In the UK you vehicles silencer must meet requirements that meet the Road traffics act but also MOT

Here are the guide lines laid down by the Ministry of Transport

Information

Method of Inspection

Reason for rejection

1. Examine the condition of the whole exhaust system, including the silencers and mountings, for security, deterioration and completeness

2. With the engine running a. check the exhaust system for leaks

Note: A durable repair to an exhaust system which effectively prevents leaks is acceptable providing the system is structurally sound
b. assess subjectively the effectiveness of the silencer in reducing exhaust noise to a level considered to be average for the vehicle.

1.
a. A part of the system missing or excessively deteriorated

b. an exhaust system mounting missing, or In such condition that it does not fully support the exhaust system.

Note: An alternative mounting device is acceptable providing the system is secure and the fixing is not likely to fail prematurely-

2.
a. A major leak of exhaust gases from any part of the system

Note: A minor exhaust leak from, for example, a connection joint or a pin hole, is not a Reason for Rejection

b. a silencer in such condition, or of such a type, that the noise emitted from the vehicle is clearly unreasonably above the level expected from a similar vehicle with a silencer in average condition.

When an engine is running it produces hundreds of explosions every second. These bangs are caused by the engine igniting compressed fuel in the chamber and then releasing high pressure gasses from the cylinder directly into the exhaust to be transported to the back of the car to be emitted into the atmosphere.

Trying to cover up this noise is very difficult and even the best sound deadening compounds can only manage to make the noise acceptable to within the few yards.

Modern exhausts do not try to deaden the sound with insulation. Instead they produce a noise that is the exact opposite of the noise produced by an engine.

Every vehicle produces noise, but as every make of car is designed differently, the noise they produce is slightly different. Or in other words, noise comes from the engine at a particular frequency which varies from car to car and model to model.

The best way to deaden this noise is to find this frequency and attempt to produce the exact opposite frequency. When these two noises come together they cancel out and as a result no sound is produced.

This is how most modern silencers work. The pressure waves from the engine are forced into chambers which are designed to reflect the pressure waves towards one another. By reversing the direction of the waves to meet new waves coming from the engine they cancel each other out this deadening the sound.

Most exhaust systems are tuned as finely as a piano, so the just the right frequency waves are reflected back to deaden the sound, but unlike a concert piano, a car does not always have the same pitch. As you accelerate, the pitch of the engine will increase, manufactures of exhausts will attempt to allow for this by producing baffles which can work across many different frequencies but it is impossible to get rid of all of the sound produced by the engine. This is why when you are driving and suddenly put your foot down the exhaust will appear to be louder. It is simply because the tuning frequency of your silencer has been passed and less sound deadening waves are being produced.

A noisy exhaust is a sure sign that there are problems with your system and it is recommended to get it looked at by our trained specialists as soon as possible.

Your silencer is one of the most important parts of your car! and often overlooked and ignored ,a faulty exhaust will result in:

• Loss of power
• Increase in fuel consumption
• Damage to the valves of your engine
• Poor tunning of your engine
• Polutants released into atmosphere
• Increased noise levels
• Risk of falling off altogether damageing your car or others on the road
• Failing MOT
• Polutants being drawn into the cockpit of your car
• A fixed penalty from the boys in blue