This letter is just one example of the many queries we receive on the subject of noisy exhausts. Given that there have been some recent changes to the Inspection Manual in this section, I thought it was a good time to cover this area of the test.
It is likely that many of these ‘big bore’ exhaust systems don’t meet the requirements of Regulation 54(2) of the Road Vehicles (Construction and Use) Regulation 1986 as amended: ‘Every exhaust system and silencer shall be maintained in good and efficient working order and shall not be altered to increase the noise made by the escape of exhaust gases.’ I don’t think this needs any further explanation, do you?!
Clearly the fitting of aftermarket exhaust systems is becoming an issue. As I have responsibility for MOT technical standards, I need to make sure that the guidance given in the Inspection Manual is clear, and that the Reason for Rejection (RfR) does not confuse.
The Method of Inspection in the manual is to ‘assess subjectively the effectiveness of the silencer in reducing exhaust noise to a level considered to be average for the vehicle’. This is open to some interpretation and we quickly realised that the Method of Inspection gave no indication of what engine speed should be achieved before the noise of the exhaust is assessed.
To remedy this, we have added a note to the MoI which now requires testers to assess the level of exhaust noise at all engine speeds required for the exhaust emissions check. We have also made a slight amendment to the RfR, which now makes it clear that the comparison is with a similar vehicle with a standard silencer in average condition.
All this reminds me of another story of a complaint from an angry father, whose son had been given a prohibition by the police for a noisy exhaust shortly after the vehicle had passed the test. Our investigation found that his son had removed the offending silencer following an emission test failure. So the retest was carried out with a standard system. We don’t know what the outcome was after we told his father!