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Exhaust: Mufflers / Silencers
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I was ten years old when my mum managed to buy the family's first car - a bright blue Lada Riva. The muffler fell off that car at least twice - and wow, that sounded great to me as a kid. It was her own fault - she's totally incapable of slowing down for speed bumps. But if you've ever heard a car without a muffler, then you'll know how incredibly loud even the smallest engine is. In British English we use the word silencer for a muffler - it's exactly the same thing.
What does a muffler do?
The muffler reduces and adjusts the noise of the exhaust system. Mufflers are the final piece of the exhaust system, just before the tail pipe. There will be one or more muffler on every vehicle. On the Mazda MX5 Miata illustrated here there is a muffler and a
- which is a form of muffler designed to change the sound of the exhaust.
Mufflers are very interesting pieces of engineering - they are able to reduce the sound waves coming out of an exhaust, without restricting the flow of exhaust gases. Keep reading to learn about the different types of mufflers and how they work.
[Image of exhaust system showing muffler. ]
Why is a muffler needed?
Vehicles produce noise from a lot of places:
- The whistle of the
sucking in hundreds of litres of air every minute
Vehicle noise regulations
When there were only a handful of cars on the road, drivers were free to more or less produce as much noise as they liked. But with so many vehicles on the road now there are strict rules on the amount of noise that a car can make - and these rules are steadily tightening.
For production vehicles these regulations come from governments - generally that means the EU in Europe and federal and state governments in the US.
In Europe, the current rule is that cars produced since 2016 can make a maximum sound level of 74dB. Cars for the disabled are allowed another 1dB and bulletproof vehicles get an extra 2dB. That's a big reduction.
While individually we might want a loud exhaust, collectively no-one wants the constant sound of traffic in their lives, and animals definitely don't.
But even for racing cars there are usually rules - either set by the track or the sport governing body. Typically a race car is limited to a sound level of 100dB at around 100 feet. That is very loud. Formula One has no limits on noise.
How a muffler works
The most important thing to understand with mufflers is that we are dealing with two different things:
- The exhaust gases are physically flowing through the exhaust. The muffler should restrict these gases as little as possible so that they move quickly out of the tailpipe.
- Sound waves are traveling through the gas inside the exhaust. Sound waves and pressure waves are the same thing - they are caused by the movement of the gas but they travel through it.
You can feel this difference when you slam a door. You'll hear the sound instantly but it takes some time for you to feel the movement of air on your skin.
The muffler reduces the amplitude (loudness) of sound waves in two ways.
Absorption : some of the exhaust gas enters a soft material, typically fibre-glass, where the energy of sound waves is absorbed and their strength reduced.
Reflection : Sound waves can be bounced off surfaces and reflected into each other. This is known as destructive interference . Where the peaks of sound-waves cancel each other out - just like in noise cancelling headphones.
How a turbo muffler works
[Diagram of turbo muffler]
There are three main types of muffler, and the most common by far is a design known as a
Exhaust gases enter on one side of the muffler, travel through a series of pipes and chambers, and then exit through the other side.
The gases typically travel in an S-shape on their way through the muffler. Along the way there are perforations in the pipes which allow the gas and sound waves to enter a chamber. The perforations are specific sizes and are engineered to break up the sound waves.
The chambers are made up from a series of baffles which are spaced to coincide with certain frequencies of sounds. As the sound waves reflect around inside these chambers, they will collide and cancel each other out using destructive interference.
A turbo muffler is very effective at silencing but this comes at the cost of restricting the exhaust gas flow as it makes its way through the system. This causes a small amount of back pressure.
Why is back pressure bad?
Generally back pressure is undesirable in an exhaust system - especially for naturally aspirated engines that rely only on suction to pull air and fuel into the cylinder.
With back pressure present, a part of the exhaust gas will remain in the cylinder after the exhaust stroke. On the next intake stroke, this exhaust gas takes up space intended for fuel and air - so there is less material to burn and the engine is less efficient than it could be. In addition, the exhaust gas that remains is hot and causes the intake charge to heat up - reducing its density and meaning that there is even less fuel and air in the cylinder for combustion.
This is less of an issue on forced induction - turbo-charged and super-charged - engines because the intake charge is forced in under pressure.
[Image of chambered muffler]
From the outside, a chambered muffler looks identical to a turbo muffler, but it's internals are simpler. Instead of the gas traveling in an S-shape, it travels directly from one side to the other through a series of chambers and baffles.
A chambered muffler is less restrictive on the flow of exhaust gases, giving minimal back pressure and so this design is found on performance vehicles and in the aftermarket. It has less of a silencing effect on the exhaust and gives a deep rumbling sound.
Straight through (absorption) mufflers
The original design of muffler is a straight through muffler. It's seen on hot rods or at the side of classics like the AC Cobra. In this design a perforated pipe runs straight through the muffler, wrapped in sound absorbent fibre-glass like rockwool. The idea behind this muffler design is the same as filling a room with soft-furnishings to reduce noise.
As the sound waves travel through the pipe, they will escape through the perforations where they are reduced in two ways
- The energy of sound waves vibrates the fibres of the material and is converted into heat energy which is radiated by the muffler housing.
- As the sound waves bounce around in the absorbent material, they are knocked out of phase - when they rejoin the main flow of gas they will tend to interfere with other sound waves, reducing their amplitude.
A straight-through muffler creates almost no back pressure - in fact a well-engineered straight-through muffler can improve the flow of exhaust gases compared to a straight pipe. But their noise reduction is limited compared to the other two types and they are not generally found on production cars.
Mufflers typically fail from corrosion. Like all exhaust parts, they have a hard life being exposed to hot corrosive gases on the inside, and the weather on the outside.
Water is a byproduct of combustion - it comes out of the cylinder as hot steam. When a engine starts the exhaust system is cold and this steam will condense into water. Since the muffler is furthest away from the engine it takes the longest to heat up. Water will build up inside until the muffler warms up and the water is boiled into steam and goes out with the rest of the exhaust gases.
If a car makes only very short journeys there is a risk that the muffler never warms up and water builds up inside, rusting the muffler from the inside out. Mufflers will often incorporate a pinhole at the bottom to drain out any built up water.
The reality is that this is probably why my mum destroyed so many mufflers on her Lada, but it's much more satisfying telling her that she's a speed demon and can't help herself but launch off every speed bump she sees.
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