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I'm not sure the 991 engine is the same head and header as the 997, certainly the rest of the plumbing is quite different. I'm hoping that "liberating" the exhaust gases will be rewarded with a little giddy up and some better sound track.
Yeah, it is a fair amount different from the 997. That could be one reason eliminating the center muffler does almost nothing to improve the sound of a 991. The side mufflers will still cancel out quite a bit of the noise.