I was thinking about noise outside the car ... which is what we need to manage to avoid citation by local EPA - my MY03 STi was pinged remotely by noise-police and I was required to revert to OEM zorst pending A$22K fine (I already had 2-strikes, re-installed OEM zorst - noise tested OK and re-installed 4" dump and 3" system twice an EPA knew it).
Not sure what regulations you need to comply with on the road or the track, but here, we are required to comply wit 75Dba @ 30 metres drive by under WOT for track day and 95Dba @ 30m on a race day.
Road regulations are 85Dba with noise meter microphone 1m fro exhaust outlet at 45 degrees and engine free rev'd to 65% of OEM redline at which time the test official side-steps off the throttle ... HFC are NFG as they cause a crackle which spikes the noise over the limit.
Anyway - good luck with your project .... I'll be fitting a 95-litre bladder tank to my car when the cage goes in.
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