Everything you need to know about modifying exhausts.
By Michael Knowling
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In Part Two of Out the Exhaust "Out the Exhaust" we discussed how to select exhaust components and design your own custom system. Here we'll introduce variable exhaust valves, factory exhaust manifolds and extractors, aftermarket extractors, turbo manifolds, turbines, wastegates and heat insulation...
Variable Exhaust Valves
Variable exhaust valves are becoming common in factory vehicles and, to a lesser extent, in the aftermarket.
Many OE exhaust systems now incorporate a variable butterfly valve in an attempt to reduce noise at low load, while maintaining maximum flow at high load. The butterfly valve angle is determined by the ECU, taking input from throttle position and/or engine load. The variable exhaust valve fitted to the Australian-delivered Subaru Liberty B4, however, is opened solely on the basis of road speed.
The location of the valve can vary - it can be on the exit to the rear muffler or on a secondary entrance to the rear muffler.
In the aftermarket there are two different approaches to a variable exhaust - the VariFlow system and the Mr Gasket Exhaust Cut-Out.
The VariFlow system involves inserting a large valve body into part of the exhaust system. At low engine load an internal butterfly valve remains at a partially closed angle, helping reduce noise output. At high engine load, the valve is opened to allow maximum gas flow and performance.
The VariFlow kit is sold with the valve body (available in various diameters), control box and all necessary electric switches and pneumatics. See "Pure Pipe Perfection 2 - Introducing the Secret Weapon..." for our in-depth description and evaluation.
The Mr Gasket Exhaust Cut-Out is quite different - it can be used either as a 'switch' between two different exhaust systems or as a bypass around the restrictive components (mufflers etc) of the main exhaust. The Cut-Out system comprises a cast iron Y-piece with a cable-operated internal flapper valve, which pivots to block one of the two opposing Y junctions. Its main use is in street/strip cars to allow driving to the track and competing without the need to 'drop' (disconnect) a muffled exhaust system.
Another approach is to configure the valve to bypass restrictive components of the exhaust system. For example, you could place the valve before a pair of restrictive mufflers and have a bypass pipe merge back into the main pipe further downstream. This would allow a quiet, muffled exhaust and - once the flapper valve pivots - a free-flowing bypass system for maximum power.
Note that the Mr Gasket Cut-Out is available to suit only 2-inch OD pipes.
Factory Exhaust Manifolds and Extractors
Most factory exhaust manifolds are made from cast iron. Cast iron manifolds are relatively cheap to manufacture and are typically very durable (though some are prone to cracking).
Some high-performance factory vehicles - such as the Peugeot 306 GTi-6, Lexus GS300, Mazda MX-5/Miata, HSV, Tickford and Ralliart Mitsubishi Magna - come equipped with tubular stainless steel pipes (aka extractors).
The advantage of extractors - where a separate pipe is dedicated to each cylinder - is the eliminated chance of one cylinder's exhaust pulse interfering with the exhaust flow out of the remaining cylinders. This enables exhaust gasses to be effectively scavenged from each combustion chamber and evacuated as quickly as possible. The design and packaging (fit in the engine bay) of good extractors also allows closer-to-optimal pipe lengths for better resonance tuning, smooth radius bends for reduced resistance to flow and less restrictive collectors.
The flow of an OE cast iron manifold is often quite poor, but OE extractors vary hugely from car to car. Some exhaust manifolds (such as those found on early-90s Ford Falcon XR6s) flow less-than-ideally due to their compromised manufacturing processes. This can often be identified by poor collector cone construction. On the other hand, HSV presently computer models and tests up to 80 different extractor designs before making a selection. As a result, the chance of finding extra power and torque by replacing many good OE extractors with aftermarket items is quite small.
Aftermarket Extractors
Aftermarket extractors are invariably made from 409-grade stainless steel - the same as most OE extractors. This ensures most aftermarket extractors will last a long time - often, the life of the car.
Interestingly, one of Australia's biggest extractor manufacturers, Pacemaker, doesn't employ computer simulation to design products; rather, it draws from experience and performs extensive chassis dyno testing. We're told they start the process with a clean slate - absolutely no attention is paid to the design of the OE manifold or extractors.
There are two main types of extractor designs - tuned length and interference (unequal length). There are numerous theories on how each of these designs operate and perform, but - at the end of the day - the dyno sheets are more important. Pacemaker suggests there's not much power difference between the two designs across most vehicles.
