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Your motorcycle’s exhaust is an important component of the engine.
It’s the final step in the combustion cycle, and manages the harmful fumes and gasses created as the engine runs.
The exhaust also affects the motorcycle’s performance.
Learn all about it below.
How does a motorcycle exhaust work?
The basic principle of working of a motorcycle exhaust has not changed much from the time of two-stroke engines.
Let’s see how it works.
After the completion of the power stroke, the piston of the motorcycle engine pushes up the exhaust gas through the exhaust port into the exhaust manifold which connects to the exhaust pipe.
The hot exhaust gas enters the exhaust pipe at high pressure (~80-100 PSI) and velocity.
The exhaust gas undergoes an expansion when it enters the wide part of the exhaust pipe and sends a low-pressure wave to the engine cylinder.
This low-pressure wave increases the pressure difference at the transfer port (inlet valve) and aids in the pumping of fresh air gasoline mixture into the cylinder.
The exhaust then enters the constant diameter section.Meanwhile, the piston reaches the bottom dead center (BDC) and rises back for the compression stroke.
Now the transfer port (inlet valve) is closed, but the exhaust port (outlet valve) is still open, and under this situation, the rising piston pushes the new air fuel mixture into the combustion chamber.
Tthe exhaust gas is now entering an inverse or inverted cone pipe and sends a positive pressure wave to the cylinder.
The positive wave of pressure pushes back the fresh air gasoline mixture back into the cylinder just in time before the piston closes the exhaust port.
The exhaust pipe of the modern four-stroke motorcycles is more engineered and sophisticated to meet the requirements of the high power engines, customer requirements, and emission & noise control laws.
However, it still follows the basic principle and aids in improving the performance of the engine.
In a multi-cylinder four-stroke engine, the hot exhaust gas enters the exhaust header pipe at around 100 PSI, travels down to a junction, enters the bigger section, and expands.
This expansion results in a negative wave to the engine cylinder that aids entry of fresh air gasoline mixture into the cylinder and also draws out the remaining traces of the exhaust from the cylinder.
The exhaust pipe of a motorcycle looks simple when you see it, however, the length of the exhaust pipe, its diameter over the different cross-section of the length is an important consideration for its designers, it affects the backpressure on the engine and hence its performance.
The exhaust pipe is built to control a few different functions: the exhaust gas flows out of the exhaust port of the cylinder into the header pipe and passes through a catalytic converter, pipe, and muffler before mixing with the atmospheric air.
Parts of the Motorcycle Exhaust
The pipes (including the header pipe which connects to the exhaust port with a flange and gasket) are normally made of stainless steel or steel (chrome-plated) and the size, length, and shape depend on the design of the individual motorcycles.
In the catalytic converter, the molecules of the toxic gases in the exhaust gas including carbon monoxide and Nitrogen oxide split into inert, nonhazardous gas particles Nitrogen, Carbon Dioxide, and water, before mixing with the atmospheric air.
The catalytic converter is a rectangular or cylindrical steel box normally fitted after the header of an exhaust pipe, or in some design it may be fitted in the muffler
You can find a honeycomb-shaped ceramic structure coated with catalysts (Platinum, Palladium, or Rhodium) inside the catalytic converter.
Due to the presence of the ‘catalysts’, the molecules of the toxic gases split into inert nonhazardous gas particles.
Catalytic converters are the efficient and economical method for controlling the pollution from the exhaust gases of automobiles including motorcycles.
The catalytic converters need a high temperature to do their function, hence they are placed nearer to the engine (in some designs, it is placed in the muffler).
The muffler has baffles and complex pipe shapes inside it absorb the noise and emit a pleasant sound conforming to the state law.
The muffler is meant to reduce the noise, however, some designs may use the muffler to modify the noise of the exhaust into a pleasant sound.
Makes the motorcycle operate quieter.
An exhaust guard is an optional item, it protects your leg from the heat of the exhaust pipe and protects the exhaust from dirt.
Replacing the Stock Exhaust
NOTE: State laws govern the exhaust pipes that can be used on a motorcycle. These laws may differ from state to state.
The major reasons for motorcycle riders to replace their stock exhaust with performance exhaust are:
- To improve the performance of their motorcycle with more torque and horsepower.
- Performance exhaust pipes are available in different finishes, add aesthetics, and improves the look of the motorcycle.
- In some models, changing the layout of the stock exhaust pipe with a performance exhaust pipe may increase the comfort of the rider and the passenger.
- Performance exhaust pipes sound better.
- Performance exhaust pipe can weigh less.
You must know clearly what you want when you are doing modification/replacement of your stock exhaust and you have the following options:
- Replace the stock exhaust completely with a performance exhaust.
- Replace only the stock exhaust header (retaining the muffler as it is).
- Replace the muffler of the stock exhaust pipe with an aftermarket slip-on muffler.
- Add the heat shield to the exposed stock exhaust pipes.
In the earlier days, one of the main intentions of replacing a stock exhaust of the motorcycle with a performance exhaust was to improve the engine performance.
Today, stock exhaust pipes have gone through sufficient Research & Development, and engineering, and they not only conform to the laws of the state but also protect optimum engine performance.
This leaves a very marginal scope for improving the engine performance by replacing it with a performance exhaust
There are important things to remember when you decide to go for an aftermarket exhaust for your motorcycle:
- Check local laws regarding aftermarket performance exhaust.
- Check the warranty conditions regarding aftermarket exhausts.
- Consider the cost of installation including rejetting of carburetor jet/needle system.
Is it legal to replace the stock exhaust with an aftermarket performance exhaust pipe?
Aftermarket performance exhaust are not legal in many states like California.
Check with your state laws and the manufacturer’s recommendations on this subject.