The Victorian EPA has released technical guidelines to assist those who wish to customise their vehicle. Check out: http://www.epa.vic.gov.au/News/default.asp?artID=E15A3 Note in paticular sections: 1. Intake systems, 2. Fuel systems, 7. Engine management systems, 9 Exhaust manifolds, 10. Exhaust systems and 11. Niose limits. Maximum penalty for breaches of the Act is over $240K but I would assume that individuals would be small beer.