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From what I'v heard... DC is good for headers. I have a DC 4 2 1, and I love it. Install was a breeze. 4-1 is better for racing, because all of your horsepower gains will be near redline. 4-2-1 is better for street, because you'll get gains all over, but mostly mid-range and up high. I'm not to sure about horsepower gains, it kinda depends on what you've already done to the engine. I think on a stock Si, its about 8-12 HP (I might be wrong). If you already have and CAI and exhast the gains will be much better.
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