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Inlet and Exhaust - Exhaust System

 - The main job of the exhaust is to expel all burnt gas from the cylinders.
 - It must also help to draw fresh mixture into the combustion chamber.
 - It must also drive the turbine on turbo cars.

The main job of the exhaust is to rid the cylinders of exhaust gas and encourage inflow of air/fuel to fill the cylinders. Any exhaust gas left behind takes up cylinder volume which prevents max amount of fresh air/fuel in to produce max power. This also means that heat is transferred to the inlet charge and charge density is reduced. During the combustion phase, stale exhaust gas can block flame propagation causing further power loss as well as increasing exhaust emissions of unburnt hydrocarbons.

Exhaust Turbo Performance.
 - The maximum exhaust energy should be transferred to the turbine.

In the case of turbo engines, there is the additional concern of spinning the turbine. The free flow of exhaust gas is essential to initially spin the turbine and to keep it spinning at top boost. Energy expended overcoming flow restrictions means less energy to spin the turbine. Because of this every effort must be made to transfer the max exhaust energy to the turbo, over a wide band of engine operating conditions.

Any efficiency decrease here will increase turbo lag. Any improvements will profoundly influence the shape of the power curve and the feel of the engine. When dealing with factory turbo car its in this area that the most gains can initially be made. You stand to gain more horsepower per euro than in any other area of the car, without compromising engine reliability.

The Exhaust Manifold.
 - Maximise flow by ensuring internal surfaces are smooth, incl gaskets.

 - Machine away slag and extrude hone hard to reach areas.

Care must be taken not to weaken the stock manifold, but bearing this in mind there is still a lot that can be done to improve flow. First make sure the internal surfaces are nice and smooth. This encourages flow and discourages carbon buildup. Casting slag should be removed as should any sharp edges or joins. The manifold should match the exhaust ports and the manifold gasget should be trimmed back as much as possible. The manifold to turbo joint should also be kept smooth. Areas that cannot be reached with a grinder should be extrude honed. But be careful not to overly increase the internal size of the runners, because this will result in lower velocity exhaust gas hitting the turbine.

The outside of the manifold should also be smoothed out and any jagged parts left behind due to the casting process should be removed and all sharp points should be smoothed over, such as joins etc. this ensures that the constant heating and cooling of the manifold does not cause cracking due to local temperature variations.

If each runner is joined due to the way the manifold was cast, then they should be cut with a hacksaw so that they are not in contact. You might think that they are joined to increase strength but they are only joined like this because its easier to cast them this way at the factory. The joining of the runners actually puts more stress on the manifold due to expansion and contraction and cutting them apart drastically reduces this stress and it also reduces the chances of a leaking or blown manifold gasket.

Gasket Selection.
 - Use good sealing insulator or conductor gaskets where appropriate.

The gasket is used to seal the joints to prevent exhaust gas leakage. However, it does have a secondary role which is very important and often overlooked. The gasket acts a heat insulator or as a heat conductor depending on its material and thickness. For some joins the gasket should act as a conductor and so copper can be used as the gasket material. Areas such as the exhaust to turbo inlet operate at similar temperatures. The join between the cylinder head and the exhaust manifold is a different matter. We don’t want the exhaust to lose heat energy by heating the cylinder head and we don’t want the cylinder head to be heated by the exhaust making cooling more difficult. Therefore, this gasket should be a good insulator while still giving a good seal against exhaust gas. A metal reinforced gasket with fibre composite an both sides will insulate well but won’t last indefinitely. A fiber thermal barrier sandwiched between two light plates made of steel or copper may last longer but doesn’t insulate as well. A similar gasket should be used on the turbo exit to minimise heat loss from the turbo to the dump pipe and to keep the dump pipe cool. The external wastegate should also use an insulating gasket on the entry side to insulate the diaphragm from heat. Use a conducting gasket on the exit side of the external wastegate to allow heat to be drawn away from the wastegate.

Tube Manifolds.
 - Tuned length pipes can be used to maximise exhaust gas scavenging.
 - Steam piping is strong but heavy.
 - Stainless steel is strong and light, but turbos will need additional bracing.

