This is from the LTSA site....
Assessing noise levels of modified exhaust systems
This Infosheet clarifies the inspection requirements for modified exhaust systems (reason for rejection number 9 in the Exhaust section of the Vehicle Inspection Requirements Manual).
The Infosheet suggests an assessment method that may be used if you are presented with a vehicle for a Warrant of Fitness inspection and you suspect the vehicle’s exhaust system has been modified and is louder than it should be (see Clarification).
Please note: An amendment to the Land Transport Act 1998 that comes into force on 2 May 2003 includes, under section 96(1B), provision for the Police to ‘green-sticker’ a vehicle that has been impounded for unauthorised street racing. With the increased focus on street racer vehicles there is also likely to be an increase in the number 'green stickered' under section 115 of the Act, for vehicle defects. One of these defects may be a noisy modified exhaust system. A vehicle that has been ‘green-stickered’ under section 96(1B) or for a noisy exhaust may only be inspected for a new warrant by a vehicle inspector who is employed by a Transport Service Delivery (TSD) Agent (New Zealand Automobile Association Inc, On Road NZ Ltd, Vehicle Inspection New Zealand Ltd, or Vehicle Testing New Zealand Ltd).
A vehicle’s exhaust system and silencer must be effective and in good working order (required by regulation 81 of the Traffic Regulations 1976). As stated in 11-1 of the Vehicle Inspection Requirements Manual, a reason for rejection is if “the noise output is noticeably and significantly higher than should be expected for the vehicle.”
When the vehicle leaves the manufacturing plant it will have been fitted with an effective exhaust system and silencer. This is your point of reference for the performance of any replacement exhaust systems. A modified exhaust system should not make the vehicle louder than it would have been when the vehicle was manufactured with its original exhaust system.
Suggested assessment method
Increase the engine speed to about half the maximum permitted limit indicated on the vehicle’s tachometer (rev counter). This is the test speed. (A ‘red line’ on the tachometer usually shows the maximum permitted limit.)
Hold the engine at this test speed for about five seconds and then reduce to idling speed.
Follow this with a series of three or four short and sharp speed increases, from idle to test speed.
If the vehicle doesn’t have a tachometer use your judgement to determine the engine speed, but follow the same test sequence.
If you suspect the vehicle’s exhaust system has been modified and the noise is louder than you’d expect from its original equipment, you should reject the vehicle.
Give the vehicle owner the LTSA leaflet “Your vehicle has failed its WoF because of a noisy exhaust” explaining why their vehicle has been rejected and what their options are for correcting the noisy exhaust.
Hmm, not good....
Diehard AU Series fan