Free Flow Exhaust System

Updated : 01-07-2017 Published : by :
Mechanical Engineering Seminars

An exhaust system is usually piping used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes...

An exhaust system is usually piping used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system design, the exhaust gas may flow through one or more of:

  • Header

  • A turbocharger to increase engine power.

  • A catalytic converter to reduce air pollution.

  • A muffler (North America) / silencer (Europe), to reduce noise.


The Header is an assembly designed to collect the exhaust gas from two or more cylinders into one pipe. Manifolds are often made of cast iron in stock production cars, and may have material-saving design features such as to use the least metal, to occupy the least space necessary, or have the lowest production cost.

In order to explain the effect of exhaust tuning on performance, let’s take a quick look at the 4-stroke engine cycle. The first step in the 4-stroke process is the intake stroke. With the intake valve open, the piston travels down the cylinder pulling a fresh air and fuel mixture into the cylinder (intake stroke). When the piston nears bottom dead center, the intake valve closes and the piston travels up the cylinder compressing the air/fuel charge (compression stroke). With the piston at the top of the stroke, the spark plug fires and ignites the compressed mixture causing essentially a closed explosion. The pressure of the ignited fuel pushes the piston down the cylinder transferring power to the piston, rod and finally the crankshaft (power stroke). After bottom dead center, the exhaust valve opens and the piston is pushed up the cylinder forcing the exhaust gases out the exhaust port and manifold (exhaust stroke).

As the exhaust valve opens, the relatively high cylinder pressure (70 – 90 psi), initiates exhaust blowdown and a large pressure wave travels down the exhaust pipe. As the valve continues to open, the exhaust gases begin flowing through the valve seat. The exhaust gases flow at an average speed of over 350 ft/sec, while the pressure wave travels at the speed of sound of around 1,700 ft/sec.

As one can see, there are two main phenomenons occurring in the exhaust, gas particle flow and pressure wave propagation. The objective of the exhaust is to remove as many gas particles as possible during the exhaust stroke. The proper handling of the pressure waves in the exhaust can help us to this end, and even help us “supercharge” the engine.

As the exhaust pressure wave arrives at the end of the exhaust pipe, part of the wave is reflected back towards the cylinder as a negative pressure (or vacuum) wave. This negative wave if timed properly to arrive at the cylinder during the overlap period can help scavenge the residual exhaust gases in the cylinder and also can initiate the flow of intake charge into the cylinder. Since the pressure waves travel at near the speed of sound, the timing of the negative wave can be controlled by the primary pipe length for a particular rpm.

If you've ever heard a engine running without a muffler, you know what a huge difference a muffler can make to the noise level. Inside a muffler, you'll find a deceptively simple set of tubes with some holes in them. These tubes and chambers are actuall¬y as finely tuned as a musical instrument. They are designed to reflect the sound waves produced by the engine in such a way that they partially cancel themselves out

In an engine, pulses are created when an exhaust valve opens and a burst of high-pressure gas suddenly enters the exhaust system. The molecules in this gas collide with the lower-pressure molecules in the pipe, causing them to stack up on each other. They in turn stack up on the molecules a little further down the pipe, leaving an area of low pressure behind. In this way, the sound wave makes its way down the pipe much faster than the actual gases do.

It turns out that it is possible to add two or more sound waves together and get less sound.

It is possible to produce a sound wave that is exactly the opposite of another wave. If the two waves are in phase, they add up to a wave with the same frequency but twice the amplitude. This is called constructive interference. But, if they are exactly out of phase, they add up to zero. This is called destructive interference.

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