For the ultimate in exhaust manifolds we have to ditch the cast manifold and fabricate a multi branch manifold using tuned length pipes. When turbos are located close to the head, then steam piping can be used to go from the head to the turbo. It is very strong, bends are readily available, it is will support the weight of a turbo in the same way that a cast item will and it has good heat retention qualities because of the thickness of the walls. The downside is that it is heavy. Alternatively, stainless steel can be used and when the turbo is located further away from the head, then steam piping is not really an option, so stainless steel or something more exotic must be used. Stainless steel is light and strong and still retains good heat resistance because it is a poor conductor. Also, a ceramic costing can be applied to the inside surface to enhance its heat resistance. All bends should be formed in a mandrel bender. Welding bends together should be avoided because it promotes cracking during its service life and slag invariable falls to the inside of the pipe causing flow blockage. The plates at the cylinder head and turbo mounting faces should be at least 10mm thick. For heavy turbos, a support system must be employed to carry the turbo. Supporting it by the manifold alone wont suffice. Hanging the turbo from a spherical bearing in the top of a tripod, which is solidly fixed to the engine is the usual method used….  The turbo cannot be rigidly fixed to the engine… it must be able to move as the exhaust expands and contracts.

Manifold Dimensions.
 - Choose the minimum pipe size that will give the flow needed.

Anything that adds weight, complexity should be avoided. If you are free to choose any size turbo and any boost level you like, then a pipe length of 20in should be used. The diameter of the piping will be dictated by the size of the engine and whether you want top end power or low and mid range power. Its always best to choose a diameter as small as possible to keep exhaust velocities high and therefore, keep spool-up times low and responsiveness high. Only when all out top end power is important should bigger diameter pipes be used. The following diameters can be used as a guide:

4cyl single turbo:
1300cc – 1.24 to 1.5in
1600cc – 1.25 to 1.625in
2000cc – 1.375 to 1.75in
2200cc – 1.375 to 1.75in
2400cc – 1.5 to 1.875in

6cyl single turbo
2000cc – 1.125 to 1.5in
2300cc – 1.25 to 1.5in
2500cc – 1.25 to 1.625in

6cyl twin turbo
2600cc – 1.25 to 1.625in
3000cc – 1.375 to 1.75in
3500cc – 1.5 to 1.75in
4000cc – 1.625 to 1.75in

8cyl twin turbo
4000cc – 1.375 to 1.62in
5000cc – 1.5 to 1.625in
5700cc – 1.5 to 1.75in
7000cc – 1.75 to 2.125in
8200cc – 1.875 to 2.375in

In 4cyl engines it is normal practice to join all four pipes at the turbo inlet. But if space doesn’t easily permit this then a 4 to 2 to 1 optin can be employed. If so then its better to keep the primary pipes 12 to 16in long. Two primaries from cylinders one and four should be joined( same for cylinders two and three). Then both secondaries can be joined into the turbo inlet.

Connecting an External Wastegate.
 - It should tee off at a gradual angle and maximise flow.

It is important to tee off the wastegate so that it doesn’t overly affect the smooth exhaust flow into the turbo. At the same time the design should maximise flow through the wastegate when it is opened.

The best place to locate the wastegate is after all the exhaust tubes have come together at the turbo, or even on the turbine inlet itself. It should not tee off at an acute angle, otherwise flow will be turbulent.

External Wastegate Dump Pipe.
 - If emissions and  noiseregulations allow, keep the dump pipe separate.

The wastegate pipe should remain separate and not rejoin the main exhaust. Where the pipe must rejoin the main exhaust it should do so as far away from the turbo as possible because there is always turbulence at the join site. Sometimes the pipe must be rejoined before the cat to satisfy emissions laws. If this is the case then it should join just before the cat and if permitted the cat should be moved further back along the exhaust system (even on internal wastegate designs, it is a good idea to move the cat back as far as possible).

Internal Wastegate Dump Pipe.
Internal wastegates are used to minimise cost/weight and because they are more compact and are often built into the turbo main casing. However, it is much more difficult to achieve non-turbulent flow. Sometimes there is nothing that can be done to minimise this turbulent flow. But if the exit of the wastegate port runs parallel with the extruder bore then some simple dump pipe designs will give good minimal turbulent flow.

Exhaust Pipe Size.
 - Bigger diameter bores are not always better.

