EXHAUST SYSTEM

DESIGNING AND BUILDING AN EXHAUST SYSTEM

by "Bad Ass" Bre

A performance muffler

The main purpose of an exhaust system is undoubtedly to route the spent exhaust gas out of the car's engine. Along the way the exhaust gasses may be used to drive a turbocharger and now-a-days it will most definitely incorporate a catalyst converter to reduce carbon dioxide emissions. But on a high performance car, such as a modified street car, or a modified race car, the exhaust system is much more important as it has a direct affect on engine performance and engine power. As a result, the exhaust system, and particularly the exhaust header design, plays an important part in both engine tuning and car tuning.

In general terms, an exhaust system consists of an exhaust manifold (which is sometimes called an exhaust header), a front pipe, a catalyst converter, a main muffler or silencer, and a tail pipe with an exhaust tip. In terms of tuning the exhaust system, the muffler is the easiest to deal with it's simply a matter of replacing the stock muffler with a free-flow or high performance muffler, such as a Flowmaster muffler. The result is a free flow exhaust system. However, the performance muffler must have an inlet and an outlet pipe that is the same size (diameter) as your front pipe and your tail pipe. Your front pipe and your tail pipe should also have the same diameter. The rest of the exhaust system is much more complicated as you need consider back pressure, your engine's power band, and your engine's maximum usable RPM.

BACK PRESSURE

The amount of back pressure produced by the exhaust system is crucial as too much back pressure will have a negative effect on your engine's top-end performance as it will restrict the flow rate of the exhaust gasses at high RPM. The result would be the engine not being able to expel the spent exhaust gasses fast enough to prevent spent exhaust gasses from contaminating the fresh air/fuel mixture that is drawn into the engine on the next intake stroke. Ultimately, this will result in reduced engine power! Therefore, attaching a little 1-inch pea-shooter to your engine instead of an exhaust system is not such a good idea! But then neither is fitting a 10-inch sewage pipe. If the exhaust pipe is too large, you will get reduced flow velocity of the exhaust gasses. The flow velocity of the exhaust gasses assists with the scavenging of the spent exhaust gasses as well as the amount of air/fuel mixture that can be drawn into the combustion chamber on the next intake stroke. This is because the flow velocity of the exhaust creates a low pressure immediately behind it that sucks more gasses out of the combustion chamber. The trick is thus to get the back pressure just right.

BASIC DESIGN

Our exhaust header design page will have more specific information, but generally speaking, a 2¼ inch exhaust pipe is ideal for an exhaust system for a 4-cylinder street car, but a 2½ inch exhaust pipe is a better fit for a 6-cylinder street car. However, a 2000cc 4-cylinder modified race car would do much better with a 3-inch exhaust pipe! The size of the exhaust header primary pipes is also important as it influences both back pressure and flow velocity; while the length of the primary pipes affect the power band of your engine. The size and length of the primary pipes of the exhaust manifold, as well as your exhaust header design depends on your engine's displacement and maximum usable RPM, as well as the power band you want from the engine.

In our next section we take a closer look at ideal primary pipe length and diameter of the exhaust manifold, at the exhaust header design and at anti-reversion techniques.

THE EXHAUST HEADER

As we've mentioned in our introduction to building exhaust systems, the exhaust manifold design, or exhaust header design has a major affect on engine performance, and plays an important role in car tuning. In terms of exhaust header design, both the primary pipe diameter and primary pipe length will determine the engine's overall power band as well as its peak power point. But, before you begin with your exhaust header design, you need to take into account the number of cylinders, the engine capacity, and the maximum usable RPM as these will all influence your design.

A 4 into 2 performance exhaust header

NATURALLY ASPIRATED STREET CARS

The primary pipe diameter and the primary pipe length of exhaust manifold are the two important factors in the exhaust header design. To a large extent, engine capacity, the number of cylinders and the maximum usable RPM will influence primary pipe diameter and primary pipe length. Thus, a 1600cc 4-cylinder or a 2400cc 6-cylinder naturally aspirated modified street car with a maximum usable RPM of 5,500 RPM, should have an exhaust manifold with a primary pipe diameter of about 1½ inch and a primary pipe length of 34-36 inches, while a 2000cc 4-cylinder naturally aspirated modified race car should have an exhaust manifold with a primary pipe diameter of about 1¾ inch and a primary pipe length of about 32 inches that feeds into a 2½ inch collector. Ideally, the primary pipe lengths should have the same lengths but can be within 2 inches of each other. On a four-cylinder engine, the and all four primary pipes should join together in a single collector before feeding into the front pipe of the exhaust system, while on a six-cylinder engine, the primary pipes from cylinders 1, 2, and 3 should join into one collector and the primary pipes from cylinders 4, 5, and 6 should join into another collector. A Y-pipe could then be used to join the two collectors before feeding into the front pipe.

