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My Dart weighs in the area of 3300 lbs race ready. I am running McCandless (Pro Parts) headers currently. Engine is stock stroke 440 zero decked with 915 heads (11.25 compression). Cam is DC 590. Thought the extensions might be worth exploring?

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Every car(Mopars only ) that I have added collector extensions on have ran faster, SB, BB or Hemi, stick or automatic trans I did find on my1963 415 HP M.W. NHRA stocker that I could add six inches primary extensions or add 12 inches of collector extensions, but not both, and get the same results That was on a set of the older two inch Hooker race headers with 3.5 inch collectors with no collector flanges, the extensions extensions slid over the collectors try the extensions on yours, make sure their longer than you think you need and then cut them off where needed