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i have catless downpipe paired with the standard cat back exhaust, for the CEL question i installed the MHD stage 0 and just flag the "aftermarket downpipe" and no CEL or issue (now stage 1).
The sound is a little bit raspy with too content of high frequency and not drone effect even an 35600rpm, very confortable set up and on full throttle is very satisfying.
For the perfect sound you have to upgrade only the axle back OEM muffler made by resonator chambers (works well at lower frequencies), because you need more low frequency tone you have to switch on absorptive type muffler.
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2015 BMW M135i F20 LCI, man box, adaptive, HK, decat, MHD Stage 1, F21 underbrace retrofit, MP LSD, M4 LCA & TS, M Performance BBK, MSL oil Tstat, MPPK Airbox, CDV delete, Wagner EVO II, FTP CP, MST V2 turbo inlet, M2 sump and pumps.
... "beauty is the splendor of truth"...
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