ALL ROUND RACE PERFORMANCE

The best exhaust header design for all round race performance would have 1⅝ inch primary pipes that are 32 inches long. These dimensions provide the best power curve over the widest RPM range and would be ideal for rally cars. An exhaust manifold with longer primary pipes would provide better top-end power but with less pulling power, while an exhaust manifold with shorter primary pipes would provide better low-end torque. On a turbocharged engine, an exhaust manifold with short primary pipes will help with acceleration until boost pressure is reached and the turbocharger spools up. Thus, you should have the exhaust manifold tuned to your specific needs during the design stage.

ANTI-REVERSION HEADERS

In our page on cylinder head porting, Henry (aka Double H) explains that the primary pipes in the exhaust manifold should at match the exhaust port diameter on the cylinder head; but to reduce reversion, a primary pipe that is slightly larger than the exhaust port is better. Reversion is the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. As we mentioned in engine tuning basics, the exhaust valves are still open when the intake stroke begins. This presents the potential for exhaust gasses to be drawn back into the combustion chamber when the piston moves down the cylinder. Any exhaust gases that are drawn into the combustion chamber will displace the air/fuel mixture being drawn in through the intakes valves and will increase the temperature in the combustion chamber, thus reducing the volumetric efficiency of the engine, as well as engine power. Preventing reversion will reduce the contamination of the air/fuel mixture by the spent exhaust gasses and will improve the efficiency of the engine. An anti-reversion header or AR header that is specifically designed to inhibit reversion would be your best choice. Anti-reversion headers have a built-in lip that restricts exhaust gas flow back into the combustion chamber.

Ultimately, determining the correct primary pipe diameter and primary length that provides the best engine characteristics and performance will require that you have your car dyno-tuned.

In our next section we'll look specifically at turbo exhaust header design.

TURBO EXHAUST SYSTEMS

The same rules regarding the exhaust header design that apply to naturally aspirated engines also apply to turbocharged engines but with a few rather significant differences. If you haven't yet read our introduction to performance exhaust systems and our guide to effective exhaust header design, then do so before reading this section as this section builds on the information in our previous pages on exhaust systems.

TURBO EXHAUST HEADERS

A log-type exhaust header

In our guide to exhaust header design, we did not mention log-type headers as these headers are always less effective than exhaust headers with equal length primary pipes that joint together in a collector. However, on a turbocharged engine, you may not have enough space for an equal length header and a turbocharger. This space limitation would necessitate the use of a log-type header. In addition, the primary pipes of the exhaust manifold must come together at the collector before it feeds into the turbocharger and the size of the collector will be determined by the size of the turbocharger's turbine inlet.

In some cases you may even need to retain the stock cast-iron exhaust manifold. If this is the case, you should examine the stock exhaust manifold closely for imperfections that could restrict exhaust gas flow, much the same as you would do when porting the cylinder head. The aim would be to make the internal surface of the exhaust manifold as smooth as possible while keeping the shape and size of the primary pipes as uniform as possible, without weakening the manifold. By smoothing down the internal surface, you would not only improve exhaust gas flow, which would be crucial to reducing turbo lag, but you'd also reduce carbon build-up. Remember, however, that widening the primary pipes would reduce exhaust gas velocity and would result in a thinner manifold wall, both of which would have a negative effect on turbo lag! A thinner manifold wall would have greater exhaust heat loss, which would mean a reduction in the heat energy that is used to drive the turbine.

An exhaust manifold for a turbo engine.

When designing your own turbo exhaust header, you need to ensure that your header is strong enough to support the weight of the turbocharger, and that is can withstand the heat buildup caused by the turbocharger. This means that you have a choice of two materials when designing the exhaust header: steam pipe and bends, or stainless steel tubing. Stainless steel tubing may be easier to bend and shape, and would require less welding and grinding but you should ensure that the bends are formed in a mandrel bender that does not deform the inner radius of the bends.

