Robbie.
If you are to use the car in competition you will need to refer to, and comply with the MSA 'Blue Book' rules for motor sport.
The section for noise tests for rally cars is under section 'R' for Rallying.
I have put an extract of it below.
4.1.1. This test must be conducted by an Environmental Scrutineer using a sound meter as prescribed in the Technical Regulations (J.5.17, G.10.1 to G.11.2).
4.1.2. Any car reading over 98dB(A) in this test will be prohibited from starting, (except in Special Stage Rallies where a level of 100dB(A) is Permitted) unless the SRs specify lower levels.
4.1.3. All measurements refer to the 0.5m test (J.5.16).
The link to all of the sections in the Blue Book is below
http://www.msauk.org/site/cms/contentvi ... rticle=876
Do you have a company near you called 'Longlife Exhausts' as they would make a system up for you with whatever sound level you want?
All the Gear, no idea