The 4BW has no secondary cats in the mid-pipe—the emissions/CEL conversation centers on the primary cats in the downpipes.
G-Sport Gen2 400 cell cats are great for better performance with near-stock efficiency and without catless-type odors, but GM’s O2 sensors are sensitive enough that almost anything non-stock will eventually throw a CEL after a number of miles. You can solve this with tuning or a DTC-Assassin type of clearing device, but tuning usually means potential powertrain warranty issues if something bad happens (although not extremely likely to happen with mild mods, but never say never), and your cat “readiness” status may affect emissions testing depending on your state.
My advice: check your state’s emissions requirements first (OBD only vs. OBD + sniffer, etc; also how many “not ready” statuses are allowed). In Texas for example, one “not ready” status was acceptable and I always passed my 335i even with aftermarket downpipes and basic “EPA” cats (it was tuned though, so no lights on dash). Then either accept the potential CEL if it happens or plan to tune it out. I’d be skeptical that any aftermarket downpipes on a modern GM with dual wideband O2s will never throw a code.
Worth noting: CPD makes similar high quality pipes with G-Sport cats (he will build them with Gen2 E400s if requested, however I can’t confirm the model# of the G-Sports he uses and if it’s the same 5” variant previously discussed from ARH), has been doing exhaust kits for the 4BW and ATS-V for a while, and offers optional extras like crosstubes for rasp control and Zybar heat coating. Austin is very responsive on IG if you have questions or want to cross-shop first. With that said, the ARH downpipes also look like a high-quality setup.
I have been shopping for downpipes as well for a while now and honestly did not find ARH until your post. If you go ARH, please post plenty of sound clips and let us know how it goes. I hope it all works out!
americanracingheaders.com
www.gsportbygesi.com
