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I am planning on doing a quad tip setup on my 2014 is350 f sport at the moment. I just ordered my remark axle back exhaust. I want to install the 2017+ rear diffuser to fit a quad tip setup. I have found a good deal on ebay for the diffuser and checked the part numbers with previous posts about this mod. I want to make sure that this will fit before I go ahead and order it. If anyone could double check me to make sure this will fit my 2014 is I would greatly appreciate it! https://www.ebay.com/itm/New-Air-Dam...oAAOSw9D9gAWOh
The rear bumper on 2014-2020 IS's are all the same, regardless of trim and engine.
While the rear diffuser did change in 2017 going forward, the mounting points on the bumper are the same for all diffusers on the 3IS.
So basically any diffuser made for a 3IS will fit any 3IS.
If you have ordered a quad tip exhaust, then the only fitment issue is will the tips fit the diffuser.