Exhaust Mod Issues

[BlackRST]

I did the $20 exhaust mod a few days ago on a whim. I had the parts laying around. I did like the change in tone, it was enough for me but my gosh my MPG's dropped like crazy.  So i removed the flapper valve block but now im not sure if the darn thing closes properly. Ive left the resonators covered still not sure if that would make a huge difference or not? Also getting that helicopter sound as well.  

[topgear1224]

What engine?

The helicopter sound is the AFM/DFM disabling cylinders. I noticed it even in stock form.

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[BlackRST]

5.3 from the RST


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