how is that possible that it could be worse?? never heard that, thought it would be some gain
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Aftermarket exhausts only show gains where the factory exhaust was poorly designed. If it's well-designed, as the Mazda6's is, then there is no room for aftermarket to improve and, depending on how they design it, it may actually be
less efficient (though probably not by much, if any).
Any company that's claiming gains on the Mazda6s exhaust is lying. They've all been tried and dyno-tested independently by buyers and there's been little to no measurable gains with any of them.
If you've got an i4 though, just don't get the Magnaflow. It's a good-sounding exhaust on the v6 but it sounds horrible on the i4. And that's not just my opinion, I'd say the vast majority will agree with that. Many people have bought it only to take it right back off.