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2017+ manuals got a new intake manifold, slight change to header, and a new tune to accommodate those changes resulting in a total gain of 5hp and 5ft lbs (at the crank not wheels) so even assuming the header was responsible for 80% of that so 4hp or 3.5whp. So take typical expected gains from a header on a pre 17 and subtract 3.5whp for new expected gains.
Also keep in mind unless drastically changing the design of the header like a 4-1 design, or a 4-2-1 design that utilizes the OP to effectively increase runner length of the 4-2 pipe, then roughly 80% of the gain from a new header is from removing the cats.
E85 especially with flex fuel is great by itself or in addition to any other modifications.
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