Some of the critical aspects of extractor design include pipe diameter and the quality of the collectors, bends and the head flange.
We're told the optimum diameter and length of extractor pipes can only be determined through testing; a general rule is that a too-large diameter pipe will reduce low rpm torque due to lower exhaust gas velocity.
Another key area is the collector cone - where two or more primary pipes merge into one. The aim here is to have the smoothest possible flow passage - Pacemaker achieves this with its patented pipe-over-cone technique.
Of course, smooth mandrel-formed bends are important in extractors to ensure maximum gas flow and the least turbulence- cheap press bent extractors should be avoided.
The head flange of a good extractor must be 8 to 10mm thick in order to prevent warping. The flange should also be laser cut to ensure accuracy, and each pipe collector should fit inside its laser-cut hole. Good quality extractors have the pipe entry matched to the shape of the exhaust port.
The amount of power gained from a quality set of aftermarket extractors varies from car to car. As a rough guide, you can pick up an average 10 percent extra power - though this can go up or down depending how 'bad' the OE manifold was. The final power tally also depends on how the car's ECU will react to modifications.
And cost? Well, for good quality extractors to suit the average in-line 4-cylinder you're looking at around AUS$300, or around AUS$550 for most V8s.
Fabricated Turbo Manifolds
A fabricated exhaust manifold is required when a naturally aspirated engine is being fitted with a turbocharger, or when a factory turbo engine is receiving a turbocharger much larger than the original.
A fabricated manifold can be configured to suit the size and shape of the new turbine housing. It allows flexibility of turbocharger positioning and, potentially, it reduces backpressure on the engine.
Fabricated exhaust manifolds are usually built from heavy-duty steam pipe or stainless steel pipe. Steam piping is available in preformed bends and various diameters and can be MIG or arc welded to form the manifold. Stainless steel manifolds can also be bought in preformed bends and different diameters, but must usually be TIG welded together.
At just over two-millimetres thick, stainless pipes are much thinner than four-millimetre thick steam tube. Note, though, neither steam pipe nor stainless steel fabricated manifolds are as durable as a factory cast iron item - they tend to crack.
More on that in a moment...
Fabricated turbo manifolds follow the same ideals as conventional extractors. Each cylinder should have its own runner for effective cylinder scavenging and it should use mandrel bends. As a rule, smaller diameter runners will aid turbine response (giving boost faster) but won't give as much airflow potential.
During fabrication, particular care must be paid to ensure to no large welding dags penetrate through to the inside of the pipe - these impede flow and have the potential to break off and damage the turbine.
Fabricating a custom tubular exhaust manifold is extremely labour intensive - depending on the layout, it may take nearly two days to piece together from start to finish. As a result, custom manifolds are not cheap to buy - expect the average steam pipe turbo manifold for an in-line four-cylinder to cost around $800, while a stainless steel variant is closer $1250. A six-cylinder turbo manifold costs around $1000 or $1400 depending whether steam pipe or stainless pipe is used.
As mentioned, fabricated manifolds are also more prone to cracking than the majority of cast iron variants. Depending on the quality of fabrication and whether any turbo bracing is being used, a custom manifold should last up to 100,000km; a lot also depends on air-fuel mixtures and boost levels.
In terms of power, replacing a factory exhaust manifold with a fabricated manifold is unlikely to be responsible for a dramatic power gain. One go-fast workshop told us they fabricated a manifold for a Nissan 200SX and its 'before' and 'after' chassis dyno runs revealed absolutely no gain. However, a larger fabricated manifold will often be necessary to support future modifications.
The upshot is custom turbo manifolds may not be a good investment unless you want to bolt on a monster turbo or you're conducting a turbo conversion on a naturally aspirated car.
High Flow Turbines
Much of the overall backpressure on a turbo engine comes from the turbine.
The engine backpressure posed by the turbocharger is considerably greater than the boost pressure that it generates. This is due to the efficiency loss across the turbine, centre core assembly and compressor.
To reduce this backpressure - and potentially improve power - a common approach is to fit a less restrictive turbine. This involves modifying or replacing the original turbine wheel and/or modifying or replacing the turbine housing.
