of the system. This serves to put more backpressure on the cat converter and mufflers prior (making them muffle more effectively) and, in addition, the smaller pipe diameter tends to be quieter.
Of course, the traditional approach to reducing exhaust noise is to add more straight-through mufflers (aka resonators) - these are usually inserted mid way along the system. If, however, there's no more space for fitting extra mufflers, you can only resort to changing the existing mufflers in hope of finding ones with better noise absorption qualities.
As discussed, obtaining the desired exhaust noise requires trial and error.
If you're constructing a new system on a tight budget, it makes sense to keep the number of components to a minimum. On a modern turbocar, you should aim to fit just a cat converter and single muffler. On a naturally aspirated car, you'll likely require a cat and two mufflers.
The pipe diameter you select should - again - be the largest you can afford. Large pipe costs very little more than smaller stuff - it's the cost of the associated large diameter mandrel bends, cat converter and muffler(s) that can stretch the budget.
Wherever budget permits, go for mandrel bends all the way - at minimum, use mandrels for all the tight curves in the system.
Note that the all-important dump pipe of a turbocar can be tackled in a completely different way when money is a concern - the factory cast iron dump pipe can often have its internals ground back for improved flow. The standard dump pipe on the Holden Commodore VL Turbo, for example, has a raised lip just near its exit flange - this can be easily removed using an air grinder.
If you're watching dollars, don't bother going for stainless steel pipes or mufflers - these add tremendously to cost of the system and offer nothing more than durability. Ask yourself; do I really expect to keep this same car for more than ten years?
Finally, go out and buy the cheapest large diameter cat and straight-through muffler(s) you can afford. As mentioned, there's little flow difference between aftermarket cat converters, but straight-throughs are the must-have type of muffler.
Potential Gains
The systems designed by Melbourne's APS (Air Power Systems) are a good example of well thought out exhausts. These systems are made from quality 304-grade stainless for maximum durability and offer an excellent blend of performance and noise.
Features of these systems are high-flow dump pipes from the back of the turbocharger, full use of mandrel bends, a high-flow cat converter, one or two straight-through resonators and a straight-through rear muffler. Three-inch pipe diameter maintained throughout the system, however their MY01 Subaru Impreza WRX system incorporates a deliberate taper to just under 2½-inches toward the rear section, while their Nissan S15 200SX/Silvia system uses a smaller diameter rear muffler outlet. This taper - presumably - contributes to these system's low noise levels and ADR emission compliance.
Fitted to a current S15 Nissan 200SX/Silvia, the APS exhaust system gives a strong power increase all the way through the rev range, with peak power bolstered around 13 percent. Note, however, fitting the exhaust simultaneously gave up to 1.8 psi extra boost pressure and air-fuel ratios went slightly leaner.
In contrast to the Nissan S15, we've seen the fitment of a similar APS exhaust to a MY01 Subaru Impreza WRX give zero extra power. Yes, zero. A considerable 19 percent power was gained at low rpm, but the Subaru refused to make any more top-end power. APS claims their system reduces backpressure from 9.6 psi down to 3.5 psi but, yet, peak power didn't budge on the vehicle we tested. This all comes down to the countermeasures built into the new generation engine management system - as mentioned in Part One.
As a very large generalisation, however, installing a low restriction exhaust as outlined will give the average vehicle about 10 - 15 percent more power. Not bad for a bolt on modification...
In Part Three of Out the Exhaust we'll cover 'peripheral' exhaust stuff - like ceramic coatings and variable flow valves - and we'll talk about extractor/exhaust manifold design ...