There is no easy way to figure out what the ideal exhaust size should be. The only foolproof way is to fit several exhaust sizes and test them on the dyno. But some good guidelines can narrow down the choices. Sometimes good results can be got from using pipes no larger than 15% of the extruder bore. Because of the big weights of big diameter exhausts and big diameter silencers, bore sizes should be kept as small as possible while still allowing maximum flow. A good guide, based on the experience of tuners is as follows:

1300-1600cc – 2.25 to 2.5in
2000-2300cc – 2.75 to 3.0in
2500-3000cc – 3.5 or 2x2.5in
3500-4000cc – 4.0 or 2x2.75in
5000-5700cc – 2x3.0in

These figures are based on engines developing 120-150bhp per litre. Rally engines developing 200-250bhp per liter need pipes that are 0.5 to 1.0in bigger.   The tailpipe can be reduced by 1/4in without suffering penalty.

But testing is the only way to go. On group A two litre turbo engines, a 3in pipe will give excellent power but a pipe 3.5in will cause a power drop and at 4in the power comes back. So testing is the only reliable method for choosing max power setups.   With turbos, the biggest flow restriction is at the turbine wheel and housing. The use of a bigger housing, without suffering further turbo lag will give better gains than exhaust change. Then next most restrictive area is at the manifold between the turbo and the head, so improvements here will also give good gains.

Problems with Exhaust Changes.
 - On turbo cars the boost can change without changing the boost control.

Exhaust changes can make it more difficult to control boost. The exhaust flow can be so great that the internal wastegate is too small to bleed off sufficient exhaust gas to keep boost down to the desired level. On the GTR the boost level usually changes from 0.8bar to 1.1bar when changing the exhaust and induction system, without employing any boost control setup. This is OK on the GTR because it can easily cope with the additional boost. But other cars, such as the Subaru WRX could experience reliability problems or even increased detonation because of uncontrollable boost rises without suitable measures such as remaps to protect the engine.

Blower and Nitrous Exhausts.
 - Tuned exhausts can induce a vacuum which will suck out the gases.

Blower and nitrous engines are completely different from turbo engines and behave more like NA engines than turbo engines when in comes to the exhaust. The exhaust responds more readily to conventional exhaust tuning techniques using free flow or acoustic wave tuning principles. We want to make use of wave movement to create a vacuum which will pull exhaust gas out of the cylinder. This translates into more powerful and efficient engines. When the engine has very good exhaust porting and suitable cam timing, proper exhaust tuning will exert a vacuum on the cylinders and they will begin to draw in fresh inlet charge even thought the cylinder is still rising at the completion of the exhaust stroke. This extends the time available for the cylinders to completely fill with air/fuel mixture, leading to higher power outputs. We can therefore, achieve our target horsepower figure without advancing the cam timing as much. This gives a more flexible engine with a broad power range.

Pumping Losses.
 - This refers to the power consumed in pumping the exhaust gas out of the engine.

Power is consumed because exhaust gases have to be pumped out of the engine. As soon as the crank rotates past BDC, the piston starts its way up the cylinder barrel on the exhaust stroke. During the exhaust stroke it must ram the exhaust gas out past the exhaust valve and through the exhaust system. This consumes power provided by another cylinder on its power stroke. This means that there is less power available at the flywheel. There are methods available to cut these pumping losses to a minimum. One way is to open the exhaust valve very early. This provides a net gain in the upper half of the powerband but causes poor fuel economy at cruise. A better and more considered route is to ensure that the entire exhaust flow route is free of obstacles and incorporates any elements that promote positive flow.

Exhaust Pressure Waves.
 - The cylinder outlets should be mated to maximise wave resonance tuning.

As mentioned above, wave resonance tuning  is the primary means of increasing flow. This means connecting cylinders in a particular order using exhaust tubing of a specific length to take advantage of exhaust pressure waves to pull exhaust gas out of the cylinders and during valve overlap, suck fresh air/fuel into the cylinders. When we look at the firing order of the engine, we join cylinders so that exhaust gas from one cylinder won’t cancel the pressure wave from another cylinder.

Exhaust Header Design.
 - Design the headers to take advantage of the firing order.

 
A typical 4cyl engine has a firing order or 1-3-4-2. Number one cylinder will tend to draw exhaust gas from number three cylinder if all four cylinders are connected together. Number three will draw from number four which in turn draws from number two and so on. To overcome this, a tube type header with branches around 10in long should be used The design can be a four into one or a four into two into one. For top end power use the four into one design and allow plenty of dyno time to get the design right.