INTEGRATING THE WASTEGATE

A major difference in the design of the exhaust manifold for a turbo exhaust system is the integration of the turbocharger's wastegate. As we've mentioned in our section on turbocharger boost control, the wastegate is used to control boost pressure created by the turbocharger, and to prevent it from creating too much boost pressure. For this reason, the wastegate should be integrated into the exhaust manifold in such a way that it is exposed to as much of the pressure in the exhaust manifold as possible. This means that the wastegate should be located either after the collector where all the primary pipes join together, or after the last exhaust port on a log-type manifold. The wastegate should also be located at an angle that neither restricts nor interferes with exhaust gas flow as efficient exhaust gas flow is required to reduce turbo lag. In other words, the exhaust gas must be able to flow to the wastegate so that the wastegate can experience the correct exhaust pressure in the system without interrupting the exhaust gas flow.

THE TAILPIPE

Determining Exhaust Diameter based on Power Output.

There are also a few important aspects of a turbo engine that you must take into account with regards to your tail pipe. Firstly, the turbo increases the amount of air/fuel mixture that is fed into the combustion chamber and consequently increases the amount of exhaust gas that must be expelled from the engine. Secondly, the exhaust gasses of the turbo engine are much higher than a naturally aspirated engine; therefore the exhaust on a turbo engine will be more prone to heat expansion. The flange that is attached to the turbine outlet can experience temperatures of up to 1500°F! For this reason the flange should be beefed up and a minimum flange thickness off a ½ inch with additional bracing is recommended. The rest of the exhaust system needs to make allowance for heat expansion and should incorporate swaged joints.

The size of the tailpipe or diameter is also complicated by the size of your turbo and the boost you are running. Some tuners recommend a tail pipe that is 10% larger than the turbine outlet. This takes turbo size into account but not boost pressure! I personally prefer basing my tail pipe size on the amount of power produced by the engine. The diagram on the right is a good starting point for selecting a tail pipe diameter for your turbocharged engine. As with naturally aspirated cars, arriving at the ideal tail pipe diameter, as well as the ideal primary pipe diameter and length, will require some time on the dyno-tuner.

EXHAUST WRAP

Exhaust header wrap

When discussing the air intake system, Double H mentions that the air in the engine compartment can be 30 degrees higher than ambient air temperature for a normally aspirated car and 50 degrees higher for a turbocharged car! And higher air temperature is a big performance killer as it is less dense and contains less air molecules than cooler air. Most of this increase in temperature is cause by the exhaust system.

There are three things you can do to reduce the heat soak from the exhaust system:

• Wrap the exhaust;
• Powder coat the exhaust; or
• Get cold air induction.

Double H discusses cold air induction systems in our section on air intake systems so I won't be repeating that here, but I will look at wrapping the exhaust and powder coating the exhaust. That's not to say you can't powder coat and wrap the exhaust!

POWDER COATING

Powder coating is also referred to as ceramic coating was developed to protect spacecrafts on re-entry into the earth's atmosphere. It is a ceramic compound that can be applied to most surfaces that must withstand high temperatures of up to 2,000°F and can be applied to piston tops, combustion chambers and valve faces! The reason for this is because ceramic is a very poor conductor of heat. However, applying the powder coating may be a bit of a problem as you need to cure the ceramic coating in a powder coating oven at 500 to 700°F for the ceramic compound to bond with the metal surface. If you do have the required powder coating equipment, you can do this yourself.

Once applied and cured, the exhaust will radiate up to 40% less heat! Unfortunately, that's not as good as applying an exhaust wrap, though it does look much neater!

EXHAUST WRAP

Exhaust wrap like bandage and is much easier to apply than powder coating. However, if you soak the exhaust wrap in water, squeeze out the excess water, and apply the wrap while it's still wet, you will be able to apply the wrap tighter and neater. You will need a few metal ties to hold the wrap in position and it is easier to start at the port end of the header. When you apply the wrap, make sure that the overlap is constantly half the width of the exhaust wrap and be careful not to have too many layers at the collectors. You also need enough wrap to cover the down pipe but the rest of the exhaust doesn't need to be wrapped.

Once you applied the wrap to the header and down pipe, leave it in the sun to dry; then spray the wrap with silicone based spray to protect it from moisture and oil. Once that's done, you can expect about 60% less heat soak in the engine compartment!

DISADVANTAGES

Some people claim that coating or wrapping the exhaust will not affect the exhaust itself. That's not entirely true. Powder coating or wrapping the exhaust will reduce its longevity but the exhaust header should still last at least 30 years! And this applies to both powder coating and wrapping! But ultimately, neither powder coating nor wrapping the exhaust beats a good cold air induction system at keeping the temperature of the intake air down!

(Credit to Custom Car US)

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