Discussing such ACL issues doesn't mean your supplier has actually done anything wrong. By the sounds of it, they are helping you with your issue. So if you're happy with their service, that's what matters
And good on you for supporting a local business (which in these pandemic times is needed)
But briefly discussing such ACL stuff may help someone else who stumbles on this thread some time in the future. That someone who has payed $$$ for a complete exhaust and sufferes maddening drone but their supplier washes their hands of the situation unless they fork out more $$ may appreciate the heads up (and the other avenue they could persue if their supplier plays hardball and refuses to resolve the issue).
You make some valid points SL
Summary:
1: After some discussion with my mechanic/tuner
I chose and had fitted a full system, 1 3/4 extractors, high-flow cats (100 cell), 3 inch catback + highflow rear mufflers + bi-modals. In hindsight I didn't do my homework and should've questioned more. I expected it to be loud and it was, but buying blind carries some risks. The 3 inch system sounded impressive in both open and closed mode but
droned badly between 1600 - 2200rpm
2: Took it back to the exhaust shop, they fitted a 2 1/2 system with "quiet" rear mufflers. This change reduced the overall
volume by about 30% but
droned just as bad, particularly bad between 1800 to 2200 rpm. Sixth gear at 110kph on the freeway sits around 2000rpm, horrible.
3: Back to the shop again, fitted more restrictive rear mufflers(
AFM)? Result, the overall volume has decreased again by about another 30%,
drones a
little less but still horrible between 1800 - 2200rpm. I still like the
sound (except between 1800-2200)however the volume levels have now dropped by 50 to 60% and any more significant drop in volume will more closely resemble the factory system, which essentially renders the catback change a pointless excercise.
It would appear obvious by the above that the rear mufflers are not really the problem, sure they decrease the overall volume but do not attenuate the drone. I planned to have my factory rear mufflers fitted if step 3 failed but I suspect it will be quieter again and the drone will remain.
NAAF pretty much aluded to this when he said the
merge collector is the issue.
It'll be back to the shop again for further mods, hopefully not another rear muffler change.
FYI I must take some responsibility as I'm aware all aftermarket exhausts do not meet adr and as such are sold as off-street/track use.
The exhaust shop, despite having not yet solved my issue have complied without question by my requests for a fix. It does surprise that they haven't suggested a front muffler as NAAF stated, maybe I should tell them, but as a somewhat uninformed customer it would feel odd streering them towards the path of a fix.