Quote:
Originally Posted by 2SS Capt
E85 is probably your easiest route...
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E85 will reduce his gas mileage by 25-30%, which he probably doesn't want, given his OP.
Keeping it "not too loud" rules out headers, too, he doesn't want FI or opening up the engine either, nor anything that changes drivability, which leaves only the intake tract... all I can think of is a cold air intake, maybe a new intake manifold, plus a tune, which will not add a lot of power. The car being an M6, nothing to gain around the transmission either.
All in all, with these very strict constraints, I'd just add a CAI and a tune, lean back and enjoy the car practically as-is with the 5-10 added hp. Not really worth the money IMO, but getting an MPVI device and being able to tweak the car in the future may have some value. 2016 M6s are relatively cheap to tune, too, it only takes 2 credits (vs 9 for my 2018 A8)