The main purpose of an exhaust system is undoubtedly to route the bunt air/fuel mixture out of the car's engine. Along the way it 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 does much more than that as it also affects engine performance and engine tuning!
An exhaust system generally consists of an exhaust manifold (which is also called an exhaust header), a front pipe, a catalyst converter, a main muffler or silencer, and a tail pipe. Of these items, the muffler is the easiest to deal with — simply replace the stock muffler with a performance muffler, such as a Flowmaster muffler, to create a free flow exhaust system. However, the performance muffler must have an inlet and an outlet that is the same size (diameter) as your front pipe and your tail pipe. Your front pipe and your tail pipe should be the same size. The rest of the exhaust system is complicated by questions of back pressure, your engine's power band, and your engine's maximum usable RPM.
BACK PRESSURE
Back pressure is an important consideration because too much back pressure will adversely affect top-end engine performance as it will restrict the flow rate of the exhaust gasses at high RPM. The car's engine will not be able to expel the burnt air/fuel mixture at the required rate. The burnt air/fuel mixture remaining in the cylinder at the next intake stroke will contaminate the fresh air/fuel mixture and will rob the engine of power. Thus, fitting a 1 inch pea-shooter to your engine in place of the exhaust pipe is not 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 exhaust fumes 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.
BASIC DESIGN
Generally speaking, when designing an exhaust system for a 4-cylinder engine, a 2¼ inch exhaust pipe is ideal but for a 6-cylinder engine, a 2½ inch pipe is ideal, though a 2000cc 4-cylinder race engine could do with a 3 inch exhaust pipe. The size of the exhaust header primary pipes of also influences 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 and your exhaust header design depends on your engine's power band, displacement and maximum usable RPM.
The Exhaust Header
As I've mentioned in our introduction to exhaust systems, the exhaust manifold design, or exhaust header design can have a major affect on engine performance. The primary pipe diameter and primary pipe length of the exhaust header has a significant affect on the engine's power band and peak power. When design the exhaust header, you need to take into account the number of cylinders, the engine capacity, and the maximum usable RPM.
NORMALLY ASPIRATED STREET CAR
When designing the exhaust header, remember that a 1600cc 4-cylinder or 2400cc 6-cylinder normally aspirated street racer with a maximum usable RPM of 5,500 should have a header with a primary pipe diameter of about 1½ inch and a primary pipe length of 34-36 inches, while a 2000cc 4-cylinder normally aspirated race engine should have a header 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. The primary pipe lengths should be within 2 inches of each other and all four primary pipes on a 4-cylinder should join together in a single collector before feeding into the exhaust pipe. A 6-cylinder engine should have two collectors with cylinders 1, 2, and 3 joining into one collector and cylinders 4, 5, and 6 joining into the other collector. A Y-pipe could then be used to join the two collectors before feeding into the exhaust pipe.
ALL ROUND RACE PERFORMANCE
For all round race performance, a header with 1⅝ inch primaries that are 32 inches in length usually provides the best power curve over the widest RPM range. Shorter primary pipes provide better low-end torque while longer primary pipes provide better top-end power but at the expense of acceleration. On a turbo engine, a header with short primary pipes will help with acceleration until boost pressure is reached and the turbo kicks in.
ANTI-REVERSION
Each primary pipe should at least match the exhaust port diameter or should be slightly larger. A primary pipe that is slightly larger than the exhaust port is better as it inhibits reversion, which is the flow of exhaust gasses back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. The exhaust valve is still open when the intake stroke begins. Preventing reversion will reduce the contamination of the air/fuel mixture by exhaust fumes. An anti-reversion (AR) header that is designed to inhibit reversion would be your best choice. AR headers have a built-in lip that restrict exhaust gas flow back into the cylinder.
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.
Turbo Exhaust Systems
The same rules regarding the exhaust header design that apply to normally aspirated engines also apply to turbo engines but with a few rather significant twists.
An exhaust header with equal length primary pipes that joint together in a collector is always better than a log-type header in which short primary pipes branch into a thicker log pipe. However, on a turbo engine, space limitations may necessitate the use of a log-type header. In addition, the primary pipes of the header will be determined by the size of the turbine inlet.
A major twist in the header design of a turbo exhaust system is the integration of the wastegate. The wastegate is used to control boost pressure and to prevent over boosting. For this reason, the wastegate should be integrated into the header so that it is exposed to as much of the pressure in the exhaust as possible. This means that the wastegate should be located at or after the collector where all the primary pipes join together, or after the last exhaust port on a log-type header. Also, the wastegate should be located at an angle that does not restrict exhaust gas flow. The exhaust gas must be able to flow to the wastegate so that the wastegate can experience the correct exhaust pressure in the system.
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 normally 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 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 bhp produced by the engine. As with normally 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.
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