The rails will allow you to improve flow into the rail bores where the stock rails have a little inlet tube attached to them. But you can also do a return up front with the stock rails like our S0 kit does, we sell the billet fitting and fuel pump module modifications separate if you wanted to build your own engine bay return off the stock rails. I have tested this type of setup to about 600rwhp with E85, and I suggest new rails for better plumbing beyond that. But if using gasoline rather than E85, 700 *might* be a reality on the stock rails.
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