Baffle Collector Design.
 - Two popular collector designs… baffle and merge collectors.
 - Baffle collectors produce sharper waves to aid suction from the cylinders.
 - More suitable for lower powered engines.

Baffle collectors are used to join two into one. The primary tubes terminate abruptly into an open tapering chamber that feeds into the tailpipe. When the primaries terminate like this, a pressure wave moves up one primary as a result of the gas being expelled from the other primary. If this wave arrives back at the exhaust port at the right time it aids in the suction of exhaust gas out of the cylinder and also sucks fresh mix into the cylinder. Depending on primary pipe length, an opposite pressure wave will arrive just as the exhaust valve is about to close to reduce the amount of fuel/air that overspills into the exhaust. This is the most positive aspect of using the Baffle Collector as a primary to secondary joining method. The downside is that at certain rpms the waves arrive at the wrong times and cause the opposite ot happen. Ie impeding exhaust flow out of the head. Therefore, this type of collector is best suited to road engines and mildly tuned race engines.

Merge Collector Design.
 - More graduated join.
 - Produces a lower intensity wave.
 - Engine relies more on exhaust gas momentum to expel the gases.
 - Better for high power engines.

Merge collectors are better for heavily modded engines with long duration, big overlap cams.

The primary pipes do not terminate abruptly but instead merge into the tailpipe. This reduces the intensity of the pressure waves in the primaries which tunes out the negative aspects of baffle waves.

Of course they don’t get the maximum effect of the good feedback waveseither.
However, on max power with high end engines, the tuner can rely more on exhaust gas momentum or inertia to rid the cylinders of exhaust gas. With this design, the primaries must join at a taper of about 10deg. Some engines respond better to 12deg and others prefer 8.5 to 9deg.

Tailpipe Design.
-Can use a tailpipe or use CATs, baffles and silencers.

After the header, we have to consider the rest of the exhaust system. On all out competition cars this can mean just a tail pipe. On road cars to usually means a cat and one or more silencers. Less tailpipe length will increase top end power and longer tail pipes will yield more low and mid range power. This is why some engines will have a tailpipe not much longer than the collector. However, most tailpipes go right to the back of the car. Tailpipe diameter is determined on the dyno and if a cat is used the diameter after the cat needs to be larger. A guide to blown and nitrous engines is:

80 to 120bhp – 1.875in
110 to 140bhp – 2in
130 to 150bhp – 2.125in
140 to 185bhp – 2.25in
180 to 220bhp – 2.5in
210 to 265bhp – 2.75in
250 to 320bhp – 3.0in
280 to 360bhp – 3.5in
400 to 500bhp – 4.0in
480 to 630bhp – 4.5in
580 to 750bhp – 5in  

As always the entire system must have a minimum of bending and adequate bore size. When ground clearance is a problem then oval exhaust bores can be used with no penalty in flow. You can reduce the diameter by ¼ in at the end (beyond the real wheels) without incurring any flow restrictions.

Silencer Design.
 - Straight through or reverse flow silencers are best.

A well designed silencer, placed to the rear of the car won’t drop power by anymore that 3-5%. The closer to the engine they are fitted, the more flow restricting occurs and the more power is lost. Straight through or reverse flow silencers are the best for power applications. Straight through provides the best sound deadening and are good for turbo cars. However, on NA cars two silencers or a silencer and a resonator must be used to stop popping occurring on overrun. The quietest silencers have an open resonating chamber in the middle. Reverse flow silencers are different and don’t contain any sound deadening materials. This makes them lighter, but poorly designed ones are loud and cause power loss. The main advantage is that they are good at suppressing popping when you lift off the throttle.

CAT Problems.
 - Don’t use CATs if possible.
 - Otherwise use performance CATs with twice the flow of regular CATs.

CATS that melt due to excessive temperatures caused by combustion in the exhaust will cause a problem by blocking the exhaust flow. However, even healthy CATs can cause problems. A poorly designed CAT can impede exhaust flow due to turbulence caused by the gas having to enter the honeycomb at 30deg angles or more and having to exit the honeycomb at the same angle. To overcome this a performance cat must have a gentle entry and exit taper of about 10deg. A performance cat will have double the flow rate of a regular cat.