i live in tas ,have a worked a15 with twin deloutoes (40mm),and was told extractors and a 1.75 inch exhast was just right for back pressure ,which is what i got (i dont know if they
just had some 1.75 pipe that they were trying to get rid of though)i hope this helps.... by the way ,a mate of mine in a cheve impala can make his exhaust flame by flicking a switch but
wont tell me how ,does any one know how such a system is rigged up i think he uses a spark plug some how?????