In most instances, the factory turbine wheel can be back-cut, whereby the turbine blades are machined. This alteration in blade shape and length helps to ease backpressure but, as we've been told by Honeywell-Garrett's Geoff Watson, "back-cutting is not good for trying to make big changes."
Where larger changes are required, you'll probably need to change to a less restrictive turbine wheel and/or turbine housing. The turbine housing can be replaced as a unit or, in some cases, the internals of an existing housing can be machined.
The Trade-Off...
Fitting a less restrictive turbine can have a hugely negative effect on drivability.
Increasing the potential for gas flow through the turbine allows more peak power, but it comes at the expense of throttle response and low rpm torque. Sure, you may pick up kilowatts at the top-end, but you sacrifice a lot down low as well - keep this in mind, because a streetcar spends ninety-nine percent of the time at low-to-medium revs.
Wastegates
The wastegate's role is simple - to bypass excess exhaust gas that would otherwise over-speed the turbocharger and cause overboosting.
Most production turbo cars are equipped with a turbocharger incorporating an internal wastegate. These integral bypass passages are generally less than 20mm in diameter and, as a result, aren't capable of flowing a huge volume of exhaust gas. This can cause high-load backpressure on the engine and - even worse - lead to the possibility of an overboost on a modified engine.
Enlarging and smoothing the internal contours of the bypass passage is one method of improving the flow through an internal wastegate. Expect only a small increase in flow, however - due to the relatively thin casting, the diameter of the bypass cannot be greatly enlarged.
In instances where maximum wastegate flow is required, an external wastegate must be fitted.
An external wastegate serves the same function as an internal wastegate, except it's a separate assembly with bypass pipes typically between 32 and 52mm in diameter (ie much larger than any internal wastegate). The fitment of an external wastegate requires a large diameter pipe to be tee'd into the manifold prior to the turbine housing. This usually requires the fabrication of an all-new custom exhaust manifold.
The traditional application for an external wastegate is on monster horsepower engine, where a huge volume of exhaust gas must be wasted to prevent overboost.
Another application - one that's rarely used - is to team an external wastegate with a 'mismatched' turbo that has a high-flow compressor but a restrictive turbine. The idea behind this is simple - the small, restrictive turbine can be spooled up rapidly to give boost pressure from low rpm. Once the maximum boost pressure has been reached - which will be quite early in the rev range - the big wastegate starts to open, bypassing the large volume of gas that would otherwise overwhelm the small turbine.
The result is rapid boost response with little high-load backpressure.
The standard 1995 Audi S2 and S4 appear to follow this concept; these engines have extremely small turbines (helping to produce peak torque at less than 2000 rpm) combined with large external wastegates. We can't think of any other reason why Audi would opt for an expensive and heavy external wastegate over an internal wastegate - internal wastegates are used on many factory cars that produce more than the 169kW of these engines.
Food for thought...
Heat Insulation
Heat insulation - such as ceramic coatings and thermo-wraps - has recently become very popular.
When applied to extractor pipes or exhaust manifolds, heat insulation is said to offer significantly reduced engine bay temperature and improved power. Furthermore, ceramic coatings are said to improve the lifespan of the core metal, though thermo-wraps reputedly accelerate pipe corrosion. Also, unlike paint, a ceramic coating won't flake or discolour.
The theory behind the claimed power increase is centred on the fact that exhaust gasses are kept hotter. This larger volume of the hotter gases increases exhaust velocity that leads to improved combustion chamber scavenging.
In reality, you can expect the power gain derived from the hotter exhaust gasses to be minimal, but the associated reduction of underbonnet temperature can lead to a power increase of its own - especially when induction air is being drawn from the engine bay.
While ceramic heat insulation is most commonly applied to extractors, it can also be applied to turbo manifolds, turbine housings and exhaust piping. An example is Mazda's Australian-developed MX-5 Turbo, which uses a ceramic coating on its manifold, turbine housing and dump pipe. The coating is used in place of conventional heat shields.
The cost of ceramic coating varies, depending on the amount of surface preparation required and the complexity of the component, but - as a guide - a set of 6-cylinder extractors costs around AUS$230 and a cast iron 4-cylinder turbo manifold costs approximately AUS$200. These prices are to have the ceramic coating applied to both the inner and outer pipe walls.
In contrast, thermo-wraps - such as Thermo-Tec - can be bought in rolls 2-inches wide and 50-feet long for somewhere around AUS$100.
