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I have a mild 400, .528 solid cam, old DC Stage 4 heads. I can get a good deal on a set of headers with 2 1/8 primaries. What would I be giving up it terms of torque and horsepower using these headers vs a 1 3/4" primary? Full exhaust street car. Thank you

As others have said, you would get killed in torque below 5500 rpm. And below 4000 rpm would be really bad. There is no way I would run a header that big in dia in your combo.